Source file src/runtime/panic.go

     1  // Copyright 2014 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package runtime
     6  
     7  import (
     8  	"internal/abi"
     9  	"internal/goarch"
    10  	"internal/runtime/atomic"
    11  	"internal/stringslite"
    12  	"runtime/internal/sys"
    13  	"unsafe"
    14  )
    15  
    16  // throwType indicates the current type of ongoing throw, which affects the
    17  // amount of detail printed to stderr. Higher values include more detail.
    18  type throwType uint32
    19  
    20  const (
    21  	// throwTypeNone means that we are not throwing.
    22  	throwTypeNone throwType = iota
    23  
    24  	// throwTypeUser is a throw due to a problem with the application.
    25  	//
    26  	// These throws do not include runtime frames, system goroutines, or
    27  	// frame metadata.
    28  	throwTypeUser
    29  
    30  	// throwTypeRuntime is a throw due to a problem with Go itself.
    31  	//
    32  	// These throws include as much information as possible to aid in
    33  	// debugging the runtime, including runtime frames, system goroutines,
    34  	// and frame metadata.
    35  	throwTypeRuntime
    36  )
    37  
    38  // We have two different ways of doing defers. The older way involves creating a
    39  // defer record at the time that a defer statement is executing and adding it to a
    40  // defer chain. This chain is inspected by the deferreturn call at all function
    41  // exits in order to run the appropriate defer calls. A cheaper way (which we call
    42  // open-coded defers) is used for functions in which no defer statements occur in
    43  // loops. In that case, we simply store the defer function/arg information into
    44  // specific stack slots at the point of each defer statement, as well as setting a
    45  // bit in a bitmask. At each function exit, we add inline code to directly make
    46  // the appropriate defer calls based on the bitmask and fn/arg information stored
    47  // on the stack. During panic/Goexit processing, the appropriate defer calls are
    48  // made using extra funcdata info that indicates the exact stack slots that
    49  // contain the bitmask and defer fn/args.
    50  
    51  // Check to make sure we can really generate a panic. If the panic
    52  // was generated from the runtime, or from inside malloc, then convert
    53  // to a throw of msg.
    54  // pc should be the program counter of the compiler-generated code that
    55  // triggered this panic.
    56  func panicCheck1(pc uintptr, msg string) {
    57  	if goarch.IsWasm == 0 && stringslite.HasPrefix(funcname(findfunc(pc)), "runtime.") {
    58  		// Note: wasm can't tail call, so we can't get the original caller's pc.
    59  		throw(msg)
    60  	}
    61  	// TODO: is this redundant? How could we be in malloc
    62  	// but not in the runtime? runtime/internal/*, maybe?
    63  	gp := getg()
    64  	if gp != nil && gp.m != nil && gp.m.mallocing != 0 {
    65  		throw(msg)
    66  	}
    67  }
    68  
    69  // Same as above, but calling from the runtime is allowed.
    70  //
    71  // Using this function is necessary for any panic that may be
    72  // generated by runtime.sigpanic, since those are always called by the
    73  // runtime.
    74  func panicCheck2(err string) {
    75  	// panic allocates, so to avoid recursive malloc, turn panics
    76  	// during malloc into throws.
    77  	gp := getg()
    78  	if gp != nil && gp.m != nil && gp.m.mallocing != 0 {
    79  		throw(err)
    80  	}
    81  }
    82  
    83  // Many of the following panic entry-points turn into throws when they
    84  // happen in various runtime contexts. These should never happen in
    85  // the runtime, and if they do, they indicate a serious issue and
    86  // should not be caught by user code.
    87  //
    88  // The panic{Index,Slice,divide,shift} functions are called by
    89  // code generated by the compiler for out of bounds index expressions,
    90  // out of bounds slice expressions, division by zero, and shift by negative.
    91  // The panicdivide (again), panicoverflow, panicfloat, and panicmem
    92  // functions are called by the signal handler when a signal occurs
    93  // indicating the respective problem.
    94  //
    95  // Since panic{Index,Slice,shift} are never called directly, and
    96  // since the runtime package should never have an out of bounds slice
    97  // or array reference or negative shift, if we see those functions called from the
    98  // runtime package we turn the panic into a throw. That will dump the
    99  // entire runtime stack for easier debugging.
   100  //
   101  // The entry points called by the signal handler will be called from
   102  // runtime.sigpanic, so we can't disallow calls from the runtime to
   103  // these (they always look like they're called from the runtime).
   104  // Hence, for these, we just check for clearly bad runtime conditions.
   105  //
   106  // The panic{Index,Slice} functions are implemented in assembly and tail call
   107  // to the goPanic{Index,Slice} functions below. This is done so we can use
   108  // a space-minimal register calling convention.
   109  
   110  // failures in the comparisons for s[x], 0 <= x < y (y == len(s))
   111  //
   112  //go:yeswritebarrierrec
   113  func goPanicIndex(x int, y int) {
   114  	panicCheck1(getcallerpc(), "index out of range")
   115  	panic(boundsError{x: int64(x), signed: true, y: y, code: boundsIndex})
   116  }
   117  
   118  //go:yeswritebarrierrec
   119  func goPanicIndexU(x uint, y int) {
   120  	panicCheck1(getcallerpc(), "index out of range")
   121  	panic(boundsError{x: int64(x), signed: false, y: y, code: boundsIndex})
   122  }
   123  
   124  // failures in the comparisons for s[:x], 0 <= x <= y (y == len(s) or cap(s))
   125  //
   126  //go:yeswritebarrierrec
   127  func goPanicSliceAlen(x int, y int) {
   128  	panicCheck1(getcallerpc(), "slice bounds out of range")
   129  	panic(boundsError{x: int64(x), signed: true, y: y, code: boundsSliceAlen})
   130  }
   131  
   132  //go:yeswritebarrierrec
   133  func goPanicSliceAlenU(x uint, y int) {
   134  	panicCheck1(getcallerpc(), "slice bounds out of range")
   135  	panic(boundsError{x: int64(x), signed: false, y: y, code: boundsSliceAlen})
   136  }
   137  
   138  //go:yeswritebarrierrec
   139  func goPanicSliceAcap(x int, y int) {
   140  	panicCheck1(getcallerpc(), "slice bounds out of range")
   141  	panic(boundsError{x: int64(x), signed: true, y: y, code: boundsSliceAcap})
   142  }
   143  
   144  //go:yeswritebarrierrec
   145  func goPanicSliceAcapU(x uint, y int) {
   146  	panicCheck1(getcallerpc(), "slice bounds out of range")
   147  	panic(boundsError{x: int64(x), signed: false, y: y, code: boundsSliceAcap})
   148  }
   149  
   150  // failures in the comparisons for s[x:y], 0 <= x <= y
   151  //
   152  //go:yeswritebarrierrec
   153  func goPanicSliceB(x int, y int) {
   154  	panicCheck1(getcallerpc(), "slice bounds out of range")
   155  	panic(boundsError{x: int64(x), signed: true, y: y, code: boundsSliceB})
   156  }
   157  
   158  //go:yeswritebarrierrec
   159  func goPanicSliceBU(x uint, y int) {
   160  	panicCheck1(getcallerpc(), "slice bounds out of range")
   161  	panic(boundsError{x: int64(x), signed: false, y: y, code: boundsSliceB})
   162  }
   163  
   164  // failures in the comparisons for s[::x], 0 <= x <= y (y == len(s) or cap(s))
   165  func goPanicSlice3Alen(x int, y int) {
   166  	panicCheck1(getcallerpc(), "slice bounds out of range")
   167  	panic(boundsError{x: int64(x), signed: true, y: y, code: boundsSlice3Alen})
   168  }
   169  func goPanicSlice3AlenU(x uint, y int) {
   170  	panicCheck1(getcallerpc(), "slice bounds out of range")
   171  	panic(boundsError{x: int64(x), signed: false, y: y, code: boundsSlice3Alen})
   172  }
   173  func goPanicSlice3Acap(x int, y int) {
   174  	panicCheck1(getcallerpc(), "slice bounds out of range")
   175  	panic(boundsError{x: int64(x), signed: true, y: y, code: boundsSlice3Acap})
   176  }
   177  func goPanicSlice3AcapU(x uint, y int) {
   178  	panicCheck1(getcallerpc(), "slice bounds out of range")
   179  	panic(boundsError{x: int64(x), signed: false, y: y, code: boundsSlice3Acap})
   180  }
   181  
   182  // failures in the comparisons for s[:x:y], 0 <= x <= y
   183  func goPanicSlice3B(x int, y int) {
   184  	panicCheck1(getcallerpc(), "slice bounds out of range")
   185  	panic(boundsError{x: int64(x), signed: true, y: y, code: boundsSlice3B})
   186  }
   187  func goPanicSlice3BU(x uint, y int) {
   188  	panicCheck1(getcallerpc(), "slice bounds out of range")
   189  	panic(boundsError{x: int64(x), signed: false, y: y, code: boundsSlice3B})
   190  }
   191  
   192  // failures in the comparisons for s[x:y:], 0 <= x <= y
   193  func goPanicSlice3C(x int, y int) {
   194  	panicCheck1(getcallerpc(), "slice bounds out of range")
   195  	panic(boundsError{x: int64(x), signed: true, y: y, code: boundsSlice3C})
   196  }
   197  func goPanicSlice3CU(x uint, y int) {
   198  	panicCheck1(getcallerpc(), "slice bounds out of range")
   199  	panic(boundsError{x: int64(x), signed: false, y: y, code: boundsSlice3C})
   200  }
   201  
   202  // failures in the conversion ([x]T)(s) or (*[x]T)(s), 0 <= x <= y, y == len(s)
   203  func goPanicSliceConvert(x int, y int) {
   204  	panicCheck1(getcallerpc(), "slice length too short to convert to array or pointer to array")
   205  	panic(boundsError{x: int64(x), signed: true, y: y, code: boundsConvert})
   206  }
   207  
   208  // Implemented in assembly, as they take arguments in registers.
   209  // Declared here to mark them as ABIInternal.
   210  func panicIndex(x int, y int)
   211  func panicIndexU(x uint, y int)
   212  func panicSliceAlen(x int, y int)
   213  func panicSliceAlenU(x uint, y int)
   214  func panicSliceAcap(x int, y int)
   215  func panicSliceAcapU(x uint, y int)
   216  func panicSliceB(x int, y int)
   217  func panicSliceBU(x uint, y int)
   218  func panicSlice3Alen(x int, y int)
   219  func panicSlice3AlenU(x uint, y int)
   220  func panicSlice3Acap(x int, y int)
   221  func panicSlice3AcapU(x uint, y int)
   222  func panicSlice3B(x int, y int)
   223  func panicSlice3BU(x uint, y int)
   224  func panicSlice3C(x int, y int)
   225  func panicSlice3CU(x uint, y int)
   226  func panicSliceConvert(x int, y int)
   227  
   228  var shiftError = error(errorString("negative shift amount"))
   229  
   230  //go:yeswritebarrierrec
   231  func panicshift() {
   232  	panicCheck1(getcallerpc(), "negative shift amount")
   233  	panic(shiftError)
   234  }
   235  
   236  var divideError = error(errorString("integer divide by zero"))
   237  
   238  //go:yeswritebarrierrec
   239  func panicdivide() {
   240  	panicCheck2("integer divide by zero")
   241  	panic(divideError)
   242  }
   243  
   244  var overflowError = error(errorString("integer overflow"))
   245  
   246  func panicoverflow() {
   247  	panicCheck2("integer overflow")
   248  	panic(overflowError)
   249  }
   250  
   251  var floatError = error(errorString("floating point error"))
   252  
   253  func panicfloat() {
   254  	panicCheck2("floating point error")
   255  	panic(floatError)
   256  }
   257  
   258  var memoryError = error(errorString("invalid memory address or nil pointer dereference"))
   259  
   260  func panicmem() {
   261  	panicCheck2("invalid memory address or nil pointer dereference")
   262  	panic(memoryError)
   263  }
   264  
   265  func panicmemAddr(addr uintptr) {
   266  	panicCheck2("invalid memory address or nil pointer dereference")
   267  	panic(errorAddressString{msg: "invalid memory address or nil pointer dereference", addr: addr})
   268  }
   269  
   270  // Create a new deferred function fn, which has no arguments and results.
   271  // The compiler turns a defer statement into a call to this.
   272  func deferproc(fn func()) {
   273  	gp := getg()
   274  	if gp.m.curg != gp {
   275  		// go code on the system stack can't defer
   276  		throw("defer on system stack")
   277  	}
   278  
   279  	d := newdefer()
   280  	d.link = gp._defer
   281  	gp._defer = d
   282  	d.fn = fn
   283  	d.pc = getcallerpc()
   284  	// We must not be preempted between calling getcallersp and
   285  	// storing it to d.sp because getcallersp's result is a
   286  	// uintptr stack pointer.
   287  	d.sp = getcallersp()
   288  
   289  	// deferproc returns 0 normally.
   290  	// a deferred func that stops a panic
   291  	// makes the deferproc return 1.
   292  	// the code the compiler generates always
   293  	// checks the return value and jumps to the
   294  	// end of the function if deferproc returns != 0.
   295  	return0()
   296  	// No code can go here - the C return register has
   297  	// been set and must not be clobbered.
   298  }
   299  
   300  var rangeDoneError = error(errorString("range function continued iteration after function for loop body returned false"))
   301  var rangePanicError = error(errorString("range function continued iteration after loop body panic"))
   302  var rangeExhaustedError = error(errorString("range function continued iteration after whole loop exit"))
   303  var rangeMissingPanicError = error(errorString("range function recovered a loop body panic and did not resume panicking"))
   304  
   305  //go:noinline
   306  func panicrangestate(state int) {
   307  	switch abi.RF_State(state) {
   308  	case abi.RF_DONE:
   309  		panic(rangeDoneError)
   310  	case abi.RF_PANIC:
   311  		panic(rangePanicError)
   312  	case abi.RF_EXHAUSTED:
   313  		panic(rangeExhaustedError)
   314  	case abi.RF_MISSING_PANIC:
   315  		panic(rangeMissingPanicError)
   316  	}
   317  	throw("unexpected state passed to panicrangestate")
   318  }
   319  
   320  // deferrangefunc is called by functions that are about to
   321  // execute a range-over-function loop in which the loop body
   322  // may execute a defer statement. That defer needs to add to
   323  // the chain for the current function, not the func literal synthesized
   324  // to represent the loop body. To do that, the original function
   325  // calls deferrangefunc to obtain an opaque token representing
   326  // the current frame, and then the loop body uses deferprocat
   327  // instead of deferproc to add to that frame's defer lists.
   328  //
   329  // The token is an 'any' with underlying type *atomic.Pointer[_defer].
   330  // It is the atomically-updated head of a linked list of _defer structs
   331  // representing deferred calls. At the same time, we create a _defer
   332  // struct on the main g._defer list with d.head set to this head pointer.
   333  //
   334  // The g._defer list is now a linked list of deferred calls,
   335  // but an atomic list hanging off:
   336  //
   337  //		g._defer => d4 -> d3 -> drangefunc -> d2 -> d1 -> nil
   338  //	                             | .head
   339  //	                             |
   340  //	                             +--> dY -> dX -> nil
   341  //
   342  // with each -> indicating a d.link pointer, and where drangefunc
   343  // has the d.rangefunc = true bit set.
   344  // Note that the function being ranged over may have added
   345  // its own defers (d4 and d3), so drangefunc need not be at the
   346  // top of the list when deferprocat is used. This is why we pass
   347  // the atomic head explicitly.
   348  //
   349  // To keep misbehaving programs from crashing the runtime,
   350  // deferprocat pushes new defers onto the .head list atomically.
   351  // The fact that it is a separate list from the main goroutine
   352  // defer list means that the main goroutine's defers can still
   353  // be handled non-atomically.
   354  //
   355  // In the diagram, dY and dX are meant to be processed when
   356  // drangefunc would be processed, which is to say the defer order
   357  // should be d4, d3, dY, dX, d2, d1. To make that happen,
   358  // when defer processing reaches a d with rangefunc=true,
   359  // it calls deferconvert to atomically take the extras
   360  // away from d.head and then adds them to the main list.
   361  //
   362  // That is, deferconvert changes this list:
   363  //
   364  //		g._defer => drangefunc -> d2 -> d1 -> nil
   365  //	                 | .head
   366  //	                 |
   367  //	                 +--> dY -> dX -> nil
   368  //
   369  // into this list:
   370  //
   371  //	g._defer => dY -> dX -> d2 -> d1 -> nil
   372  //
   373  // It also poisons *drangefunc.head so that any future
   374  // deferprocat using that head will throw.
   375  // (The atomic head is ordinary garbage collected memory so that
   376  // it's not a problem if user code holds onto it beyond
   377  // the lifetime of drangefunc.)
   378  //
   379  // TODO: We could arrange for the compiler to call into the
   380  // runtime after the loop finishes normally, to do an eager
   381  // deferconvert, which would catch calling the loop body
   382  // and having it defer after the loop is done. If we have a
   383  // more general catch of loop body misuse, though, this
   384  // might not be worth worrying about in addition.
   385  //
   386  // See also ../cmd/compile/internal/rangefunc/rewrite.go.
   387  func deferrangefunc() any {
   388  	gp := getg()
   389  	if gp.m.curg != gp {
   390  		// go code on the system stack can't defer
   391  		throw("defer on system stack")
   392  	}
   393  
   394  	d := newdefer()
   395  	d.link = gp._defer
   396  	gp._defer = d
   397  	d.pc = getcallerpc()
   398  	// We must not be preempted between calling getcallersp and
   399  	// storing it to d.sp because getcallersp's result is a
   400  	// uintptr stack pointer.
   401  	d.sp = getcallersp()
   402  
   403  	d.rangefunc = true
   404  	d.head = new(atomic.Pointer[_defer])
   405  
   406  	return d.head
   407  }
   408  
   409  // badDefer returns a fixed bad defer pointer for poisoning an atomic defer list head.
   410  func badDefer() *_defer {
   411  	return (*_defer)(unsafe.Pointer(uintptr(1)))
   412  }
   413  
   414  // deferprocat is like deferproc but adds to the atomic list represented by frame.
   415  // See the doc comment for deferrangefunc for details.
   416  func deferprocat(fn func(), frame any) {
   417  	head := frame.(*atomic.Pointer[_defer])
   418  	if raceenabled {
   419  		racewritepc(unsafe.Pointer(head), getcallerpc(), abi.FuncPCABIInternal(deferprocat))
   420  	}
   421  	d1 := newdefer()
   422  	d1.fn = fn
   423  	for {
   424  		d1.link = head.Load()
   425  		if d1.link == badDefer() {
   426  			throw("defer after range func returned")
   427  		}
   428  		if head.CompareAndSwap(d1.link, d1) {
   429  			break
   430  		}
   431  	}
   432  
   433  	// Must be last - see deferproc above.
   434  	return0()
   435  }
   436  
   437  // deferconvert converts the rangefunc defer list of d0 into an ordinary list
   438  // following d0.
   439  // See the doc comment for deferrangefunc for details.
   440  func deferconvert(d0 *_defer) {
   441  	head := d0.head
   442  	if raceenabled {
   443  		racereadpc(unsafe.Pointer(head), getcallerpc(), abi.FuncPCABIInternal(deferconvert))
   444  	}
   445  	tail := d0.link
   446  	d0.rangefunc = false
   447  
   448  	var d *_defer
   449  	for {
   450  		d = head.Load()
   451  		if head.CompareAndSwap(d, badDefer()) {
   452  			break
   453  		}
   454  	}
   455  	if d == nil {
   456  		return
   457  	}
   458  	for d1 := d; ; d1 = d1.link {
   459  		d1.sp = d0.sp
   460  		d1.pc = d0.pc
   461  		if d1.link == nil {
   462  			d1.link = tail
   463  			break
   464  		}
   465  	}
   466  	d0.link = d
   467  	return
   468  }
   469  
   470  // deferprocStack queues a new deferred function with a defer record on the stack.
   471  // The defer record must have its fn field initialized.
   472  // All other fields can contain junk.
   473  // Nosplit because of the uninitialized pointer fields on the stack.
   474  //
   475  //go:nosplit
   476  func deferprocStack(d *_defer) {
   477  	gp := getg()
   478  	if gp.m.curg != gp {
   479  		// go code on the system stack can't defer
   480  		throw("defer on system stack")
   481  	}
   482  	// fn is already set.
   483  	// The other fields are junk on entry to deferprocStack and
   484  	// are initialized here.
   485  	d.heap = false
   486  	d.rangefunc = false
   487  	d.sp = getcallersp()
   488  	d.pc = getcallerpc()
   489  	// The lines below implement:
   490  	//   d.panic = nil
   491  	//   d.fd = nil
   492  	//   d.link = gp._defer
   493  	//   d.head = nil
   494  	//   gp._defer = d
   495  	// But without write barriers. The first three are writes to
   496  	// the stack so they don't need a write barrier, and furthermore
   497  	// are to uninitialized memory, so they must not use a write barrier.
   498  	// The fourth write does not require a write barrier because we
   499  	// explicitly mark all the defer structures, so we don't need to
   500  	// keep track of pointers to them with a write barrier.
   501  	*(*uintptr)(unsafe.Pointer(&d.link)) = uintptr(unsafe.Pointer(gp._defer))
   502  	*(*uintptr)(unsafe.Pointer(&d.head)) = 0
   503  	*(*uintptr)(unsafe.Pointer(&gp._defer)) = uintptr(unsafe.Pointer(d))
   504  
   505  	return0()
   506  	// No code can go here - the C return register has
   507  	// been set and must not be clobbered.
   508  }
   509  
   510  // Each P holds a pool for defers.
   511  
   512  // Allocate a Defer, usually using per-P pool.
   513  // Each defer must be released with freedefer.  The defer is not
   514  // added to any defer chain yet.
   515  func newdefer() *_defer {
   516  	var d *_defer
   517  	mp := acquirem()
   518  	pp := mp.p.ptr()
   519  	if len(pp.deferpool) == 0 && sched.deferpool != nil {
   520  		lock(&sched.deferlock)
   521  		for len(pp.deferpool) < cap(pp.deferpool)/2 && sched.deferpool != nil {
   522  			d := sched.deferpool
   523  			sched.deferpool = d.link
   524  			d.link = nil
   525  			pp.deferpool = append(pp.deferpool, d)
   526  		}
   527  		unlock(&sched.deferlock)
   528  	}
   529  	if n := len(pp.deferpool); n > 0 {
   530  		d = pp.deferpool[n-1]
   531  		pp.deferpool[n-1] = nil
   532  		pp.deferpool = pp.deferpool[:n-1]
   533  	}
   534  	releasem(mp)
   535  	mp, pp = nil, nil
   536  
   537  	if d == nil {
   538  		// Allocate new defer.
   539  		d = new(_defer)
   540  	}
   541  	d.heap = true
   542  	return d
   543  }
   544  
   545  // popDefer pops the head of gp's defer list and frees it.
   546  func popDefer(gp *g) {
   547  	d := gp._defer
   548  	d.fn = nil // Can in theory point to the stack
   549  	// We must not copy the stack between the updating gp._defer and setting
   550  	// d.link to nil. Between these two steps, d is not on any defer list, so
   551  	// stack copying won't adjust stack pointers in it (namely, d.link). Hence,
   552  	// if we were to copy the stack, d could then contain a stale pointer.
   553  	gp._defer = d.link
   554  	d.link = nil
   555  	// After this point we can copy the stack.
   556  
   557  	if !d.heap {
   558  		return
   559  	}
   560  
   561  	mp := acquirem()
   562  	pp := mp.p.ptr()
   563  	if len(pp.deferpool) == cap(pp.deferpool) {
   564  		// Transfer half of local cache to the central cache.
   565  		var first, last *_defer
   566  		for len(pp.deferpool) > cap(pp.deferpool)/2 {
   567  			n := len(pp.deferpool)
   568  			d := pp.deferpool[n-1]
   569  			pp.deferpool[n-1] = nil
   570  			pp.deferpool = pp.deferpool[:n-1]
   571  			if first == nil {
   572  				first = d
   573  			} else {
   574  				last.link = d
   575  			}
   576  			last = d
   577  		}
   578  		lock(&sched.deferlock)
   579  		last.link = sched.deferpool
   580  		sched.deferpool = first
   581  		unlock(&sched.deferlock)
   582  	}
   583  
   584  	*d = _defer{}
   585  
   586  	pp.deferpool = append(pp.deferpool, d)
   587  
   588  	releasem(mp)
   589  	mp, pp = nil, nil
   590  }
   591  
   592  // deferreturn runs deferred functions for the caller's frame.
   593  // The compiler inserts a call to this at the end of any
   594  // function which calls defer.
   595  func deferreturn() {
   596  	var p _panic
   597  	p.deferreturn = true
   598  
   599  	p.start(getcallerpc(), unsafe.Pointer(getcallersp()))
   600  	for {
   601  		fn, ok := p.nextDefer()
   602  		if !ok {
   603  			break
   604  		}
   605  		fn()
   606  	}
   607  }
   608  
   609  // Goexit terminates the goroutine that calls it. No other goroutine is affected.
   610  // Goexit runs all deferred calls before terminating the goroutine. Because Goexit
   611  // is not a panic, any recover calls in those deferred functions will return nil.
   612  //
   613  // Calling Goexit from the main goroutine terminates that goroutine
   614  // without func main returning. Since func main has not returned,
   615  // the program continues execution of other goroutines.
   616  // If all other goroutines exit, the program crashes.
   617  func Goexit() {
   618  	// Create a panic object for Goexit, so we can recognize when it might be
   619  	// bypassed by a recover().
   620  	var p _panic
   621  	p.goexit = true
   622  
   623  	p.start(getcallerpc(), unsafe.Pointer(getcallersp()))
   624  	for {
   625  		fn, ok := p.nextDefer()
   626  		if !ok {
   627  			break
   628  		}
   629  		fn()
   630  	}
   631  
   632  	goexit1()
   633  }
   634  
   635  // Call all Error and String methods before freezing the world.
   636  // Used when crashing with panicking.
   637  func preprintpanics(p *_panic) {
   638  	defer func() {
   639  		text := "panic while printing panic value"
   640  		switch r := recover().(type) {
   641  		case nil:
   642  			// nothing to do
   643  		case string:
   644  			throw(text + ": " + r)
   645  		default:
   646  			throw(text + ": type " + toRType(efaceOf(&r)._type).string())
   647  		}
   648  	}()
   649  	for p != nil {
   650  		switch v := p.arg.(type) {
   651  		case error:
   652  			p.arg = v.Error()
   653  		case stringer:
   654  			p.arg = v.String()
   655  		}
   656  		p = p.link
   657  	}
   658  }
   659  
   660  // Print all currently active panics. Used when crashing.
   661  // Should only be called after preprintpanics.
   662  func printpanics(p *_panic) {
   663  	if p.link != nil {
   664  		printpanics(p.link)
   665  		if !p.link.goexit {
   666  			print("\t")
   667  		}
   668  	}
   669  	if p.goexit {
   670  		return
   671  	}
   672  	print("panic: ")
   673  	printpanicval(p.arg)
   674  	if p.recovered {
   675  		print(" [recovered]")
   676  	}
   677  	print("\n")
   678  }
   679  
   680  // readvarintUnsafe reads the uint32 in varint format starting at fd, and returns the
   681  // uint32 and a pointer to the byte following the varint.
   682  //
   683  // The implementation is the same with runtime.readvarint, except that this function
   684  // uses unsafe.Pointer for speed.
   685  func readvarintUnsafe(fd unsafe.Pointer) (uint32, unsafe.Pointer) {
   686  	var r uint32
   687  	var shift int
   688  	for {
   689  		b := *(*uint8)(fd)
   690  		fd = add(fd, unsafe.Sizeof(b))
   691  		if b < 128 {
   692  			return r + uint32(b)<<shift, fd
   693  		}
   694  		r += uint32(b&0x7F) << (shift & 31)
   695  		shift += 7
   696  		if shift > 28 {
   697  			panic("Bad varint")
   698  		}
   699  	}
   700  }
   701  
   702  // A PanicNilError happens when code calls panic(nil).
   703  //
   704  // Before Go 1.21, programs that called panic(nil) observed recover returning nil.
   705  // Starting in Go 1.21, programs that call panic(nil) observe recover returning a *PanicNilError.
   706  // Programs can change back to the old behavior by setting GODEBUG=panicnil=1.
   707  type PanicNilError struct {
   708  	// This field makes PanicNilError structurally different from
   709  	// any other struct in this package, and the _ makes it different
   710  	// from any struct in other packages too.
   711  	// This avoids any accidental conversions being possible
   712  	// between this struct and some other struct sharing the same fields,
   713  	// like happened in go.dev/issue/56603.
   714  	_ [0]*PanicNilError
   715  }
   716  
   717  func (*PanicNilError) Error() string { return "panic called with nil argument" }
   718  func (*PanicNilError) RuntimeError() {}
   719  
   720  var panicnil = &godebugInc{name: "panicnil"}
   721  
   722  // The implementation of the predeclared function panic.
   723  // The compiler emits calls to this function.
   724  //
   725  // gopanic should be an internal detail,
   726  // but widely used packages access it using linkname.
   727  // Notable members of the hall of shame include:
   728  //   - go.undefinedlabs.com/scopeagent
   729  //   - github.com/goplus/igop
   730  //
   731  // Do not remove or change the type signature.
   732  // See go.dev/issue/67401.
   733  //
   734  //go:linkname gopanic
   735  func gopanic(e any) {
   736  	if e == nil {
   737  		if debug.panicnil.Load() != 1 {
   738  			e = new(PanicNilError)
   739  		} else {
   740  			panicnil.IncNonDefault()
   741  		}
   742  	}
   743  
   744  	gp := getg()
   745  	if gp.m.curg != gp {
   746  		print("panic: ")
   747  		printpanicval(e)
   748  		print("\n")
   749  		throw("panic on system stack")
   750  	}
   751  
   752  	if gp.m.mallocing != 0 {
   753  		print("panic: ")
   754  		printpanicval(e)
   755  		print("\n")
   756  		throw("panic during malloc")
   757  	}
   758  	if gp.m.preemptoff != "" {
   759  		print("panic: ")
   760  		printpanicval(e)
   761  		print("\n")
   762  		print("preempt off reason: ")
   763  		print(gp.m.preemptoff)
   764  		print("\n")
   765  		throw("panic during preemptoff")
   766  	}
   767  	if gp.m.locks != 0 {
   768  		print("panic: ")
   769  		printpanicval(e)
   770  		print("\n")
   771  		throw("panic holding locks")
   772  	}
   773  
   774  	var p _panic
   775  	p.arg = e
   776  
   777  	runningPanicDefers.Add(1)
   778  
   779  	p.start(getcallerpc(), unsafe.Pointer(getcallersp()))
   780  	for {
   781  		fn, ok := p.nextDefer()
   782  		if !ok {
   783  			break
   784  		}
   785  		fn()
   786  	}
   787  
   788  	// If we're tracing, flush the current generation to make the trace more
   789  	// readable.
   790  	//
   791  	// TODO(aktau): Handle a panic from within traceAdvance more gracefully.
   792  	// Currently it would hang. Not handled now because it is very unlikely, and
   793  	// already unrecoverable.
   794  	if traceEnabled() {
   795  		traceAdvance(false)
   796  	}
   797  
   798  	// ran out of deferred calls - old-school panic now
   799  	// Because it is unsafe to call arbitrary user code after freezing
   800  	// the world, we call preprintpanics to invoke all necessary Error
   801  	// and String methods to prepare the panic strings before startpanic.
   802  	preprintpanics(&p)
   803  
   804  	fatalpanic(&p)   // should not return
   805  	*(*int)(nil) = 0 // not reached
   806  }
   807  
   808  // start initializes a panic to start unwinding the stack.
   809  //
   810  // If p.goexit is true, then start may return multiple times.
   811  func (p *_panic) start(pc uintptr, sp unsafe.Pointer) {
   812  	gp := getg()
   813  
   814  	// Record the caller's PC and SP, so recovery can identify panics
   815  	// that have been recovered. Also, so that if p is from Goexit, we
   816  	// can restart its defer processing loop if a recovered panic tries
   817  	// to jump past it.
   818  	p.startPC = getcallerpc()
   819  	p.startSP = unsafe.Pointer(getcallersp())
   820  
   821  	if p.deferreturn {
   822  		p.sp = sp
   823  
   824  		if s := (*savedOpenDeferState)(gp.param); s != nil {
   825  			// recovery saved some state for us, so that we can resume
   826  			// calling open-coded defers without unwinding the stack.
   827  
   828  			gp.param = nil
   829  
   830  			p.retpc = s.retpc
   831  			p.deferBitsPtr = (*byte)(add(sp, s.deferBitsOffset))
   832  			p.slotsPtr = add(sp, s.slotsOffset)
   833  		}
   834  
   835  		return
   836  	}
   837  
   838  	p.link = gp._panic
   839  	gp._panic = (*_panic)(noescape(unsafe.Pointer(p)))
   840  
   841  	// Initialize state machine, and find the first frame with a defer.
   842  	//
   843  	// Note: We could use startPC and startSP here, but callers will
   844  	// never have defer statements themselves. By starting at their
   845  	// caller instead, we avoid needing to unwind through an extra
   846  	// frame. It also somewhat simplifies the terminating condition for
   847  	// deferreturn.
   848  	p.lr, p.fp = pc, sp
   849  	p.nextFrame()
   850  }
   851  
   852  // nextDefer returns the next deferred function to invoke, if any.
   853  //
   854  // Note: The "ok bool" result is necessary to correctly handle when
   855  // the deferred function itself was nil (e.g., "defer (func())(nil)").
   856  func (p *_panic) nextDefer() (func(), bool) {
   857  	gp := getg()
   858  
   859  	if !p.deferreturn {
   860  		if gp._panic != p {
   861  			throw("bad panic stack")
   862  		}
   863  
   864  		if p.recovered {
   865  			mcall(recovery) // does not return
   866  			throw("recovery failed")
   867  		}
   868  	}
   869  
   870  	// The assembler adjusts p.argp in wrapper functions that shouldn't
   871  	// be visible to recover(), so we need to restore it each iteration.
   872  	p.argp = add(p.startSP, sys.MinFrameSize)
   873  
   874  	for {
   875  		for p.deferBitsPtr != nil {
   876  			bits := *p.deferBitsPtr
   877  
   878  			// Check whether any open-coded defers are still pending.
   879  			//
   880  			// Note: We need to check this upfront (rather than after
   881  			// clearing the top bit) because it's possible that Goexit
   882  			// invokes a deferred call, and there were still more pending
   883  			// open-coded defers in the frame; but then the deferred call
   884  			// panic and invoked the remaining defers in the frame, before
   885  			// recovering and restarting the Goexit loop.
   886  			if bits == 0 {
   887  				p.deferBitsPtr = nil
   888  				break
   889  			}
   890  
   891  			// Find index of top bit set.
   892  			i := 7 - uintptr(sys.LeadingZeros8(bits))
   893  
   894  			// Clear bit and store it back.
   895  			bits &^= 1 << i
   896  			*p.deferBitsPtr = bits
   897  
   898  			return *(*func())(add(p.slotsPtr, i*goarch.PtrSize)), true
   899  		}
   900  
   901  	Recheck:
   902  		if d := gp._defer; d != nil && d.sp == uintptr(p.sp) {
   903  			if d.rangefunc {
   904  				deferconvert(d)
   905  				popDefer(gp)
   906  				goto Recheck
   907  			}
   908  
   909  			fn := d.fn
   910  
   911  			// TODO(mdempsky): Instead of having each deferproc call have
   912  			// its own "deferreturn(); return" sequence, we should just make
   913  			// them reuse the one we emit for open-coded defers.
   914  			p.retpc = d.pc
   915  
   916  			// Unlink and free.
   917  			popDefer(gp)
   918  
   919  			return fn, true
   920  		}
   921  
   922  		if !p.nextFrame() {
   923  			return nil, false
   924  		}
   925  	}
   926  }
   927  
   928  // nextFrame finds the next frame that contains deferred calls, if any.
   929  func (p *_panic) nextFrame() (ok bool) {
   930  	if p.lr == 0 {
   931  		return false
   932  	}
   933  
   934  	gp := getg()
   935  	systemstack(func() {
   936  		var limit uintptr
   937  		if d := gp._defer; d != nil {
   938  			limit = d.sp
   939  		}
   940  
   941  		var u unwinder
   942  		u.initAt(p.lr, uintptr(p.fp), 0, gp, 0)
   943  		for {
   944  			if !u.valid() {
   945  				p.lr = 0
   946  				return // ok == false
   947  			}
   948  
   949  			// TODO(mdempsky): If we populate u.frame.fn.deferreturn for
   950  			// every frame containing a defer (not just open-coded defers),
   951  			// then we can simply loop until we find the next frame where
   952  			// it's non-zero.
   953  
   954  			if u.frame.sp == limit {
   955  				break // found a frame with linked defers
   956  			}
   957  
   958  			if p.initOpenCodedDefers(u.frame.fn, unsafe.Pointer(u.frame.varp)) {
   959  				break // found a frame with open-coded defers
   960  			}
   961  
   962  			u.next()
   963  		}
   964  
   965  		p.lr = u.frame.lr
   966  		p.sp = unsafe.Pointer(u.frame.sp)
   967  		p.fp = unsafe.Pointer(u.frame.fp)
   968  
   969  		ok = true
   970  	})
   971  
   972  	return
   973  }
   974  
   975  func (p *_panic) initOpenCodedDefers(fn funcInfo, varp unsafe.Pointer) bool {
   976  	fd := funcdata(fn, abi.FUNCDATA_OpenCodedDeferInfo)
   977  	if fd == nil {
   978  		return false
   979  	}
   980  
   981  	if fn.deferreturn == 0 {
   982  		throw("missing deferreturn")
   983  	}
   984  
   985  	deferBitsOffset, fd := readvarintUnsafe(fd)
   986  	deferBitsPtr := (*uint8)(add(varp, -uintptr(deferBitsOffset)))
   987  	if *deferBitsPtr == 0 {
   988  		return false // has open-coded defers, but none pending
   989  	}
   990  
   991  	slotsOffset, fd := readvarintUnsafe(fd)
   992  
   993  	p.retpc = fn.entry() + uintptr(fn.deferreturn)
   994  	p.deferBitsPtr = deferBitsPtr
   995  	p.slotsPtr = add(varp, -uintptr(slotsOffset))
   996  
   997  	return true
   998  }
   999  
  1000  // The implementation of the predeclared function recover.
  1001  // Cannot split the stack because it needs to reliably
  1002  // find the stack segment of its caller.
  1003  //
  1004  // TODO(rsc): Once we commit to CopyStackAlways,
  1005  // this doesn't need to be nosplit.
  1006  //
  1007  //go:nosplit
  1008  func gorecover(argp uintptr) any {
  1009  	// Must be in a function running as part of a deferred call during the panic.
  1010  	// Must be called from the topmost function of the call
  1011  	// (the function used in the defer statement).
  1012  	// p.argp is the argument pointer of that topmost deferred function call.
  1013  	// Compare against argp reported by caller.
  1014  	// If they match, the caller is the one who can recover.
  1015  	gp := getg()
  1016  	p := gp._panic
  1017  	if p != nil && !p.goexit && !p.recovered && argp == uintptr(p.argp) {
  1018  		p.recovered = true
  1019  		return p.arg
  1020  	}
  1021  	return nil
  1022  }
  1023  
  1024  //go:linkname sync_throw sync.throw
  1025  func sync_throw(s string) {
  1026  	throw(s)
  1027  }
  1028  
  1029  //go:linkname sync_fatal sync.fatal
  1030  func sync_fatal(s string) {
  1031  	fatal(s)
  1032  }
  1033  
  1034  // throw triggers a fatal error that dumps a stack trace and exits.
  1035  //
  1036  // throw should be used for runtime-internal fatal errors where Go itself,
  1037  // rather than user code, may be at fault for the failure.
  1038  //
  1039  // NOTE: temporarily marked "go:noinline" pending investigation/fix of
  1040  // issue #67274, so as to fix longtest builders.
  1041  //
  1042  // throw should be an internal detail,
  1043  // but widely used packages access it using linkname.
  1044  // Notable members of the hall of shame include:
  1045  //   - github.com/bytedance/sonic
  1046  //   - github.com/cockroachdb/pebble
  1047  //   - github.com/dgraph-io/ristretto
  1048  //   - github.com/outcaste-io/ristretto
  1049  //   - github.com/pingcap/br
  1050  //   - gvisor.dev/gvisor
  1051  //   - github.com/sagernet/gvisor
  1052  //
  1053  // Do not remove or change the type signature.
  1054  // See go.dev/issue/67401.
  1055  //
  1056  //go:linkname throw
  1057  //go:nosplit
  1058  func throw(s string) {
  1059  	// Everything throw does should be recursively nosplit so it
  1060  	// can be called even when it's unsafe to grow the stack.
  1061  	systemstack(func() {
  1062  		print("fatal error: ")
  1063  		printindented(s) // logically printpanicval(s), but avoids convTstring write barrier
  1064  		print("\n")
  1065  	})
  1066  
  1067  	fatalthrow(throwTypeRuntime)
  1068  }
  1069  
  1070  // fatal triggers a fatal error that dumps a stack trace and exits.
  1071  //
  1072  // fatal is equivalent to throw, but is used when user code is expected to be
  1073  // at fault for the failure, such as racing map writes.
  1074  //
  1075  // fatal does not include runtime frames, system goroutines, or frame metadata
  1076  // (fp, sp, pc) in the stack trace unless GOTRACEBACK=system or higher.
  1077  //
  1078  //go:nosplit
  1079  func fatal(s string) {
  1080  	// Everything fatal does should be recursively nosplit so it
  1081  	// can be called even when it's unsafe to grow the stack.
  1082  	systemstack(func() {
  1083  		print("fatal error: ")
  1084  		printindented(s) // logically printpanicval(s), but avoids convTstring write barrier
  1085  		print("\n")
  1086  	})
  1087  
  1088  	fatalthrow(throwTypeUser)
  1089  }
  1090  
  1091  // runningPanicDefers is non-zero while running deferred functions for panic.
  1092  // This is used to try hard to get a panic stack trace out when exiting.
  1093  var runningPanicDefers atomic.Uint32
  1094  
  1095  // panicking is non-zero when crashing the program for an unrecovered panic.
  1096  var panicking atomic.Uint32
  1097  
  1098  // paniclk is held while printing the panic information and stack trace,
  1099  // so that two concurrent panics don't overlap their output.
  1100  var paniclk mutex
  1101  
  1102  // Unwind the stack after a deferred function calls recover
  1103  // after a panic. Then arrange to continue running as though
  1104  // the caller of the deferred function returned normally.
  1105  //
  1106  // However, if unwinding the stack would skip over a Goexit call, we
  1107  // return into the Goexit loop instead, so it can continue processing
  1108  // defers instead.
  1109  func recovery(gp *g) {
  1110  	p := gp._panic
  1111  	pc, sp, fp := p.retpc, uintptr(p.sp), uintptr(p.fp)
  1112  	p0, saveOpenDeferState := p, p.deferBitsPtr != nil && *p.deferBitsPtr != 0
  1113  
  1114  	// Unwind the panic stack.
  1115  	for ; p != nil && uintptr(p.startSP) < sp; p = p.link {
  1116  		// Don't allow jumping past a pending Goexit.
  1117  		// Instead, have its _panic.start() call return again.
  1118  		//
  1119  		// TODO(mdempsky): In this case, Goexit will resume walking the
  1120  		// stack where it left off, which means it will need to rewalk
  1121  		// frames that we've already processed.
  1122  		//
  1123  		// There's a similar issue with nested panics, when the inner
  1124  		// panic supercedes the outer panic. Again, we end up needing to
  1125  		// walk the same stack frames.
  1126  		//
  1127  		// These are probably pretty rare occurrences in practice, and
  1128  		// they don't seem any worse than the existing logic. But if we
  1129  		// move the unwinding state into _panic, we could detect when we
  1130  		// run into where the last panic started, and then just pick up
  1131  		// where it left off instead.
  1132  		//
  1133  		// With how subtle defer handling is, this might not actually be
  1134  		// worthwhile though.
  1135  		if p.goexit {
  1136  			pc, sp = p.startPC, uintptr(p.startSP)
  1137  			saveOpenDeferState = false // goexit is unwinding the stack anyway
  1138  			break
  1139  		}
  1140  
  1141  		runningPanicDefers.Add(-1)
  1142  	}
  1143  	gp._panic = p
  1144  
  1145  	if p == nil { // must be done with signal
  1146  		gp.sig = 0
  1147  	}
  1148  
  1149  	if gp.param != nil {
  1150  		throw("unexpected gp.param")
  1151  	}
  1152  	if saveOpenDeferState {
  1153  		// If we're returning to deferreturn and there are more open-coded
  1154  		// defers for it to call, save enough state for it to be able to
  1155  		// pick up where p0 left off.
  1156  		gp.param = unsafe.Pointer(&savedOpenDeferState{
  1157  			retpc: p0.retpc,
  1158  
  1159  			// We need to save deferBitsPtr and slotsPtr too, but those are
  1160  			// stack pointers. To avoid issues around heap objects pointing
  1161  			// to the stack, save them as offsets from SP.
  1162  			deferBitsOffset: uintptr(unsafe.Pointer(p0.deferBitsPtr)) - uintptr(p0.sp),
  1163  			slotsOffset:     uintptr(p0.slotsPtr) - uintptr(p0.sp),
  1164  		})
  1165  	}
  1166  
  1167  	// TODO(mdempsky): Currently, we rely on frames containing "defer"
  1168  	// to end with "CALL deferreturn; RET". This allows deferreturn to
  1169  	// finish running any pending defers in the frame.
  1170  	//
  1171  	// But we should be able to tell whether there are still pending
  1172  	// defers here. If there aren't, we can just jump directly to the
  1173  	// "RET" instruction. And if there are, we don't need an actual
  1174  	// "CALL deferreturn" instruction; we can simulate it with something
  1175  	// like:
  1176  	//
  1177  	//	if usesLR {
  1178  	//		lr = pc
  1179  	//	} else {
  1180  	//		sp -= sizeof(pc)
  1181  	//		*(*uintptr)(sp) = pc
  1182  	//	}
  1183  	//	pc = funcPC(deferreturn)
  1184  	//
  1185  	// So that we effectively tail call into deferreturn, such that it
  1186  	// then returns to the simple "RET" epilogue. That would save the
  1187  	// overhead of the "deferreturn" call when there aren't actually any
  1188  	// pending defers left, and shrink the TEXT size of compiled
  1189  	// binaries. (Admittedly, both of these are modest savings.)
  1190  
  1191  	// Ensure we're recovering within the appropriate stack.
  1192  	if sp != 0 && (sp < gp.stack.lo || gp.stack.hi < sp) {
  1193  		print("recover: ", hex(sp), " not in [", hex(gp.stack.lo), ", ", hex(gp.stack.hi), "]\n")
  1194  		throw("bad recovery")
  1195  	}
  1196  
  1197  	// Make the deferproc for this d return again,
  1198  	// this time returning 1. The calling function will
  1199  	// jump to the standard return epilogue.
  1200  	gp.sched.sp = sp
  1201  	gp.sched.pc = pc
  1202  	gp.sched.lr = 0
  1203  	// Restore the bp on platforms that support frame pointers.
  1204  	// N.B. It's fine to not set anything for platforms that don't
  1205  	// support frame pointers, since nothing consumes them.
  1206  	switch {
  1207  	case goarch.IsAmd64 != 0:
  1208  		// on x86, fp actually points one word higher than the top of
  1209  		// the frame since the return address is saved on the stack by
  1210  		// the caller
  1211  		gp.sched.bp = fp - 2*goarch.PtrSize
  1212  	case goarch.IsArm64 != 0:
  1213  		// on arm64, the architectural bp points one word higher
  1214  		// than the sp. fp is totally useless to us here, because it
  1215  		// only gets us to the caller's fp.
  1216  		gp.sched.bp = sp - goarch.PtrSize
  1217  	}
  1218  	gp.sched.ret = 1
  1219  	gogo(&gp.sched)
  1220  }
  1221  
  1222  // fatalthrow implements an unrecoverable runtime throw. It freezes the
  1223  // system, prints stack traces starting from its caller, and terminates the
  1224  // process.
  1225  //
  1226  //go:nosplit
  1227  func fatalthrow(t throwType) {
  1228  	pc := getcallerpc()
  1229  	sp := getcallersp()
  1230  	gp := getg()
  1231  
  1232  	if gp.m.throwing == throwTypeNone {
  1233  		gp.m.throwing = t
  1234  	}
  1235  
  1236  	// Switch to the system stack to avoid any stack growth, which may make
  1237  	// things worse if the runtime is in a bad state.
  1238  	systemstack(func() {
  1239  		if isSecureMode() {
  1240  			exit(2)
  1241  		}
  1242  
  1243  		startpanic_m()
  1244  
  1245  		if dopanic_m(gp, pc, sp) {
  1246  			// crash uses a decent amount of nosplit stack and we're already
  1247  			// low on stack in throw, so crash on the system stack (unlike
  1248  			// fatalpanic).
  1249  			crash()
  1250  		}
  1251  
  1252  		exit(2)
  1253  	})
  1254  
  1255  	*(*int)(nil) = 0 // not reached
  1256  }
  1257  
  1258  // fatalpanic implements an unrecoverable panic. It is like fatalthrow, except
  1259  // that if msgs != nil, fatalpanic also prints panic messages and decrements
  1260  // runningPanicDefers once main is blocked from exiting.
  1261  //
  1262  //go:nosplit
  1263  func fatalpanic(msgs *_panic) {
  1264  	pc := getcallerpc()
  1265  	sp := getcallersp()
  1266  	gp := getg()
  1267  	var docrash bool
  1268  	// Switch to the system stack to avoid any stack growth, which
  1269  	// may make things worse if the runtime is in a bad state.
  1270  	systemstack(func() {
  1271  		if startpanic_m() && msgs != nil {
  1272  			// There were panic messages and startpanic_m
  1273  			// says it's okay to try to print them.
  1274  
  1275  			// startpanic_m set panicking, which will
  1276  			// block main from exiting, so now OK to
  1277  			// decrement runningPanicDefers.
  1278  			runningPanicDefers.Add(-1)
  1279  
  1280  			printpanics(msgs)
  1281  		}
  1282  
  1283  		docrash = dopanic_m(gp, pc, sp)
  1284  	})
  1285  
  1286  	if docrash {
  1287  		// By crashing outside the above systemstack call, debuggers
  1288  		// will not be confused when generating a backtrace.
  1289  		// Function crash is marked nosplit to avoid stack growth.
  1290  		crash()
  1291  	}
  1292  
  1293  	systemstack(func() {
  1294  		exit(2)
  1295  	})
  1296  
  1297  	*(*int)(nil) = 0 // not reached
  1298  }
  1299  
  1300  // startpanic_m prepares for an unrecoverable panic.
  1301  //
  1302  // It returns true if panic messages should be printed, or false if
  1303  // the runtime is in bad shape and should just print stacks.
  1304  //
  1305  // It must not have write barriers even though the write barrier
  1306  // explicitly ignores writes once dying > 0. Write barriers still
  1307  // assume that g.m.p != nil, and this function may not have P
  1308  // in some contexts (e.g. a panic in a signal handler for a signal
  1309  // sent to an M with no P).
  1310  //
  1311  //go:nowritebarrierrec
  1312  func startpanic_m() bool {
  1313  	gp := getg()
  1314  	if mheap_.cachealloc.size == 0 { // very early
  1315  		print("runtime: panic before malloc heap initialized\n")
  1316  	}
  1317  	// Disallow malloc during an unrecoverable panic. A panic
  1318  	// could happen in a signal handler, or in a throw, or inside
  1319  	// malloc itself. We want to catch if an allocation ever does
  1320  	// happen (even if we're not in one of these situations).
  1321  	gp.m.mallocing++
  1322  
  1323  	// If we're dying because of a bad lock count, set it to a
  1324  	// good lock count so we don't recursively panic below.
  1325  	if gp.m.locks < 0 {
  1326  		gp.m.locks = 1
  1327  	}
  1328  
  1329  	switch gp.m.dying {
  1330  	case 0:
  1331  		// Setting dying >0 has the side-effect of disabling this G's writebuf.
  1332  		gp.m.dying = 1
  1333  		panicking.Add(1)
  1334  		lock(&paniclk)
  1335  		if debug.schedtrace > 0 || debug.scheddetail > 0 {
  1336  			schedtrace(true)
  1337  		}
  1338  		freezetheworld()
  1339  		return true
  1340  	case 1:
  1341  		// Something failed while panicking.
  1342  		// Just print a stack trace and exit.
  1343  		gp.m.dying = 2
  1344  		print("panic during panic\n")
  1345  		return false
  1346  	case 2:
  1347  		// This is a genuine bug in the runtime, we couldn't even
  1348  		// print the stack trace successfully.
  1349  		gp.m.dying = 3
  1350  		print("stack trace unavailable\n")
  1351  		exit(4)
  1352  		fallthrough
  1353  	default:
  1354  		// Can't even print! Just exit.
  1355  		exit(5)
  1356  		return false // Need to return something.
  1357  	}
  1358  }
  1359  
  1360  var didothers bool
  1361  var deadlock mutex
  1362  
  1363  // gp is the crashing g running on this M, but may be a user G, while getg() is
  1364  // always g0.
  1365  func dopanic_m(gp *g, pc, sp uintptr) bool {
  1366  	if gp.sig != 0 {
  1367  		signame := signame(gp.sig)
  1368  		if signame != "" {
  1369  			print("[signal ", signame)
  1370  		} else {
  1371  			print("[signal ", hex(gp.sig))
  1372  		}
  1373  		print(" code=", hex(gp.sigcode0), " addr=", hex(gp.sigcode1), " pc=", hex(gp.sigpc), "]\n")
  1374  	}
  1375  
  1376  	level, all, docrash := gotraceback()
  1377  	if level > 0 {
  1378  		if gp != gp.m.curg {
  1379  			all = true
  1380  		}
  1381  		if gp != gp.m.g0 {
  1382  			print("\n")
  1383  			goroutineheader(gp)
  1384  			traceback(pc, sp, 0, gp)
  1385  		} else if level >= 2 || gp.m.throwing >= throwTypeRuntime {
  1386  			print("\nruntime stack:\n")
  1387  			traceback(pc, sp, 0, gp)
  1388  		}
  1389  		if !didothers && all {
  1390  			didothers = true
  1391  			tracebackothers(gp)
  1392  		}
  1393  	}
  1394  	unlock(&paniclk)
  1395  
  1396  	if panicking.Add(-1) != 0 {
  1397  		// Some other m is panicking too.
  1398  		// Let it print what it needs to print.
  1399  		// Wait forever without chewing up cpu.
  1400  		// It will exit when it's done.
  1401  		lock(&deadlock)
  1402  		lock(&deadlock)
  1403  	}
  1404  
  1405  	printDebugLog()
  1406  
  1407  	return docrash
  1408  }
  1409  
  1410  // canpanic returns false if a signal should throw instead of
  1411  // panicking.
  1412  //
  1413  //go:nosplit
  1414  func canpanic() bool {
  1415  	gp := getg()
  1416  	mp := acquirem()
  1417  
  1418  	// Is it okay for gp to panic instead of crashing the program?
  1419  	// Yes, as long as it is running Go code, not runtime code,
  1420  	// and not stuck in a system call.
  1421  	if gp != mp.curg {
  1422  		releasem(mp)
  1423  		return false
  1424  	}
  1425  	// N.B. mp.locks != 1 instead of 0 to account for acquirem.
  1426  	if mp.locks != 1 || mp.mallocing != 0 || mp.throwing != throwTypeNone || mp.preemptoff != "" || mp.dying != 0 {
  1427  		releasem(mp)
  1428  		return false
  1429  	}
  1430  	status := readgstatus(gp)
  1431  	if status&^_Gscan != _Grunning || gp.syscallsp != 0 {
  1432  		releasem(mp)
  1433  		return false
  1434  	}
  1435  	if GOOS == "windows" && mp.libcallsp != 0 {
  1436  		releasem(mp)
  1437  		return false
  1438  	}
  1439  	releasem(mp)
  1440  	return true
  1441  }
  1442  
  1443  // shouldPushSigpanic reports whether pc should be used as sigpanic's
  1444  // return PC (pushing a frame for the call). Otherwise, it should be
  1445  // left alone so that LR is used as sigpanic's return PC, effectively
  1446  // replacing the top-most frame with sigpanic. This is used by
  1447  // preparePanic.
  1448  func shouldPushSigpanic(gp *g, pc, lr uintptr) bool {
  1449  	if pc == 0 {
  1450  		// Probably a call to a nil func. The old LR is more
  1451  		// useful in the stack trace. Not pushing the frame
  1452  		// will make the trace look like a call to sigpanic
  1453  		// instead. (Otherwise the trace will end at sigpanic
  1454  		// and we won't get to see who faulted.)
  1455  		return false
  1456  	}
  1457  	// If we don't recognize the PC as code, but we do recognize
  1458  	// the link register as code, then this assumes the panic was
  1459  	// caused by a call to non-code. In this case, we want to
  1460  	// ignore this call to make unwinding show the context.
  1461  	//
  1462  	// If we running C code, we're not going to recognize pc as a
  1463  	// Go function, so just assume it's good. Otherwise, traceback
  1464  	// may try to read a stale LR that looks like a Go code
  1465  	// pointer and wander into the woods.
  1466  	if gp.m.incgo || findfunc(pc).valid() {
  1467  		// This wasn't a bad call, so use PC as sigpanic's
  1468  		// return PC.
  1469  		return true
  1470  	}
  1471  	if findfunc(lr).valid() {
  1472  		// This was a bad call, but the LR is good, so use the
  1473  		// LR as sigpanic's return PC.
  1474  		return false
  1475  	}
  1476  	// Neither the PC or LR is good. Hopefully pushing a frame
  1477  	// will work.
  1478  	return true
  1479  }
  1480  
  1481  // isAbortPC reports whether pc is the program counter at which
  1482  // runtime.abort raises a signal.
  1483  //
  1484  // It is nosplit because it's part of the isgoexception
  1485  // implementation.
  1486  //
  1487  //go:nosplit
  1488  func isAbortPC(pc uintptr) bool {
  1489  	f := findfunc(pc)
  1490  	if !f.valid() {
  1491  		return false
  1492  	}
  1493  	return f.funcID == abi.FuncID_abort
  1494  }
  1495  

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