vdso_linux.go

     1  // Copyright 2012 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  //go:build linux && (386 || amd64 || arm || arm64 || loong64 || mips64 || mips64le || ppc64 || ppc64le || riscv64 || s390x)
     6  
     7  package runtime
     8  
     9  import "unsafe"
    10  
    11  // Look up symbols in the Linux vDSO.
    12  
    13  // This code was originally based on the sample Linux vDSO parser at
    14  // https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/tools/testing/selftests/vDSO/parse_vdso.c
    15  
    16  // This implements the ELF dynamic linking spec at
    17  // http://sco.com/developers/gabi/latest/ch5.dynamic.html
    18  
    19  // The version section is documented at
    20  // https://refspecs.linuxfoundation.org/LSB_3.2.0/LSB-Core-generic/LSB-Core-generic/symversion.html
    21  
    22  const (
    23  	_AT_SYSINFO_EHDR = 33
    24  
    25  	_PT_LOAD    = 1 /* Loadable program segment */
    26  	_PT_DYNAMIC = 2 /* Dynamic linking information */
    27  
    28  	_DT_NULL     = 0          /* Marks end of dynamic section */
    29  	_DT_HASH     = 4          /* Dynamic symbol hash table */
    30  	_DT_STRTAB   = 5          /* Address of string table */
    31  	_DT_SYMTAB   = 6          /* Address of symbol table */
    32  	_DT_GNU_HASH = 0x6ffffef5 /* GNU-style dynamic symbol hash table */
    33  	_DT_VERSYM   = 0x6ffffff0
    34  	_DT_VERDEF   = 0x6ffffffc
    35  
    36  	_VER_FLG_BASE = 0x1 /* Version definition of file itself */
    37  
    38  	_SHN_UNDEF = 0 /* Undefined section */
    39  
    40  	_SHT_DYNSYM = 11 /* Dynamic linker symbol table */
    41  
    42  	_STT_FUNC = 2 /* Symbol is a code object */
    43  
    44  	_STT_NOTYPE = 0 /* Symbol type is not specified */
    45  
    46  	_STB_GLOBAL = 1 /* Global symbol */
    47  	_STB_WEAK   = 2 /* Weak symbol */
    48  
    49  	_EI_NIDENT = 16
    50  
    51  	// Maximum indices for the array types used when traversing the vDSO ELF structures.
    52  	// Computed from architecture-specific max provided by vdso_linux_*.go
    53  	vdsoSymTabSize     = vdsoArrayMax / unsafe.Sizeof(elfSym{})
    54  	vdsoDynSize        = vdsoArrayMax / unsafe.Sizeof(elfDyn{})
    55  	vdsoSymStringsSize = vdsoArrayMax     // byte
    56  	vdsoVerSymSize     = vdsoArrayMax / 2 // uint16
    57  	vdsoHashSize       = vdsoArrayMax / 4 // uint32
    58  
    59  	// vdsoBloomSizeScale is a scaling factor for gnuhash tables which are uint32 indexed,
    60  	// but contain uintptrs
    61  	vdsoBloomSizeScale = unsafe.Sizeof(uintptr(0)) / 4 // uint32
    62  )
    63  
    64  /* How to extract and insert information held in the st_info field.  */
    65  func _ELF_ST_BIND(val byte) byte { return val >> 4 }
    66  func _ELF_ST_TYPE(val byte) byte { return val & 0xf }
    67  
    68  type vdsoSymbolKey struct {
    69  	name    string
    70  	symHash uint32
    71  	gnuHash uint32
    72  	ptr     *uintptr
    73  }
    74  
    75  type vdsoVersionKey struct {
    76  	version string
    77  	verHash uint32
    78  }
    79  
    80  type vdsoInfo struct {
    81  	valid bool
    82  
    83  	/* Load information */
    84  	loadAddr   uintptr
    85  	loadOffset uintptr /* loadAddr - recorded vaddr */
    86  
    87  	/* Symbol table */
    88  	symtab     *[vdsoSymTabSize]elfSym
    89  	symstrings *[vdsoSymStringsSize]byte
    90  	chain      []uint32
    91  	bucket     []uint32
    92  	symOff     uint32
    93  	isGNUHash  bool
    94  
    95  	/* Version table */
    96  	versym *[vdsoVerSymSize]uint16
    97  	verdef *elfVerdef
    98  }
    99  
   100  // see vdso_linux_*.go for vdsoSymbolKeys[] and vdso*Sym vars
   101  
   102  func vdsoInitFromSysinfoEhdr(info *vdsoInfo, hdr *elfEhdr) {
   103  	info.valid = false
   104  	info.loadAddr = uintptr(unsafe.Pointer(hdr))
   105  
   106  	pt := unsafe.Pointer(info.loadAddr + uintptr(hdr.e_phoff))
   107  
   108  	// We need two things from the segment table: the load offset
   109  	// and the dynamic table.
   110  	var foundVaddr bool
   111  	var dyn *[vdsoDynSize]elfDyn
   112  	for i := uint16(0); i < hdr.e_phnum; i++ {
   113  		pt := (*elfPhdr)(add(pt, uintptr(i)*unsafe.Sizeof(elfPhdr{})))
   114  		switch pt.p_type {
   115  		case _PT_LOAD:
   116  			if !foundVaddr {
   117  				foundVaddr = true
   118  				info.loadOffset = info.loadAddr + uintptr(pt.p_offset-pt.p_vaddr)
   119  			}
   120  
   121  		case _PT_DYNAMIC:
   122  			dyn = (*[vdsoDynSize]elfDyn)(unsafe.Pointer(info.loadAddr + uintptr(pt.p_offset)))
   123  		}
   124  	}
   125  
   126  	if !foundVaddr || dyn == nil {
   127  		return // Failed
   128  	}
   129  
   130  	// Fish out the useful bits of the dynamic table.
   131  
   132  	var hash, gnuhash *[vdsoHashSize]uint32
   133  	info.symstrings = nil
   134  	info.symtab = nil
   135  	info.versym = nil
   136  	info.verdef = nil
   137  	for i := 0; dyn[i].d_tag != _DT_NULL; i++ {
   138  		dt := &dyn[i]
   139  		p := info.loadOffset + uintptr(dt.d_val)
   140  		switch dt.d_tag {
   141  		case _DT_STRTAB:
   142  			info.symstrings = (*[vdsoSymStringsSize]byte)(unsafe.Pointer(p))
   143  		case _DT_SYMTAB:
   144  			info.symtab = (*[vdsoSymTabSize]elfSym)(unsafe.Pointer(p))
   145  		case _DT_HASH:
   146  			hash = (*[vdsoHashSize]uint32)(unsafe.Pointer(p))
   147  		case _DT_GNU_HASH:
   148  			gnuhash = (*[vdsoHashSize]uint32)(unsafe.Pointer(p))
   149  		case _DT_VERSYM:
   150  			info.versym = (*[vdsoVerSymSize]uint16)(unsafe.Pointer(p))
   151  		case _DT_VERDEF:
   152  			info.verdef = (*elfVerdef)(unsafe.Pointer(p))
   153  		}
   154  	}
   155  
   156  	if info.symstrings == nil || info.symtab == nil || (hash == nil && gnuhash == nil) {
   157  		return // Failed
   158  	}
   159  
   160  	if info.verdef == nil {
   161  		info.versym = nil
   162  	}
   163  
   164  	if gnuhash != nil {
   165  		// Parse the GNU hash table header.
   166  		nbucket := gnuhash[0]
   167  		info.symOff = gnuhash[1]
   168  		bloomSize := gnuhash[2]
   169  		info.bucket = gnuhash[4+bloomSize*uint32(vdsoBloomSizeScale):][:nbucket]
   170  		info.chain = gnuhash[4+bloomSize*uint32(vdsoBloomSizeScale)+nbucket:]
   171  		info.isGNUHash = true
   172  	} else {
   173  		// Parse the hash table header.
   174  		nbucket := hash[0]
   175  		nchain := hash[1]
   176  		info.bucket = hash[2 : 2+nbucket]
   177  		info.chain = hash[2+nbucket : 2+nbucket+nchain]
   178  	}
   179  
   180  	// That's all we need.
   181  	info.valid = true
   182  }
   183  
   184  func vdsoFindVersion(info *vdsoInfo, ver *vdsoVersionKey) int32 {
   185  	if !info.valid {
   186  		return 0
   187  	}
   188  
   189  	def := info.verdef
   190  	for {
   191  		if def.vd_flags&_VER_FLG_BASE == 0 {
   192  			aux := (*elfVerdaux)(add(unsafe.Pointer(def), uintptr(def.vd_aux)))
   193  			if def.vd_hash == ver.verHash && ver.version == gostringnocopy(&info.symstrings[aux.vda_name]) {
   194  				return int32(def.vd_ndx & 0x7fff)
   195  			}
   196  		}
   197  
   198  		if def.vd_next == 0 {
   199  			break
   200  		}
   201  		def = (*elfVerdef)(add(unsafe.Pointer(def), uintptr(def.vd_next)))
   202  	}
   203  
   204  	return -1 // cannot match any version
   205  }
   206  
   207  func vdsoParseSymbols(info *vdsoInfo, version int32) {
   208  	if !info.valid {
   209  		return
   210  	}
   211  
   212  	apply := func(symIndex uint32, k vdsoSymbolKey) bool {
   213  		sym := &info.symtab[symIndex]
   214  		typ := _ELF_ST_TYPE(sym.st_info)
   215  		bind := _ELF_ST_BIND(sym.st_info)
   216  		// On ppc64x, VDSO functions are of type _STT_NOTYPE.
   217  		if typ != _STT_FUNC && typ != _STT_NOTYPE || bind != _STB_GLOBAL && bind != _STB_WEAK || sym.st_shndx == _SHN_UNDEF {
   218  			return false
   219  		}
   220  		if k.name != gostringnocopy(&info.symstrings[sym.st_name]) {
   221  			return false
   222  		}
   223  		// Check symbol version.
   224  		if info.versym != nil && version != 0 && int32(info.versym[symIndex]&0x7fff) != version {
   225  			return false
   226  		}
   227  
   228  		*k.ptr = info.loadOffset + uintptr(sym.st_value)
   229  		return true
   230  	}
   231  
   232  	if !info.isGNUHash {
   233  		// Old-style DT_HASH table.
   234  		for _, k := range vdsoSymbolKeys {
   235  			if len(info.bucket) > 0 {
   236  				for chain := info.bucket[k.symHash%uint32(len(info.bucket))]; chain != 0; chain = info.chain[chain] {
   237  					if apply(chain, k) {
   238  						break
   239  					}
   240  				}
   241  			}
   242  		}
   243  		return
   244  	}
   245  
   246  	// New-style DT_GNU_HASH table.
   247  	for _, k := range vdsoSymbolKeys {
   248  		symIndex := info.bucket[k.gnuHash%uint32(len(info.bucket))]
   249  		if symIndex < info.symOff {
   250  			continue
   251  		}
   252  		for ; ; symIndex++ {
   253  			hash := info.chain[symIndex-info.symOff]
   254  			if hash|1 == k.gnuHash|1 {
   255  				// Found a hash match.
   256  				if apply(symIndex, k) {
   257  					break
   258  				}
   259  			}
   260  			if hash&1 != 0 {
   261  				// End of chain.
   262  				break
   263  			}
   264  		}
   265  	}
   266  }
   267  
   268  func vdsoauxv(tag, val uintptr) {
   269  	switch tag {
   270  	case _AT_SYSINFO_EHDR:
   271  		if val == 0 {
   272  			// Something went wrong
   273  			return
   274  		}
   275  		var info vdsoInfo
   276  		// TODO(rsc): I don't understand why the compiler thinks info escapes
   277  		// when passed to the three functions below.
   278  		info1 := (*vdsoInfo)(noescape(unsafe.Pointer(&info)))
   279  		vdsoInitFromSysinfoEhdr(info1, (*elfEhdr)(unsafe.Pointer(val)))
   280  		vdsoParseSymbols(info1, vdsoFindVersion(info1, &vdsoLinuxVersion))
   281  	}
   282  }
   283  
   284  // vdsoMarker reports whether PC is on the VDSO page.
   285  //
   286  //go:nosplit
   287  func inVDSOPage(pc uintptr) bool {
   288  	for _, k := range vdsoSymbolKeys {
   289  		if *k.ptr != 0 {
   290  			page := *k.ptr &^ (physPageSize - 1)
   291  			return pc >= page && pc < page+physPageSize
   292  		}
   293  	}
   294  	return false
   295  }
   296
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