exec_libc2.go

     1  // Copyright 2011 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 darwin || openbsd
     6  
     7  package syscall
     8  
     9  import (
    10  	"internal/abi"
    11  	"runtime"
    12  	"unsafe"
    13  )
    14  
    15  type SysProcAttr struct {
    16  	Chroot     string      // Chroot.
    17  	Credential *Credential // Credential.
    18  	Ptrace     bool        // Enable tracing.
    19  	Setsid     bool        // Create session.
    20  	// Setpgid sets the process group ID of the child to Pgid,
    21  	// or, if Pgid == 0, to the new child's process ID.
    22  	Setpgid bool
    23  	// Setctty sets the controlling terminal of the child to
    24  	// file descriptor Ctty. Ctty must be a descriptor number
    25  	// in the child process: an index into ProcAttr.Files.
    26  	// This is only meaningful if Setsid is true.
    27  	Setctty bool
    28  	Noctty  bool // Detach fd 0 from controlling terminal
    29  	Ctty    int  // Controlling TTY fd
    30  	// Foreground places the child process group in the foreground.
    31  	// This implies Setpgid. The Ctty field must be set to
    32  	// the descriptor of the controlling TTY.
    33  	// Unlike Setctty, in this case Ctty must be a descriptor
    34  	// number in the parent process.
    35  	Foreground bool
    36  	Pgid       int // Child's process group ID if Setpgid.
    37  }
    38  
    39  // Implemented in runtime package.
    40  func runtime_BeforeFork()
    41  func runtime_AfterFork()
    42  func runtime_AfterForkInChild()
    43  
    44  // Fork, dup fd onto 0..len(fd), and exec(argv0, argvv, envv) in child.
    45  // If a dup or exec fails, write the errno error to pipe.
    46  // (Pipe is close-on-exec so if exec succeeds, it will be closed.)
    47  // In the child, this function must not acquire any locks, because
    48  // they might have been locked at the time of the fork. This means
    49  // no rescheduling, no malloc calls, and no new stack segments.
    50  // For the same reason compiler does not race instrument it.
    51  // The calls to rawSyscall are okay because they are nosplit
    52  // functions that do not grow the stack and are not race
    53  // instrumented (go:norace).
    54  //
    55  //go:norace
    56  func forkAndExecInChild(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr *ProcAttr, sys *SysProcAttr, pipe int) (pid int, err1 Errno) {
    57  	// Declare all variables at top in case any
    58  	// declarations require heap allocation (e.g., err1).
    59  	var (
    60  		r1              uintptr
    61  		nextfd          int
    62  		i               int
    63  		pgrp            _C_int
    64  		cred            *Credential
    65  		ngroups, groups uintptr
    66  	)
    67  
    68  	rlim := origRlimitNofile.Load()
    69  
    70  	// guard against side effects of shuffling fds below.
    71  	// Make sure that nextfd is beyond any currently open files so
    72  	// that we can't run the risk of overwriting any of them.
    73  	fd := make([]int, len(attr.Files))
    74  	nextfd = len(attr.Files)
    75  	for i, ufd := range attr.Files {
    76  		if nextfd < int(ufd) {
    77  			nextfd = int(ufd)
    78  		}
    79  		fd[i] = int(ufd)
    80  	}
    81  	nextfd++
    82  
    83  	// About to call fork.
    84  	// No more allocation or calls of non-assembly functions.
    85  	runtime_BeforeFork()
    86  	r1, _, err1 = rawSyscall(abi.FuncPCABI0(libc_fork_trampoline), 0, 0, 0)
    87  	if err1 != 0 {
    88  		runtime_AfterFork()
    89  		return 0, err1
    90  	}
    91  
    92  	if r1 != 0 {
    93  		// parent; return PID
    94  		runtime_AfterFork()
    95  		return int(r1), 0
    96  	}
    97  
    98  	// Fork succeeded, now in child.
    99  
   100  	// Enable tracing if requested.
   101  	if sys.Ptrace {
   102  		if runtime.GOOS == "ios" {
   103  			err1 = ENOSYS
   104  			goto childerror
   105  		}
   106  		_, _, err1 = rawSyscall6(abi.FuncPCABI0(libc_ptrace_trampoline), PTRACE_TRACEME, 0, 0, 0, 0, 0)
   107  		if err1 != 0 {
   108  			goto childerror
   109  		}
   110  	}
   111  
   112  	// Session ID
   113  	if sys.Setsid {
   114  		_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_setsid_trampoline), 0, 0, 0)
   115  		if err1 != 0 {
   116  			goto childerror
   117  		}
   118  	}
   119  
   120  	// Set process group
   121  	if sys.Setpgid || sys.Foreground {
   122  		// Place child in process group.
   123  		_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_setpgid_trampoline), 0, uintptr(sys.Pgid), 0)
   124  		if err1 != 0 {
   125  			goto childerror
   126  		}
   127  	}
   128  
   129  	if sys.Foreground {
   130  		// This should really be pid_t, however _C_int (aka int32) is
   131  		// generally equivalent.
   132  		pgrp = _C_int(sys.Pgid)
   133  		if pgrp == 0 {
   134  			r1, _, err1 = rawSyscall(abi.FuncPCABI0(libc_getpid_trampoline), 0, 0, 0)
   135  			if err1 != 0 {
   136  				goto childerror
   137  			}
   138  			pgrp = _C_int(r1)
   139  		}
   140  
   141  		// Place process group in foreground.
   142  		_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_ioctl_trampoline), uintptr(sys.Ctty), uintptr(TIOCSPGRP), uintptr(unsafe.Pointer(&pgrp)))
   143  		if err1 != 0 {
   144  			goto childerror
   145  		}
   146  	}
   147  
   148  	// Restore the signal mask. We do this after TIOCSPGRP to avoid
   149  	// having the kernel send a SIGTTOU signal to the process group.
   150  	runtime_AfterForkInChild()
   151  
   152  	// Chroot
   153  	if chroot != nil {
   154  		_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_chroot_trampoline), uintptr(unsafe.Pointer(chroot)), 0, 0)
   155  		if err1 != 0 {
   156  			goto childerror
   157  		}
   158  	}
   159  
   160  	// User and groups
   161  	if cred = sys.Credential; cred != nil {
   162  		ngroups = uintptr(len(cred.Groups))
   163  		groups = uintptr(0)
   164  		if ngroups > 0 {
   165  			groups = uintptr(unsafe.Pointer(&cred.Groups[0]))
   166  		}
   167  		if !cred.NoSetGroups {
   168  			_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_setgroups_trampoline), ngroups, groups, 0)
   169  			if err1 != 0 {
   170  				goto childerror
   171  			}
   172  		}
   173  		_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_setgid_trampoline), uintptr(cred.Gid), 0, 0)
   174  		if err1 != 0 {
   175  			goto childerror
   176  		}
   177  		_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_setuid_trampoline), uintptr(cred.Uid), 0, 0)
   178  		if err1 != 0 {
   179  			goto childerror
   180  		}
   181  	}
   182  
   183  	// Chdir
   184  	if dir != nil {
   185  		_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_chdir_trampoline), uintptr(unsafe.Pointer(dir)), 0, 0)
   186  		if err1 != 0 {
   187  			goto childerror
   188  		}
   189  	}
   190  
   191  	// Pass 1: look for fd[i] < i and move those up above len(fd)
   192  	// so that pass 2 won't stomp on an fd it needs later.
   193  	if pipe < nextfd {
   194  		if runtime.GOOS == "openbsd" {
   195  			_, _, err1 = rawSyscall(dupTrampoline, uintptr(pipe), uintptr(nextfd), O_CLOEXEC)
   196  		} else {
   197  			_, _, err1 = rawSyscall(dupTrampoline, uintptr(pipe), uintptr(nextfd), 0)
   198  			if err1 != 0 {
   199  				goto childerror
   200  			}
   201  			_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_fcntl_trampoline), uintptr(nextfd), F_SETFD, FD_CLOEXEC)
   202  		}
   203  		if err1 != 0 {
   204  			goto childerror
   205  		}
   206  		pipe = nextfd
   207  		nextfd++
   208  	}
   209  	for i = 0; i < len(fd); i++ {
   210  		if fd[i] >= 0 && fd[i] < i {
   211  			if nextfd == pipe { // don't stomp on pipe
   212  				nextfd++
   213  			}
   214  			if runtime.GOOS == "openbsd" {
   215  				_, _, err1 = rawSyscall(dupTrampoline, uintptr(fd[i]), uintptr(nextfd), O_CLOEXEC)
   216  			} else {
   217  				_, _, err1 = rawSyscall(dupTrampoline, uintptr(fd[i]), uintptr(nextfd), 0)
   218  				if err1 != 0 {
   219  					goto childerror
   220  				}
   221  				_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_fcntl_trampoline), uintptr(nextfd), F_SETFD, FD_CLOEXEC)
   222  			}
   223  			if err1 != 0 {
   224  				goto childerror
   225  			}
   226  			fd[i] = nextfd
   227  			nextfd++
   228  		}
   229  	}
   230  
   231  	// Pass 2: dup fd[i] down onto i.
   232  	for i = 0; i < len(fd); i++ {
   233  		if fd[i] == -1 {
   234  			rawSyscall(abi.FuncPCABI0(libc_close_trampoline), uintptr(i), 0, 0)
   235  			continue
   236  		}
   237  		if fd[i] == i {
   238  			// dup2(i, i) won't clear close-on-exec flag on Linux,
   239  			// probably not elsewhere either.
   240  			_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_fcntl_trampoline), uintptr(fd[i]), F_SETFD, 0)
   241  			if err1 != 0 {
   242  				goto childerror
   243  			}
   244  			continue
   245  		}
   246  		// The new fd is created NOT close-on-exec,
   247  		// which is exactly what we want.
   248  		_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_dup2_trampoline), uintptr(fd[i]), uintptr(i), 0)
   249  		if err1 != 0 {
   250  			goto childerror
   251  		}
   252  	}
   253  
   254  	// By convention, we don't close-on-exec the fds we are
   255  	// started with, so if len(fd) < 3, close 0, 1, 2 as needed.
   256  	// Programs that know they inherit fds >= 3 will need
   257  	// to set them close-on-exec.
   258  	for i = len(fd); i < 3; i++ {
   259  		rawSyscall(abi.FuncPCABI0(libc_close_trampoline), uintptr(i), 0, 0)
   260  	}
   261  
   262  	// Detach fd 0 from tty
   263  	if sys.Noctty {
   264  		_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_ioctl_trampoline), 0, uintptr(TIOCNOTTY), 0)
   265  		if err1 != 0 {
   266  			goto childerror
   267  		}
   268  	}
   269  
   270  	// Set the controlling TTY to Ctty
   271  	if sys.Setctty {
   272  		_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_ioctl_trampoline), uintptr(sys.Ctty), uintptr(TIOCSCTTY), 0)
   273  		if err1 != 0 {
   274  			goto childerror
   275  		}
   276  	}
   277  
   278  	// Restore original rlimit.
   279  	if rlim != nil {
   280  		rawSyscall(abi.FuncPCABI0(libc_setrlimit_trampoline), uintptr(RLIMIT_NOFILE), uintptr(unsafe.Pointer(rlim)), 0)
   281  	}
   282  
   283  	// Time to exec.
   284  	_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_execve_trampoline),
   285  		uintptr(unsafe.Pointer(argv0)),
   286  		uintptr(unsafe.Pointer(&argv[0])),
   287  		uintptr(unsafe.Pointer(&envv[0])))
   288  
   289  childerror:
   290  	// send error code on pipe
   291  	rawSyscall(abi.FuncPCABI0(libc_write_trampoline), uintptr(pipe), uintptr(unsafe.Pointer(&err1)), unsafe.Sizeof(err1))
   292  	for {
   293  		rawSyscall(abi.FuncPCABI0(libc_exit_trampoline), 253, 0, 0)
   294  	}
   295  }
   296  
   297  // forkAndExecFailureCleanup cleans up after an exec failure.
   298  func forkAndExecFailureCleanup(attr *ProcAttr, sys *SysProcAttr) {
   299  	// Nothing to do.
   300  }
   301
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