exec_libc2.go

Documentation: syscall

     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 && !mips64)
     6  // +build darwin openbsd,!mips64
     7  
     8  package syscall
     9  
    10  import (
    11  	"internal/abi"
    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 assembly
    52  // functions that do not grow the stack.
    53  //go:norace
    54  func forkAndExecInChild(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr *ProcAttr, sys *SysProcAttr, pipe int) (pid int, err Errno) {
    55  	// Declare all variables at top in case any
    56  	// declarations require heap allocation (e.g., err1).
    57  	var (
    58  		r1     uintptr
    59  		err1   Errno
    60  		nextfd int
    61  		i      int
    62  	)
    63  
    64  	// guard against side effects of shuffling fds below.
    65  	// Make sure that nextfd is beyond any currently open files so
    66  	// that we can't run the risk of overwriting any of them.
    67  	fd := make([]int, len(attr.Files))
    68  	nextfd = len(attr.Files)
    69  	for i, ufd := range attr.Files {
    70  		if nextfd < int(ufd) {
    71  			nextfd = int(ufd)
    72  		}
    73  		fd[i] = int(ufd)
    74  	}
    75  	nextfd++
    76  
    77  	// About to call fork.
    78  	// No more allocation or calls of non-assembly functions.
    79  	runtime_BeforeFork()
    80  	r1, _, err1 = rawSyscall(abi.FuncPCABI0(libc_fork_trampoline), 0, 0, 0)
    81  	if err1 != 0 {
    82  		runtime_AfterFork()
    83  		return 0, err1
    84  	}
    85  
    86  	if r1 != 0 {
    87  		// parent; return PID
    88  		runtime_AfterFork()
    89  		return int(r1), 0
    90  	}
    91  
    92  	// Fork succeeded, now in child.
    93  
    94  	// Enable tracing if requested.
    95  	if sys.Ptrace {
    96  		if err := ptrace(PTRACE_TRACEME, 0, 0, 0); err != nil {
    97  			err1 = err.(Errno)
    98  			goto childerror
    99  		}
   100  	}
   101  
   102  	// Session ID
   103  	if sys.Setsid {
   104  		_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_setsid_trampoline), 0, 0, 0)
   105  		if err1 != 0 {
   106  			goto childerror
   107  		}
   108  	}
   109  
   110  	// Set process group
   111  	if sys.Setpgid || sys.Foreground {
   112  		// Place child in process group.
   113  		_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_setpgid_trampoline), 0, uintptr(sys.Pgid), 0)
   114  		if err1 != 0 {
   115  			goto childerror
   116  		}
   117  	}
   118  
   119  	if sys.Foreground {
   120  		pgrp := sys.Pgid
   121  		if pgrp == 0 {
   122  			r1, _, err1 = rawSyscall(abi.FuncPCABI0(libc_getpid_trampoline), 0, 0, 0)
   123  			if err1 != 0 {
   124  				goto childerror
   125  			}
   126  
   127  			pgrp = int(r1)
   128  		}
   129  
   130  		// Place process group in foreground.
   131  		_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_ioctl_trampoline), uintptr(sys.Ctty), uintptr(TIOCSPGRP), uintptr(unsafe.Pointer(&pgrp)))
   132  		if err1 != 0 {
   133  			goto childerror
   134  		}
   135  	}
   136  
   137  	// Restore the signal mask. We do this after TIOCSPGRP to avoid
   138  	// having the kernel send a SIGTTOU signal to the process group.
   139  	runtime_AfterForkInChild()
   140  
   141  	// Chroot
   142  	if chroot != nil {
   143  		_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_chroot_trampoline), uintptr(unsafe.Pointer(chroot)), 0, 0)
   144  		if err1 != 0 {
   145  			goto childerror
   146  		}
   147  	}
   148  
   149  	// User and groups
   150  	if cred := sys.Credential; cred != nil {
   151  		ngroups := uintptr(len(cred.Groups))
   152  		groups := uintptr(0)
   153  		if ngroups > 0 {
   154  			groups = uintptr(unsafe.Pointer(&cred.Groups[0]))
   155  		}
   156  		if !cred.NoSetGroups {
   157  			_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_setgroups_trampoline), ngroups, groups, 0)
   158  			if err1 != 0 {
   159  				goto childerror
   160  			}
   161  		}
   162  		_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_setgid_trampoline), uintptr(cred.Gid), 0, 0)
   163  		if err1 != 0 {
   164  			goto childerror
   165  		}
   166  		_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_setuid_trampoline), uintptr(cred.Uid), 0, 0)
   167  		if err1 != 0 {
   168  			goto childerror
   169  		}
   170  	}
   171  
   172  	// Chdir
   173  	if dir != nil {
   174  		_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_chdir_trampoline), uintptr(unsafe.Pointer(dir)), 0, 0)
   175  		if err1 != 0 {
   176  			goto childerror
   177  		}
   178  	}
   179  
   180  	// Pass 1: look for fd[i] < i and move those up above len(fd)
   181  	// so that pass 2 won't stomp on an fd it needs later.
   182  	if pipe < nextfd {
   183  		_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_dup2_trampoline), uintptr(pipe), uintptr(nextfd), 0)
   184  		if err1 != 0 {
   185  			goto childerror
   186  		}
   187  		rawSyscall(abi.FuncPCABI0(libc_fcntl_trampoline), uintptr(nextfd), F_SETFD, FD_CLOEXEC)
   188  		pipe = nextfd
   189  		nextfd++
   190  	}
   191  	for i = 0; i < len(fd); i++ {
   192  		if fd[i] >= 0 && fd[i] < int(i) {
   193  			if nextfd == pipe { // don't stomp on pipe
   194  				nextfd++
   195  			}
   196  			_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_dup2_trampoline), uintptr(fd[i]), uintptr(nextfd), 0)
   197  			if err1 != 0 {
   198  				goto childerror
   199  			}
   200  			rawSyscall(abi.FuncPCABI0(libc_fcntl_trampoline), uintptr(nextfd), F_SETFD, FD_CLOEXEC)
   201  			fd[i] = nextfd
   202  			nextfd++
   203  		}
   204  	}
   205  
   206  	// Pass 2: dup fd[i] down onto i.
   207  	for i = 0; i < len(fd); i++ {
   208  		if fd[i] == -1 {
   209  			rawSyscall(abi.FuncPCABI0(libc_close_trampoline), uintptr(i), 0, 0)
   210  			continue
   211  		}
   212  		if fd[i] == int(i) {
   213  			// dup2(i, i) won't clear close-on-exec flag on Linux,
   214  			// probably not elsewhere either.
   215  			_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_fcntl_trampoline), uintptr(fd[i]), F_SETFD, 0)
   216  			if err1 != 0 {
   217  				goto childerror
   218  			}
   219  			continue
   220  		}
   221  		// The new fd is created NOT close-on-exec,
   222  		// which is exactly what we want.
   223  		_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_dup2_trampoline), uintptr(fd[i]), uintptr(i), 0)
   224  		if err1 != 0 {
   225  			goto childerror
   226  		}
   227  	}
   228  
   229  	// By convention, we don't close-on-exec the fds we are
   230  	// started with, so if len(fd) < 3, close 0, 1, 2 as needed.
   231  	// Programs that know they inherit fds >= 3 will need
   232  	// to set them close-on-exec.
   233  	for i = len(fd); i < 3; i++ {
   234  		rawSyscall(abi.FuncPCABI0(libc_close_trampoline), uintptr(i), 0, 0)
   235  	}
   236  
   237  	// Detach fd 0 from tty
   238  	if sys.Noctty {
   239  		_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_ioctl_trampoline), 0, uintptr(TIOCNOTTY), 0)
   240  		if err1 != 0 {
   241  			goto childerror
   242  		}
   243  	}
   244  
   245  	// Set the controlling TTY to Ctty
   246  	if sys.Setctty {
   247  		_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_ioctl_trampoline), uintptr(sys.Ctty), uintptr(TIOCSCTTY), 0)
   248  		if err1 != 0 {
   249  			goto childerror
   250  		}
   251  	}
   252  
   253  	// Time to exec.
   254  	_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_execve_trampoline),
   255  		uintptr(unsafe.Pointer(argv0)),
   256  		uintptr(unsafe.Pointer(&argv[0])),
   257  		uintptr(unsafe.Pointer(&envv[0])))
   258  
   259  childerror:
   260  	// send error code on pipe
   261  	rawSyscall(abi.FuncPCABI0(libc_write_trampoline), uintptr(pipe), uintptr(unsafe.Pointer(&err1)), unsafe.Sizeof(err1))
   262  	for {
   263  		rawSyscall(abi.FuncPCABI0(libc_exit_trampoline), 253, 0, 0)
   264  	}
   265  }
   266
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