exec_bsd.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 dragonfly || freebsd || netbsd || (openbsd && mips64)
     6  // +build dragonfly freebsd netbsd openbsd,mips64
     7  
     8  package syscall
     9  
    10  import (
    11  	"unsafe"
    12  )
    13  
    14  type SysProcAttr struct {
    15  	Chroot     string      // Chroot.
    16  	Credential *Credential // Credential.
    17  	Ptrace     bool        // Enable tracing.
    18  	Setsid     bool        // Create session.
    19  	// Setpgid sets the process group ID of the child to Pgid,
    20  	// or, if Pgid == 0, to the new child's process ID.
    21  	Setpgid bool
    22  	// Setctty sets the controlling terminal of the child to
    23  	// file descriptor Ctty. Ctty must be a descriptor number
    24  	// in the child process: an index into ProcAttr.Files.
    25  	// This is only meaningful if Setsid is true.
    26  	Setctty bool
    27  	Noctty  bool // Detach fd 0 from controlling terminal
    28  	Ctty    int  // Controlling TTY fd
    29  	// Foreground places the child process group in the foreground.
    30  	// This implies Setpgid. The Ctty field must be set to
    31  	// the descriptor of the controlling TTY.
    32  	// Unlike Setctty, in this case Ctty must be a descriptor
    33  	// number in the parent process.
    34  	Foreground bool
    35  	Pgid       int // Child's process group ID if Setpgid.
    36  }
    37  
    38  // Implemented in runtime package.
    39  func runtime_BeforeFork()
    40  func runtime_AfterFork()
    41  func runtime_AfterForkInChild()
    42  
    43  // Fork, dup fd onto 0..len(fd), and exec(argv0, argvv, envv) in child.
    44  // If a dup or exec fails, write the errno error to pipe.
    45  // (Pipe is close-on-exec so if exec succeeds, it will be closed.)
    46  // In the child, this function must not acquire any locks, because
    47  // they might have been locked at the time of the fork. This means
    48  // no rescheduling, no malloc calls, and no new stack segments.
    49  // For the same reason compiler does not race instrument it.
    50  // The calls to RawSyscall are okay because they are assembly
    51  // functions that do not grow the stack.
    52  //go:norace
    53  func forkAndExecInChild(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr *ProcAttr, sys *SysProcAttr, pipe int) (pid int, err Errno) {
    54  	// Declare all variables at top in case any
    55  	// declarations require heap allocation (e.g., err1).
    56  	var (
    57  		r1     uintptr
    58  		err1   Errno
    59  		nextfd int
    60  		i      int
    61  	)
    62  
    63  	// guard against side effects of shuffling fds below.
    64  	// Make sure that nextfd is beyond any currently open files so
    65  	// that we can't run the risk of overwriting any of them.
    66  	fd := make([]int, len(attr.Files))
    67  	nextfd = len(attr.Files)
    68  	for i, ufd := range attr.Files {
    69  		if nextfd < int(ufd) {
    70  			nextfd = int(ufd)
    71  		}
    72  		fd[i] = int(ufd)
    73  	}
    74  	nextfd++
    75  
    76  	// About to call fork.
    77  	// No more allocation or calls of non-assembly functions.
    78  	runtime_BeforeFork()
    79  	r1, _, err1 = RawSyscall(SYS_FORK, 0, 0, 0)
    80  	if err1 != 0 {
    81  		runtime_AfterFork()
    82  		return 0, err1
    83  	}
    84  
    85  	if r1 != 0 {
    86  		// parent; return PID
    87  		runtime_AfterFork()
    88  		return int(r1), 0
    89  	}
    90  
    91  	// Fork succeeded, now in child.
    92  
    93  	// Enable tracing if requested.
    94  	if sys.Ptrace {
    95  		_, _, err1 = RawSyscall(SYS_PTRACE, uintptr(PTRACE_TRACEME), 0, 0)
    96  		if err1 != 0 {
    97  			goto childerror
    98  		}
    99  	}
   100  
   101  	// Session ID
   102  	if sys.Setsid {
   103  		_, _, err1 = RawSyscall(SYS_SETSID, 0, 0, 0)
   104  		if err1 != 0 {
   105  			goto childerror
   106  		}
   107  	}
   108  
   109  	// Set process group
   110  	if sys.Setpgid || sys.Foreground {
   111  		// Place child in process group.
   112  		_, _, err1 = RawSyscall(SYS_SETPGID, 0, uintptr(sys.Pgid), 0)
   113  		if err1 != 0 {
   114  			goto childerror
   115  		}
   116  	}
   117  
   118  	if sys.Foreground {
   119  		// This should really be pid_t, however _C_int (aka int32) is
   120  		// generally equivalent.
   121  		pgrp := _C_int(sys.Pgid)
   122  		if pgrp == 0 {
   123  			r1, _, err1 = RawSyscall(SYS_GETPID, 0, 0, 0)
   124  			if err1 != 0 {
   125  				goto childerror
   126  			}
   127  
   128  			pgrp = _C_int(r1)
   129  		}
   130  
   131  		// Place process group in foreground.
   132  		_, _, err1 = RawSyscall(SYS_IOCTL, uintptr(sys.Ctty), uintptr(TIOCSPGRP), uintptr(unsafe.Pointer(&pgrp)))
   133  		if err1 != 0 {
   134  			goto childerror
   135  		}
   136  	}
   137  
   138  	// Restore the signal mask. We do this after TIOCSPGRP to avoid
   139  	// having the kernel send a SIGTTOU signal to the process group.
   140  	runtime_AfterForkInChild()
   141  
   142  	// Chroot
   143  	if chroot != nil {
   144  		_, _, err1 = RawSyscall(SYS_CHROOT, uintptr(unsafe.Pointer(chroot)), 0, 0)
   145  		if err1 != 0 {
   146  			goto childerror
   147  		}
   148  	}
   149  
   150  	// User and groups
   151  	if cred := sys.Credential; cred != nil {
   152  		ngroups := uintptr(len(cred.Groups))
   153  		groups := uintptr(0)
   154  		if ngroups > 0 {
   155  			groups = uintptr(unsafe.Pointer(&cred.Groups[0]))
   156  		}
   157  		if !cred.NoSetGroups {
   158  			_, _, err1 = RawSyscall(SYS_SETGROUPS, ngroups, groups, 0)
   159  			if err1 != 0 {
   160  				goto childerror
   161  			}
   162  		}
   163  		_, _, err1 = RawSyscall(SYS_SETGID, uintptr(cred.Gid), 0, 0)
   164  		if err1 != 0 {
   165  			goto childerror
   166  		}
   167  		_, _, err1 = RawSyscall(SYS_SETUID, uintptr(cred.Uid), 0, 0)
   168  		if err1 != 0 {
   169  			goto childerror
   170  		}
   171  	}
   172  
   173  	// Chdir
   174  	if dir != nil {
   175  		_, _, err1 = RawSyscall(SYS_CHDIR, uintptr(unsafe.Pointer(dir)), 0, 0)
   176  		if err1 != 0 {
   177  			goto childerror
   178  		}
   179  	}
   180  
   181  	// Pass 1: look for fd[i] < i and move those up above len(fd)
   182  	// so that pass 2 won't stomp on an fd it needs later.
   183  	if pipe < nextfd {
   184  		_, _, err1 = RawSyscall(SYS_DUP2, uintptr(pipe), uintptr(nextfd), 0)
   185  		if err1 != 0 {
   186  			goto childerror
   187  		}
   188  		RawSyscall(SYS_FCNTL, uintptr(nextfd), F_SETFD, FD_CLOEXEC)
   189  		pipe = nextfd
   190  		nextfd++
   191  	}
   192  	for i = 0; i < len(fd); i++ {
   193  		if fd[i] >= 0 && fd[i] < int(i) {
   194  			if nextfd == pipe { // don't stomp on pipe
   195  				nextfd++
   196  			}
   197  			_, _, err1 = RawSyscall(SYS_DUP2, uintptr(fd[i]), uintptr(nextfd), 0)
   198  			if err1 != 0 {
   199  				goto childerror
   200  			}
   201  			RawSyscall(SYS_FCNTL, uintptr(nextfd), F_SETFD, FD_CLOEXEC)
   202  			fd[i] = nextfd
   203  			nextfd++
   204  		}
   205  	}
   206  
   207  	// Pass 2: dup fd[i] down onto i.
   208  	for i = 0; i < len(fd); i++ {
   209  		if fd[i] == -1 {
   210  			RawSyscall(SYS_CLOSE, uintptr(i), 0, 0)
   211  			continue
   212  		}
   213  		if fd[i] == int(i) {
   214  			// dup2(i, i) won't clear close-on-exec flag on Linux,
   215  			// probably not elsewhere either.
   216  			_, _, err1 = RawSyscall(SYS_FCNTL, uintptr(fd[i]), F_SETFD, 0)
   217  			if err1 != 0 {
   218  				goto childerror
   219  			}
   220  			continue
   221  		}
   222  		// The new fd is created NOT close-on-exec,
   223  		// which is exactly what we want.
   224  		_, _, err1 = RawSyscall(SYS_DUP2, uintptr(fd[i]), uintptr(i), 0)
   225  		if err1 != 0 {
   226  			goto childerror
   227  		}
   228  	}
   229  
   230  	// By convention, we don't close-on-exec the fds we are
   231  	// started with, so if len(fd) < 3, close 0, 1, 2 as needed.
   232  	// Programs that know they inherit fds >= 3 will need
   233  	// to set them close-on-exec.
   234  	for i = len(fd); i < 3; i++ {
   235  		RawSyscall(SYS_CLOSE, uintptr(i), 0, 0)
   236  	}
   237  
   238  	// Detach fd 0 from tty
   239  	if sys.Noctty {
   240  		_, _, err1 = RawSyscall(SYS_IOCTL, 0, uintptr(TIOCNOTTY), 0)
   241  		if err1 != 0 {
   242  			goto childerror
   243  		}
   244  	}
   245  
   246  	// Set the controlling TTY to Ctty
   247  	if sys.Setctty {
   248  		_, _, err1 = RawSyscall(SYS_IOCTL, uintptr(sys.Ctty), uintptr(TIOCSCTTY), 0)
   249  		if err1 != 0 {
   250  			goto childerror
   251  		}
   252  	}
   253  
   254  	// Time to exec.
   255  	_, _, err1 = RawSyscall(SYS_EXECVE,
   256  		uintptr(unsafe.Pointer(argv0)),
   257  		uintptr(unsafe.Pointer(&argv[0])),
   258  		uintptr(unsafe.Pointer(&envv[0])))
   259  
   260  childerror:
   261  	// send error code on pipe
   262  	RawSyscall(SYS_WRITE, uintptr(pipe), uintptr(unsafe.Pointer(&err1)), unsafe.Sizeof(err1))
   263  	for {
   264  		RawSyscall(SYS_EXIT, 253, 0, 0)
   265  	}
   266  }
   267
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