exec_freebsd.go

     1  // Copyright 2021 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 syscall
     6  
     7  import (
     8  	"runtime"
     9  	"unsafe"
    10  )
    11  
    12  type SysProcAttr struct {
    13  	Chroot     string      // Chroot.
    14  	Credential *Credential // Credential.
    15  	Ptrace     bool        // Enable tracing.
    16  	Setsid     bool        // Create session.
    17  	// Setpgid sets the process group ID of the child to Pgid,
    18  	// or, if Pgid == 0, to the new child's process ID.
    19  	Setpgid bool
    20  	// Setctty sets the controlling terminal of the child to
    21  	// file descriptor Ctty. Ctty must be a descriptor number
    22  	// in the child process: an index into ProcAttr.Files.
    23  	// This is only meaningful if Setsid is true.
    24  	Setctty bool
    25  	Noctty  bool // Detach fd 0 from controlling terminal
    26  	Ctty    int  // Controlling TTY fd
    27  	// Foreground places the child process group in the foreground.
    28  	// This implies Setpgid. The Ctty field must be set to
    29  	// the descriptor of the controlling TTY.
    30  	// Unlike Setctty, in this case Ctty must be a descriptor
    31  	// number in the parent process.
    32  	Foreground bool
    33  	Pgid       int    // Child's process group ID if Setpgid.
    34  	Pdeathsig  Signal // Signal that the process will get when its parent dies (Linux and FreeBSD only)
    35  }
    36  
    37  const (
    38  	_P_PID = 0
    39  
    40  	_PROC_PDEATHSIG_CTL = 11
    41  )
    42  
    43  // Implemented in runtime package.
    44  func runtime_BeforeFork()
    45  func runtime_AfterFork()
    46  func runtime_AfterForkInChild()
    47  
    48  // Fork, dup fd onto 0..len(fd), and exec(argv0, argvv, envv) in child.
    49  // If a dup or exec fails, write the errno error to pipe.
    50  // (Pipe is close-on-exec so if exec succeeds, it will be closed.)
    51  // In the child, this function must not acquire any locks, because
    52  // they might have been locked at the time of the fork. This means
    53  // no rescheduling, no malloc calls, and no new stack segments.
    54  // For the same reason compiler does not race instrument it.
    55  // The calls to RawSyscall are okay because they are assembly
    56  // functions that do not grow the stack.
    57  //go:norace
    58  func forkAndExecInChild(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr *ProcAttr, sys *SysProcAttr, pipe int) (pid int, err Errno) {
    59  	// Declare all variables at top in case any
    60  	// declarations require heap allocation (e.g., err1).
    61  	var (
    62  		r1     uintptr
    63  		err1   Errno
    64  		nextfd int
    65  		i      int
    66  	)
    67  
    68  	// Record parent PID so child can test if it has died.
    69  	ppid, _, _ := RawSyscall(SYS_GETPID, 0, 0, 0)
    70  
    71  	// guard against side effects of shuffling fds below.
    72  	// Make sure that nextfd is beyond any currently open files so
    73  	// that we can't run the risk of overwriting any of them.
    74  	fd := make([]int, len(attr.Files))
    75  	nextfd = len(attr.Files)
    76  	for i, ufd := range attr.Files {
    77  		if nextfd < int(ufd) {
    78  			nextfd = int(ufd)
    79  		}
    80  		fd[i] = int(ufd)
    81  	}
    82  	nextfd++
    83  
    84  	// About to call fork.
    85  	// No more allocation or calls of non-assembly functions.
    86  	runtime_BeforeFork()
    87  	r1, _, err1 = RawSyscall(SYS_FORK, 0, 0, 0)
    88  	if err1 != 0 {
    89  		runtime_AfterFork()
    90  		return 0, err1
    91  	}
    92  
    93  	if r1 != 0 {
    94  		// parent; return PID
    95  		runtime_AfterFork()
    96  		return int(r1), 0
    97  	}
    98  
    99  	// Fork succeeded, now in child.
   100  
   101  	// Enable tracing if requested.
   102  	if sys.Ptrace {
   103  		_, _, err1 = RawSyscall(SYS_PTRACE, uintptr(PTRACE_TRACEME), 0, 0)
   104  		if err1 != 0 {
   105  			goto childerror
   106  		}
   107  	}
   108  
   109  	// Session ID
   110  	if sys.Setsid {
   111  		_, _, err1 = RawSyscall(SYS_SETSID, 0, 0, 0)
   112  		if err1 != 0 {
   113  			goto childerror
   114  		}
   115  	}
   116  
   117  	// Set process group
   118  	if sys.Setpgid || sys.Foreground {
   119  		// Place child in process group.
   120  		_, _, err1 = RawSyscall(SYS_SETPGID, 0, uintptr(sys.Pgid), 0)
   121  		if err1 != 0 {
   122  			goto childerror
   123  		}
   124  	}
   125  
   126  	if sys.Foreground {
   127  		// This should really be pid_t, however _C_int (aka int32) is
   128  		// generally equivalent.
   129  		pgrp := _C_int(sys.Pgid)
   130  		if pgrp == 0 {
   131  			r1, _, err1 = RawSyscall(SYS_GETPID, 0, 0, 0)
   132  			if err1 != 0 {
   133  				goto childerror
   134  			}
   135  
   136  			pgrp = _C_int(r1)
   137  		}
   138  
   139  		// Place process group in foreground.
   140  		_, _, err1 = RawSyscall(SYS_IOCTL, uintptr(sys.Ctty), uintptr(TIOCSPGRP), uintptr(unsafe.Pointer(&pgrp)))
   141  		if err1 != 0 {
   142  			goto childerror
   143  		}
   144  	}
   145  
   146  	// Restore the signal mask. We do this after TIOCSPGRP to avoid
   147  	// having the kernel send a SIGTTOU signal to the process group.
   148  	runtime_AfterForkInChild()
   149  
   150  	// Chroot
   151  	if chroot != nil {
   152  		_, _, err1 = RawSyscall(SYS_CHROOT, uintptr(unsafe.Pointer(chroot)), 0, 0)
   153  		if err1 != 0 {
   154  			goto childerror
   155  		}
   156  	}
   157  
   158  	// User and groups
   159  	if cred := sys.Credential; cred != nil {
   160  		ngroups := uintptr(len(cred.Groups))
   161  		groups := uintptr(0)
   162  		if ngroups > 0 {
   163  			groups = uintptr(unsafe.Pointer(&cred.Groups[0]))
   164  		}
   165  		if !cred.NoSetGroups {
   166  			_, _, err1 = RawSyscall(SYS_SETGROUPS, ngroups, groups, 0)
   167  			if err1 != 0 {
   168  				goto childerror
   169  			}
   170  		}
   171  		_, _, err1 = RawSyscall(SYS_SETGID, uintptr(cred.Gid), 0, 0)
   172  		if err1 != 0 {
   173  			goto childerror
   174  		}
   175  		_, _, err1 = RawSyscall(SYS_SETUID, uintptr(cred.Uid), 0, 0)
   176  		if err1 != 0 {
   177  			goto childerror
   178  		}
   179  	}
   180  
   181  	// Chdir
   182  	if dir != nil {
   183  		_, _, err1 = RawSyscall(SYS_CHDIR, uintptr(unsafe.Pointer(dir)), 0, 0)
   184  		if err1 != 0 {
   185  			goto childerror
   186  		}
   187  	}
   188  
   189  	// Parent death signal
   190  	if sys.Pdeathsig != 0 {
   191  		switch runtime.GOARCH {
   192  		case "386", "arm":
   193  			_, _, err1 = RawSyscall6(SYS_PROCCTL, _P_PID, 0, 0, _PROC_PDEATHSIG_CTL, uintptr(unsafe.Pointer(&sys.Pdeathsig)), 0)
   194  		default:
   195  			_, _, err1 = RawSyscall6(SYS_PROCCTL, _P_PID, 0, _PROC_PDEATHSIG_CTL, uintptr(unsafe.Pointer(&sys.Pdeathsig)), 0, 0)
   196  		}
   197  		if err1 != 0 {
   198  			goto childerror
   199  		}
   200  
   201  		// Signal self if parent is already dead. This might cause a
   202  		// duplicate signal in rare cases, but it won't matter when
   203  		// using SIGKILL.
   204  		r1, _, _ = RawSyscall(SYS_GETPPID, 0, 0, 0)
   205  		if r1 != ppid {
   206  			pid, _, _ := RawSyscall(SYS_GETPID, 0, 0, 0)
   207  			_, _, err1 := RawSyscall(SYS_KILL, pid, uintptr(sys.Pdeathsig), 0)
   208  			if err1 != 0 {
   209  				goto childerror
   210  			}
   211  		}
   212  	}
   213  
   214  	// Pass 1: look for fd[i] < i and move those up above len(fd)
   215  	// so that pass 2 won't stomp on an fd it needs later.
   216  	if pipe < nextfd {
   217  		_, _, err1 = RawSyscall(SYS_FCNTL, uintptr(pipe), F_DUP2FD_CLOEXEC, uintptr(nextfd))
   218  		if err1 != 0 {
   219  			goto childerror
   220  		}
   221  		pipe = nextfd
   222  		nextfd++
   223  	}
   224  	for i = 0; i < len(fd); i++ {
   225  		if fd[i] >= 0 && fd[i] < int(i) {
   226  			if nextfd == pipe { // don't stomp on pipe
   227  				nextfd++
   228  			}
   229  			_, _, err1 = RawSyscall(SYS_FCNTL, uintptr(fd[i]), F_DUP2FD_CLOEXEC, uintptr(nextfd))
   230  			if err1 != 0 {
   231  				goto childerror
   232  			}
   233  			fd[i] = nextfd
   234  			nextfd++
   235  		}
   236  	}
   237  
   238  	// Pass 2: dup fd[i] down onto i.
   239  	for i = 0; i < len(fd); i++ {
   240  		if fd[i] == -1 {
   241  			RawSyscall(SYS_CLOSE, uintptr(i), 0, 0)
   242  			continue
   243  		}
   244  		if fd[i] == int(i) {
   245  			// dup2(i, i) won't clear close-on-exec flag on Linux,
   246  			// probably not elsewhere either.
   247  			_, _, err1 = RawSyscall(SYS_FCNTL, uintptr(fd[i]), F_SETFD, 0)
   248  			if err1 != 0 {
   249  				goto childerror
   250  			}
   251  			continue
   252  		}
   253  		// The new fd is created NOT close-on-exec,
   254  		// which is exactly what we want.
   255  		_, _, err1 = RawSyscall(SYS_DUP2, uintptr(fd[i]), uintptr(i), 0)
   256  		if err1 != 0 {
   257  			goto childerror
   258  		}
   259  	}
   260  
   261  	// By convention, we don't close-on-exec the fds we are
   262  	// started with, so if len(fd) < 3, close 0, 1, 2 as needed.
   263  	// Programs that know they inherit fds >= 3 will need
   264  	// to set them close-on-exec.
   265  	for i = len(fd); i < 3; i++ {
   266  		RawSyscall(SYS_CLOSE, uintptr(i), 0, 0)
   267  	}
   268  
   269  	// Detach fd 0 from tty
   270  	if sys.Noctty {
   271  		_, _, err1 = RawSyscall(SYS_IOCTL, 0, uintptr(TIOCNOTTY), 0)
   272  		if err1 != 0 {
   273  			goto childerror
   274  		}
   275  	}
   276  
   277  	// Set the controlling TTY to Ctty
   278  	if sys.Setctty {
   279  		_, _, err1 = RawSyscall(SYS_IOCTL, uintptr(sys.Ctty), uintptr(TIOCSCTTY), 0)
   280  		if err1 != 0 {
   281  			goto childerror
   282  		}
   283  	}
   284  
   285  	// Time to exec.
   286  	_, _, err1 = RawSyscall(SYS_EXECVE,
   287  		uintptr(unsafe.Pointer(argv0)),
   288  		uintptr(unsafe.Pointer(&argv[0])),
   289  		uintptr(unsafe.Pointer(&envv[0])))
   290  
   291  childerror:
   292  	// send error code on pipe
   293  	RawSyscall(SYS_WRITE, uintptr(pipe), uintptr(unsafe.Pointer(&err1)), unsafe.Sizeof(err1))
   294  	for {
   295  		RawSyscall(SYS_EXIT, 253, 0, 0)
   296  	}
   297  }
   298
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