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 && !mips64)
     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 assembly
    52  // functions that do not grow the stack.
    53  //
    54  //go:norace
    55  func forkAndExecInChild(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr *ProcAttr, sys *SysProcAttr, pipe int) (pid int, err Errno) {
    56  	// Declare all variables at top in case any
    57  	// declarations require heap allocation (e.g., err1).
    58  	var (
    59  		r1     uintptr
    60  		err1   Errno
    61  		nextfd int
    62  		i      int
    63  	)
    64  
    65  	// guard against side effects of shuffling fds below.
    66  	// Make sure that nextfd is beyond any currently open files so
    67  	// that we can't run the risk of overwriting any of them.
    68  	fd := make([]int, len(attr.Files))
    69  	nextfd = len(attr.Files)
    70  	for i, ufd := range attr.Files {
    71  		if nextfd < int(ufd) {
    72  			nextfd = int(ufd)
    73  		}
    74  		fd[i] = int(ufd)
    75  	}
    76  	nextfd++
    77  
    78  	// About to call fork.
    79  	// No more allocation or calls of non-assembly functions.
    80  	runtime_BeforeFork()
    81  	r1, _, err1 = rawSyscall(abi.FuncPCABI0(libc_fork_trampoline), 0, 0, 0)
    82  	if err1 != 0 {
    83  		runtime_AfterFork()
    84  		return 0, err1
    85  	}
    86  
    87  	if r1 != 0 {
    88  		// parent; return PID
    89  		runtime_AfterFork()
    90  		return int(r1), 0
    91  	}
    92  
    93  	// Fork succeeded, now in child.
    94  
    95  	// Enable tracing if requested.
    96  	if sys.Ptrace {
    97  		if err := ptrace(PTRACE_TRACEME, 0, 0, 0); err != nil {
    98  			err1 = err.(Errno)
    99  			goto childerror
   100  		}
   101  	}
   102  
   103  	// Session ID
   104  	if sys.Setsid {
   105  		_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_setsid_trampoline), 0, 0, 0)
   106  		if err1 != 0 {
   107  			goto childerror
   108  		}
   109  	}
   110  
   111  	// Set process group
   112  	if sys.Setpgid || sys.Foreground {
   113  		// Place child in process group.
   114  		_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_setpgid_trampoline), 0, uintptr(sys.Pgid), 0)
   115  		if err1 != 0 {
   116  			goto childerror
   117  		}
   118  	}
   119  
   120  	if sys.Foreground {
   121  		// This should really be pid_t, however _C_int (aka int32) is
   122  		// generally equivalent.
   123  		pgrp := _C_int(sys.Pgid)
   124  		if pgrp == 0 {
   125  			r1, _, err1 = rawSyscall(abi.FuncPCABI0(libc_getpid_trampoline), 0, 0, 0)
   126  			if err1 != 0 {
   127  				goto childerror
   128  			}
   129  			pgrp = _C_int(r1)
   130  		}
   131  
   132  		// Place process group in foreground.
   133  		_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_ioctl_trampoline), uintptr(sys.Ctty), uintptr(TIOCSPGRP), uintptr(unsafe.Pointer(&pgrp)))
   134  		if err1 != 0 {
   135  			goto childerror
   136  		}
   137  	}
   138  
   139  	// Restore the signal mask. We do this after TIOCSPGRP to avoid
   140  	// having the kernel send a SIGTTOU signal to the process group.
   141  	runtime_AfterForkInChild()
   142  
   143  	// Chroot
   144  	if chroot != nil {
   145  		_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_chroot_trampoline), uintptr(unsafe.Pointer(chroot)), 0, 0)
   146  		if err1 != 0 {
   147  			goto childerror
   148  		}
   149  	}
   150  
   151  	// User and groups
   152  	if cred := sys.Credential; cred != nil {
   153  		ngroups := uintptr(len(cred.Groups))
   154  		groups := uintptr(0)
   155  		if ngroups > 0 {
   156  			groups = uintptr(unsafe.Pointer(&cred.Groups[0]))
   157  		}
   158  		if !cred.NoSetGroups {
   159  			_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_setgroups_trampoline), ngroups, groups, 0)
   160  			if err1 != 0 {
   161  				goto childerror
   162  			}
   163  		}
   164  		_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_setgid_trampoline), uintptr(cred.Gid), 0, 0)
   165  		if err1 != 0 {
   166  			goto childerror
   167  		}
   168  		_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_setuid_trampoline), uintptr(cred.Uid), 0, 0)
   169  		if err1 != 0 {
   170  			goto childerror
   171  		}
   172  	}
   173  
   174  	// Chdir
   175  	if dir != nil {
   176  		_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_chdir_trampoline), uintptr(unsafe.Pointer(dir)), 0, 0)
   177  		if err1 != 0 {
   178  			goto childerror
   179  		}
   180  	}
   181  
   182  	// Pass 1: look for fd[i] < i and move those up above len(fd)
   183  	// so that pass 2 won't stomp on an fd it needs later.
   184  	if pipe < nextfd {
   185  		if runtime.GOOS == "openbsd" {
   186  			_, _, err1 = rawSyscall(dupTrampoline, uintptr(pipe), uintptr(nextfd), O_CLOEXEC)
   187  		} else {
   188  			_, _, err1 = rawSyscall(dupTrampoline, uintptr(pipe), uintptr(nextfd), 0)
   189  			if err1 != 0 {
   190  				goto childerror
   191  			}
   192  			_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_fcntl_trampoline), uintptr(nextfd), F_SETFD, FD_CLOEXEC)
   193  		}
   194  		if err1 != 0 {
   195  			goto childerror
   196  		}
   197  		pipe = nextfd
   198  		nextfd++
   199  	}
   200  	for i = 0; i < len(fd); i++ {
   201  		if fd[i] >= 0 && fd[i] < i {
   202  			if nextfd == pipe { // don't stomp on pipe
   203  				nextfd++
   204  			}
   205  			if runtime.GOOS == "openbsd" {
   206  				_, _, err1 = rawSyscall(dupTrampoline, uintptr(fd[i]), uintptr(nextfd), O_CLOEXEC)
   207  			} else {
   208  				_, _, err1 = rawSyscall(dupTrampoline, uintptr(fd[i]), uintptr(nextfd), 0)
   209  				if err1 != 0 {
   210  					goto childerror
   211  				}
   212  				_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_fcntl_trampoline), uintptr(nextfd), F_SETFD, FD_CLOEXEC)
   213  			}
   214  			if err1 != 0 {
   215  				goto childerror
   216  			}
   217  			fd[i] = nextfd
   218  			nextfd++
   219  		}
   220  	}
   221  
   222  	// Pass 2: dup fd[i] down onto i.
   223  	for i = 0; i < len(fd); i++ {
   224  		if fd[i] == -1 {
   225  			rawSyscall(abi.FuncPCABI0(libc_close_trampoline), uintptr(i), 0, 0)
   226  			continue
   227  		}
   228  		if fd[i] == i {
   229  			// dup2(i, i) won't clear close-on-exec flag on Linux,
   230  			// probably not elsewhere either.
   231  			_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_fcntl_trampoline), uintptr(fd[i]), F_SETFD, 0)
   232  			if err1 != 0 {
   233  				goto childerror
   234  			}
   235  			continue
   236  		}
   237  		// The new fd is created NOT close-on-exec,
   238  		// which is exactly what we want.
   239  		_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_dup2_trampoline), uintptr(fd[i]), uintptr(i), 0)
   240  		if err1 != 0 {
   241  			goto childerror
   242  		}
   243  	}
   244  
   245  	// By convention, we don't close-on-exec the fds we are
   246  	// started with, so if len(fd) < 3, close 0, 1, 2 as needed.
   247  	// Programs that know they inherit fds >= 3 will need
   248  	// to set them close-on-exec.
   249  	for i = len(fd); i < 3; i++ {
   250  		rawSyscall(abi.FuncPCABI0(libc_close_trampoline), uintptr(i), 0, 0)
   251  	}
   252  
   253  	// Detach fd 0 from tty
   254  	if sys.Noctty {
   255  		_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_ioctl_trampoline), 0, uintptr(TIOCNOTTY), 0)
   256  		if err1 != 0 {
   257  			goto childerror
   258  		}
   259  	}
   260  
   261  	// Set the controlling TTY to Ctty
   262  	if sys.Setctty {
   263  		_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_ioctl_trampoline), uintptr(sys.Ctty), uintptr(TIOCSCTTY), 0)
   264  		if err1 != 0 {
   265  			goto childerror
   266  		}
   267  	}
   268  
   269  	// Time to exec.
   270  	_, _, err1 = rawSyscall(abi.FuncPCABI0(libc_execve_trampoline),
   271  		uintptr(unsafe.Pointer(argv0)),
   272  		uintptr(unsafe.Pointer(&argv[0])),
   273  		uintptr(unsafe.Pointer(&envv[0])))
   274  
   275  childerror:
   276  	// send error code on pipe
   277  	rawSyscall(abi.FuncPCABI0(libc_write_trampoline), uintptr(pipe), uintptr(unsafe.Pointer(&err1)), unsafe.Sizeof(err1))
   278  	for {
   279  		rawSyscall(abi.FuncPCABI0(libc_exit_trampoline), 253, 0, 0)
   280  	}
   281  }
   282
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