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