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