/* POSIX spawn interface. Linux version.
Copyright (C) 2016-2018 Free Software Foundation, Inc.
This file is part of the GNU C Library.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; if not, see
<http://www.gnu.org/licenses/>. */
#include <spawn.h>
#include <fcntl.h>
#include <paths.h>
#include <string.h>
#include <sys/resource.h>
#include <sys/wait.h>
#include <sys/param.h>
#include <sys/mman.h>
#include <not-cancel.h>
#include <local-setxid.h>
#include <shlib-compat.h>
#include <nptl/pthreadP.h>
#include <dl-sysdep.h>
#include <libc-pointer-arith.h>
#include <ldsodefs.h>
#include "spawn_int.h"
/* The Linux implementation of posix_spawn{p} uses the clone syscall directly
with CLONE_VM and CLONE_VFORK flags and an allocated stack. The new stack
and start function solves most the vfork limitation (possible parent
clobber due stack spilling). The remaining issue are:
1. That no signal handlers must run in child context, to avoid corrupting
parent's state.
2. The parent must ensure child's stack freeing.
3. Child must synchronize with parent to enforce 2. and to possible
return execv issues.
The first issue is solved by blocking all signals in child, even
the NPTL-internal ones (SIGCANCEL and SIGSETXID). The second and
third issue is done by a stack allocation in parent, and by using a
field in struct spawn_args where the child can write an error
code. CLONE_VFORK ensures that the parent does not run until the
child has either exec'ed successfully or exited. */
/* The Unix standard contains a long explanation of the way to signal
an error after the fork() was successful. Since no new wait status
was wanted there is no way to signal an error using one of the
available methods. The committee chose to signal an error by a
normal program exit with the exit code 127. */
#define SPAWN_ERROR 127
#ifdef __ia64__
# define CLONE(__fn, __stackbase, __stacksize, __flags, __args) \
__clone2 (__fn, __stackbase, __stacksize, __flags, __args, 0, 0, 0)
#else
# define CLONE(__fn, __stack, __stacksize, __flags, __args) \
__clone (__fn, __stack, __flags, __args)
#endif
/* Since ia64 wants the stackbase w/clone2, re-use the grows-up macro. */
#if _STACK_GROWS_UP || defined (__ia64__)
# define STACK(__stack, __stack_size) (__stack)
#elif _STACK_GROWS_DOWN
# define STACK(__stack, __stack_size) (__stack + __stack_size)
#endif
struct posix_spawn_args
{
sigset_t oldmask;
const char *file;
int (*exec) (const char *, char *const *, char *const *);
const posix_spawn_file_actions_t *fa;
const posix_spawnattr_t *restrict attr;
char *const *argv;
ptrdiff_t argc;
char *const *envp;
int xflags;
int err;
};
/* Older version requires that shell script without shebang definition
to be called explicitly using /bin/sh (_PATH_BSHELL). */
static void
maybe_script_execute (struct posix_spawn_args *args)
{
if (SHLIB_COMPAT (libc, GLIBC_2_2, GLIBC_2_15)
&& (args->xflags & SPAWN_XFLAGS_TRY_SHELL) && errno == ENOEXEC)
{
char *const *argv = args->argv;
ptrdiff_t argc = args->argc;
/* Construct an argument list for the shell. */
char *new_argv[argc + 2];
new_argv[0] = (char *) _PATH_BSHELL;
new_argv[1] = (char *) args->file;
if (argc > 1)
memcpy (new_argv + 2, argv + 1, argc * sizeof(char *));
else
new_argv[2] = NULL;
/* Execute the shell. */
args->exec (new_argv[0], new_argv, args->envp);
}
}
/* Function used in the clone call to setup the signals mask, posix_spawn
attributes, and file actions. It run on its own stack (provided by the
posix_spawn call). */
static int
__spawni_child (void *arguments)
{
struct posix_spawn_args *args = arguments;
const posix_spawnattr_t *restrict attr = args->attr;
const posix_spawn_file_actions_t *file_actions = args->fa;
/* The child must ensure that no signal handler are enabled because it shared
memory with parent, so the signal disposition must be either SIG_DFL or
SIG_IGN. It does by iterating over all signals and although it could
possibly be more optimized (by tracking which signal potentially have a
signal handler), it might requires system specific solutions (since the
sigset_t data type can be very different on different architectures). */
struct sigaction sa;
memset (&sa, '\0', sizeof (sa));
sigset_t hset;
__sigprocmask (SIG_BLOCK, 0, &hset);
for (int sig = 1; sig < _NSIG; ++sig)
{
if ((attr->__flags & POSIX_SPAWN_SETSIGDEF)
&& sigismember (&attr->__sd, sig))
{
sa.sa_handler = SIG_DFL;
}
else if (sigismember (&hset, sig))
{
if (__is_internal_signal (sig))
sa.sa_handler = SIG_IGN;
else
{
__libc_sigaction (sig, 0, &sa);
if (sa.sa_handler == SIG_IGN)
continue;
sa.sa_handler = SIG_DFL;
}
}
else
continue;
__libc_sigaction (sig, &sa, 0);
}
#ifdef _POSIX_PRIORITY_SCHEDULING
/* Set the scheduling algorithm and parameters. */
if ((attr->__flags & (POSIX_SPAWN_SETSCHEDPARAM | POSIX_SPAWN_SETSCHEDULER))
== POSIX_SPAWN_SETSCHEDPARAM)
{
if (__sched_setparam (0, &attr->__sp) == -1)
goto fail;
}
else if ((attr->__flags & POSIX_SPAWN_SETSCHEDULER) != 0)
{
if (__sched_setscheduler (0, attr->__policy, &attr->__sp) == -1)
goto fail;
}
#endif
if ((attr->__flags & POSIX_SPAWN_SETSID) != 0
&& __setsid () < 0)
goto fail;
/* Set the process group ID. */
if ((attr->__flags & POSIX_SPAWN_SETPGROUP) != 0
&& __setpgid (0, attr->__pgrp) != 0)
goto fail;
/* Set the effective user and group IDs. */
if ((attr->__flags & POSIX_SPAWN_RESETIDS) != 0
&& (local_seteuid (__getuid ()) != 0
|| local_setegid (__getgid ()) != 0))
goto fail;
/* Execute the file actions. */
if (file_actions != 0)
{
int cnt;
struct rlimit64 fdlimit;
bool have_fdlimit = false;
for (cnt = 0; cnt < file_actions->__used; ++cnt)
{
struct __spawn_action *action = &file_actions->__actions[cnt];
switch (action->tag)
{
case spawn_do_close:
if (__close_nocancel (action->action.close_action.fd) != 0)
{
if (!have_fdlimit)
{
__getrlimit64 (RLIMIT_NOFILE, &fdlimit);
have_fdlimit = true;
}
/* Signal errors only for file descriptors out of range. */
if (action->action.close_action.fd < 0
|| action->action.close_action.fd >= fdlimit.rlim_cur)
goto fail;
}
break;
case spawn_do_open:
{
/* POSIX states that if fildes was already an open file descriptor,
it shall be closed before the new file is opened. This avoid
pontential issues when posix_spawn plus addopen action is called
with the process already at maximum number of file descriptor
opened and also for multiple actions on single-open special
paths (like /dev/watchdog). */
__close_nocancel (action->action.open_action.fd);
int ret = __open_nocancel (action->action.open_action.path,
action->action.
open_action.oflag | O_LARGEFILE,
action->action.open_action.mode);
if (ret == -1)
goto fail;
int new_fd = ret;
/* Make sure the desired file descriptor is used. */
if (ret != action->action.open_action.fd)
{
if (__dup2 (new_fd, action->action.open_action.fd)
!= action->action.open_action.fd)
goto fail;
if (__close_nocancel (new_fd) != 0)
goto fail;
}
}
break;
case spawn_do_dup2:
if (__dup2 (action->action.dup2_action.fd,
action->action.dup2_action.newfd)
!= action->action.dup2_action.newfd)
goto fail;
break;
}
}
}
/* Set the initial signal mask of the child if POSIX_SPAWN_SETSIGMASK
is set, otherwise restore the previous one. */
__sigprocmask (SIG_SETMASK, (attr->__flags & POSIX_SPAWN_SETSIGMASK)
? &attr->__ss : &args->oldmask, 0);
args->exec (args->file, args->argv, args->envp);
/* This is compatibility function required to enable posix_spawn run
script without shebang definition for older posix_spawn versions
(2.15). */
maybe_script_execute (args);
fail:
/* errno should have an appropriate non-zero value; otherwise,
there's a bug in glibc or the kernel. For lack of an error code
(EINTERNALBUG) describing that, use ECHILD. Another option would
be to set args->err to some negative sentinel and have the parent
abort(), but that seems needlessly harsh. */
args->err = errno ? : ECHILD;
_exit (SPAWN_ERROR);
}
/* Spawn a new process executing PATH with the attributes describes in *ATTRP.
Before running the process perform the actions described in FILE-ACTIONS. */
static int
__spawnix (pid_t * pid, const char *file,
const posix_spawn_file_actions_t * file_actions,
const posix_spawnattr_t * attrp, char *const argv[],
char *const envp[], int xflags,
int (*exec) (const char *, char *const *, char *const *))
{
pid_t new_pid;
struct posix_spawn_args args;
int ec;
/* To avoid imposing hard limits on posix_spawn{p} the total number of
arguments is first calculated to allocate a mmap to hold all possible
values. */
ptrdiff_t argc = 0;
/* Linux allows at most max (0x7FFFFFFF, 1/4 stack size) arguments
to be used in a execve call. We limit to INT_MAX minus one due the
compatiblity code that may execute a shell script (maybe_script_execute)
where it will construct another argument list with an additional
argument. */
ptrdiff_t limit = INT_MAX - 1;
while (argv[argc++] != NULL)
if (argc == limit)
{
errno = E2BIG;
return errno;
}
int prot = (PROT_READ | PROT_WRITE
| ((GL (dl_stack_flags) & PF_X) ? PROT_EXEC : 0));
/* Add a slack area for child's stack. */
size_t argv_size = (argc * sizeof (void *)) + 512;
/* We need at least a few pages in case the compiler's stack checking is
enabled. In some configs, it is known to use at least 24KiB. We use
32KiB to be "safe" from anything the compiler might do. Besides, the
extra pages won't actually be allocated unless they get used. */
argv_size += (32 * 1024);
size_t stack_size = ALIGN_UP (argv_size, GLRO(dl_pagesize));
void *stack = __mmap (NULL, stack_size, prot,
MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK, -1, 0);
if (__glibc_unlikely (stack == MAP_FAILED))
return errno;
/* Disable asynchronous cancellation. */
int state;
__libc_ptf_call (__pthread_setcancelstate,
(PTHREAD_CANCEL_DISABLE, &state), 0);
/* Child must set args.err to something non-negative - we rely on
the parent and child sharing VM. */
args.err = 0;
args.file = file;
args.exec = exec;
args.fa = file_actions;
args.attr = attrp ? attrp : &(const posix_spawnattr_t) { 0 };
args.argv = argv;
args.argc = argc;
args.envp = envp;
args.xflags = xflags;
__libc_signal_block_all (&args.oldmask);
/* The clone flags used will create a new child that will run in the same
memory space (CLONE_VM) and the execution of calling thread will be
suspend until the child calls execve or _exit.
Also since the calling thread execution will be suspend, there is not
need for CLONE_SETTLS. Although parent and child share the same TLS
namespace, there will be no concurrent access for TLS variables (errno
for instance). */
new_pid = CLONE (__spawni_child, STACK (stack, stack_size), stack_size,
CLONE_VM | CLONE_VFORK | SIGCHLD, &args);
/* It needs to collect the case where the auxiliary process was created
but failed to execute the file (due either any preparation step or
for execve itself). */
if (new_pid > 0)
{
/* Also, it handles the unlikely case where the auxiliary process was
terminated before calling execve as if it was successfully. The
args.err is set to 0 as default and changed to a positive value
only in case of failure, so in case of premature termination
due a signal args.err will remain zeroed and it will be up to
caller to actually collect it. */
ec = args.err;
if (ec > 0)
/* There still an unlikely case where the child is cancelled after
setting args.err, due to a positive error value. Also there is
possible pid reuse race (where the kernel allocated the same pid
to an unrelated process). Unfortunately due synchronization
issues where the kernel might not have the process collected
the waitpid below can not use WNOHANG. */
__waitpid (new_pid, NULL, 0);
}
else
ec = -new_pid;
__munmap (stack, stack_size);
if ((ec == 0) && (pid != NULL))
*pid = new_pid;
__libc_signal_restore_set (&args.oldmask);
__libc_ptf_call (__pthread_setcancelstate, (state, NULL), 0);
return ec;
}
/* Spawn a new process executing PATH with the attributes describes in *ATTRP.
Before running the process perform the actions described in FILE-ACTIONS. */
int
__spawni (pid_t * pid, const char *file,
const posix_spawn_file_actions_t * acts,
const posix_spawnattr_t * attrp, char *const argv[],
char *const envp[], int xflags)
{
/* It uses __execvpex to avoid run ENOEXEC in non compatibility mode (it
will be handled by maybe_script_execute). */
return __spawnix (pid, file, acts, attrp, argv, envp, xflags,
xflags & SPAWN_XFLAGS_USE_PATH ? __execvpex :__execve);
}