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aa95a2414e
As shown in some buildbot issues on aarch64 and powerpc, calling
clone (VFORK) and waitpid (WNOHANG) does not guarantee the child
is ready to be collected. This patch changes the call back to 0
as before fe05e1cb6d
fix.
This change can lead to the scenario 4.3 described in the commit,
where the waitpid call can hang undefinitely on the call. However
this is also a very unlikely and also undefinied situation where
both the caller is trying to terminate a pid before posix_spawn
returns and the race pid reuse is triggered. I don't see how to
correct handle this specific situation within posix_spawn.
Checked on x86_64-linux-gnu, aarch64-linux-gnu and
powerpc64-linux-gnu.
* sysdeps/unix/sysv/linux/spawni.c (__spawnix): Use 0 instead of
WNOHANG in waitpid call.
410 lines
14 KiB
C
410 lines
14 KiB
C
/* POSIX spawn interface. Linux version.
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Copyright (C) 2016-2017 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<http://www.gnu.org/licenses/>. */
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#include <spawn.h>
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#include <fcntl.h>
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#include <paths.h>
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#include <string.h>
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#include <sys/resource.h>
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#include <sys/wait.h>
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#include <sys/param.h>
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#include <sys/mman.h>
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#include <not-cancel.h>
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#include <local-setxid.h>
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#include <shlib-compat.h>
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#include <nptl/pthreadP.h>
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#include <dl-sysdep.h>
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#include <libc-pointer-arith.h>
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#include <ldsodefs.h>
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#include "spawn_int.h"
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/* The Linux implementation of posix_spawn{p} uses the clone syscall directly
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with CLONE_VM and CLONE_VFORK flags and an allocated stack. The new stack
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and start function solves most the vfork limitation (possible parent
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clobber due stack spilling). The remaining issue are:
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1. That no signal handlers must run in child context, to avoid corrupting
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parent's state.
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2. The parent must ensure child's stack freeing.
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3. Child must synchronize with parent to enforce 2. and to possible
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return execv issues.
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The first issue is solved by blocking all signals in child, even
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the NPTL-internal ones (SIGCANCEL and SIGSETXID). The second and
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third issue is done by a stack allocation in parent, and by using a
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field in struct spawn_args where the child can write an error
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code. CLONE_VFORK ensures that the parent does not run until the
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child has either exec'ed successfully or exited. */
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/* The Unix standard contains a long explanation of the way to signal
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an error after the fork() was successful. Since no new wait status
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was wanted there is no way to signal an error using one of the
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available methods. The committee chose to signal an error by a
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normal program exit with the exit code 127. */
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#define SPAWN_ERROR 127
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#ifdef __ia64__
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# define CLONE(__fn, __stackbase, __stacksize, __flags, __args) \
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__clone2 (__fn, __stackbase, __stacksize, __flags, __args, 0, 0, 0)
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#else
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# define CLONE(__fn, __stack, __stacksize, __flags, __args) \
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__clone (__fn, __stack, __flags, __args)
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#endif
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/* Since ia64 wants the stackbase w/clone2, re-use the grows-up macro. */
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#if _STACK_GROWS_UP || defined (__ia64__)
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# define STACK(__stack, __stack_size) (__stack)
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#elif _STACK_GROWS_DOWN
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# define STACK(__stack, __stack_size) (__stack + __stack_size)
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#endif
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struct posix_spawn_args
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{
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sigset_t oldmask;
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const char *file;
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int (*exec) (const char *, char *const *, char *const *);
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const posix_spawn_file_actions_t *fa;
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const posix_spawnattr_t *restrict attr;
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char *const *argv;
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ptrdiff_t argc;
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char *const *envp;
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int xflags;
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int err;
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};
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/* Older version requires that shell script without shebang definition
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to be called explicitly using /bin/sh (_PATH_BSHELL). */
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static void
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maybe_script_execute (struct posix_spawn_args *args)
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{
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if (SHLIB_COMPAT (libc, GLIBC_2_2, GLIBC_2_15)
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&& (args->xflags & SPAWN_XFLAGS_TRY_SHELL) && errno == ENOEXEC)
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{
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char *const *argv = args->argv;
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ptrdiff_t argc = args->argc;
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/* Construct an argument list for the shell. */
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char *new_argv[argc + 1];
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new_argv[0] = (char *) _PATH_BSHELL;
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new_argv[1] = (char *) args->file;
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if (argc > 1)
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memcpy (new_argv + 2, argv + 1, argc * sizeof(char *));
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else
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new_argv[2] = NULL;
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/* Execute the shell. */
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args->exec (new_argv[0], new_argv, args->envp);
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}
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}
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/* Function used in the clone call to setup the signals mask, posix_spawn
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attributes, and file actions. It run on its own stack (provided by the
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posix_spawn call). */
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static int
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__spawni_child (void *arguments)
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{
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struct posix_spawn_args *args = arguments;
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const posix_spawnattr_t *restrict attr = args->attr;
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const posix_spawn_file_actions_t *file_actions = args->fa;
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/* The child must ensure that no signal handler are enabled because it shared
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memory with parent, so the signal disposition must be either SIG_DFL or
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SIG_IGN. It does by iterating over all signals and although it could
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possibly be more optimized (by tracking which signal potentially have a
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signal handler), it might requires system specific solutions (since the
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sigset_t data type can be very different on different architectures). */
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struct sigaction sa;
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memset (&sa, '\0', sizeof (sa));
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sigset_t hset;
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__sigprocmask (SIG_BLOCK, 0, &hset);
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for (int sig = 1; sig < _NSIG; ++sig)
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{
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if ((attr->__flags & POSIX_SPAWN_SETSIGDEF)
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&& sigismember (&attr->__sd, sig))
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{
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sa.sa_handler = SIG_DFL;
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}
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else if (sigismember (&hset, sig))
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{
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if (__nptl_is_internal_signal (sig))
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sa.sa_handler = SIG_IGN;
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else
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{
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__libc_sigaction (sig, 0, &sa);
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if (sa.sa_handler == SIG_IGN)
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continue;
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sa.sa_handler = SIG_DFL;
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}
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}
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else
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continue;
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__libc_sigaction (sig, &sa, 0);
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}
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#ifdef _POSIX_PRIORITY_SCHEDULING
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/* Set the scheduling algorithm and parameters. */
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if ((attr->__flags & (POSIX_SPAWN_SETSCHEDPARAM | POSIX_SPAWN_SETSCHEDULER))
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== POSIX_SPAWN_SETSCHEDPARAM)
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{
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if (__sched_setparam (0, &attr->__sp) == -1)
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goto fail;
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}
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else if ((attr->__flags & POSIX_SPAWN_SETSCHEDULER) != 0)
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{
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if (__sched_setscheduler (0, attr->__policy, &attr->__sp) == -1)
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goto fail;
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}
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#endif
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if ((attr->__flags & POSIX_SPAWN_SETSID) != 0
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&& __setsid () < 0)
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goto fail;
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/* Set the process group ID. */
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if ((attr->__flags & POSIX_SPAWN_SETPGROUP) != 0
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&& __setpgid (0, attr->__pgrp) != 0)
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goto fail;
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/* Set the effective user and group IDs. */
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if ((attr->__flags & POSIX_SPAWN_RESETIDS) != 0
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&& (local_seteuid (__getuid ()) != 0
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|| local_setegid (__getgid ()) != 0))
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goto fail;
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/* Execute the file actions. */
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if (file_actions != 0)
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{
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int cnt;
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struct rlimit64 fdlimit;
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bool have_fdlimit = false;
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for (cnt = 0; cnt < file_actions->__used; ++cnt)
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{
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struct __spawn_action *action = &file_actions->__actions[cnt];
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switch (action->tag)
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{
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case spawn_do_close:
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if (__close_nocancel (action->action.close_action.fd) != 0)
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{
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if (!have_fdlimit)
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{
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__getrlimit64 (RLIMIT_NOFILE, &fdlimit);
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have_fdlimit = true;
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}
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/* Signal errors only for file descriptors out of range. */
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if (action->action.close_action.fd < 0
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|| action->action.close_action.fd >= fdlimit.rlim_cur)
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goto fail;
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}
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break;
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case spawn_do_open:
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{
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/* POSIX states that if fildes was already an open file descriptor,
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it shall be closed before the new file is opened. This avoid
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pontential issues when posix_spawn plus addopen action is called
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with the process already at maximum number of file descriptor
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opened and also for multiple actions on single-open special
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paths (like /dev/watchdog). */
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__close_nocancel (action->action.open_action.fd);
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int ret = __open_nocancel (action->action.open_action.path,
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action->action.
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open_action.oflag | O_LARGEFILE,
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action->action.open_action.mode);
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if (ret == -1)
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goto fail;
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int new_fd = ret;
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/* Make sure the desired file descriptor is used. */
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if (ret != action->action.open_action.fd)
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{
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if (__dup2 (new_fd, action->action.open_action.fd)
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!= action->action.open_action.fd)
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goto fail;
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if (__close_nocancel (new_fd) != 0)
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goto fail;
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}
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}
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break;
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case spawn_do_dup2:
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if (__dup2 (action->action.dup2_action.fd,
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action->action.dup2_action.newfd)
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!= action->action.dup2_action.newfd)
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goto fail;
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break;
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}
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}
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}
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/* Set the initial signal mask of the child if POSIX_SPAWN_SETSIGMASK
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is set, otherwise restore the previous one. */
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__sigprocmask (SIG_SETMASK, (attr->__flags & POSIX_SPAWN_SETSIGMASK)
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? &attr->__ss : &args->oldmask, 0);
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args->exec (args->file, args->argv, args->envp);
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/* This is compatibility function required to enable posix_spawn run
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script without shebang definition for older posix_spawn versions
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(2.15). */
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maybe_script_execute (args);
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fail:
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/* errno should have an appropriate non-zero value; otherwise,
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there's a bug in glibc or the kernel. For lack of an error code
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(EINTERNALBUG) describing that, use ECHILD. Another option would
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be to set args->err to some negative sentinel and have the parent
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abort(), but that seems needlessly harsh. */
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args->err = errno ? : ECHILD;
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_exit (SPAWN_ERROR);
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}
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/* Spawn a new process executing PATH with the attributes describes in *ATTRP.
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Before running the process perform the actions described in FILE-ACTIONS. */
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static int
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__spawnix (pid_t * pid, const char *file,
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const posix_spawn_file_actions_t * file_actions,
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const posix_spawnattr_t * attrp, char *const argv[],
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char *const envp[], int xflags,
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int (*exec) (const char *, char *const *, char *const *))
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{
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pid_t new_pid;
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struct posix_spawn_args args;
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int ec;
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/* To avoid imposing hard limits on posix_spawn{p} the total number of
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arguments is first calculated to allocate a mmap to hold all possible
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values. */
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ptrdiff_t argc = 0;
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/* Linux allows at most max (0x7FFFFFFF, 1/4 stack size) arguments
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to be used in a execve call. We limit to INT_MAX minus one due the
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compatiblity code that may execute a shell script (maybe_script_execute)
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where it will construct another argument list with an additional
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argument. */
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ptrdiff_t limit = INT_MAX - 1;
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while (argv[argc++] != NULL)
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if (argc == limit)
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{
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errno = E2BIG;
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return errno;
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}
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int prot = (PROT_READ | PROT_WRITE
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| ((GL (dl_stack_flags) & PF_X) ? PROT_EXEC : 0));
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/* Add a slack area for child's stack. */
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size_t argv_size = (argc * sizeof (void *)) + 512;
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/* We need at least a few pages in case the compiler's stack checking is
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enabled. In some configs, it is known to use at least 24KiB. We use
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32KiB to be "safe" from anything the compiler might do. Besides, the
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extra pages won't actually be allocated unless they get used. */
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argv_size += (32 * 1024);
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size_t stack_size = ALIGN_UP (argv_size, GLRO(dl_pagesize));
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void *stack = __mmap (NULL, stack_size, prot,
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MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK, -1, 0);
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if (__glibc_unlikely (stack == MAP_FAILED))
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return errno;
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/* Disable asynchronous cancellation. */
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int state;
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__libc_ptf_call (__pthread_setcancelstate,
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(PTHREAD_CANCEL_DISABLE, &state), 0);
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/* Child must set args.err to something non-negative - we rely on
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the parent and child sharing VM. */
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args.err = 0;
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args.file = file;
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args.exec = exec;
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args.fa = file_actions;
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args.attr = attrp ? attrp : &(const posix_spawnattr_t) { 0 };
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args.argv = argv;
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args.argc = argc;
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args.envp = envp;
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args.xflags = xflags;
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__libc_signal_block_all (&args.oldmask);
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/* The clone flags used will create a new child that will run in the same
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memory space (CLONE_VM) and the execution of calling thread will be
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suspend until the child calls execve or _exit.
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Also since the calling thread execution will be suspend, there is not
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need for CLONE_SETTLS. Although parent and child share the same TLS
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namespace, there will be no concurrent access for TLS variables (errno
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for instance). */
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new_pid = CLONE (__spawni_child, STACK (stack, stack_size), stack_size,
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CLONE_VM | CLONE_VFORK | SIGCHLD, &args);
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/* It needs to collect the case where the auxiliary process was created
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but failed to execute the file (due either any preparation step or
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for execve itself). */
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if (new_pid > 0)
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{
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/* Also, it handles the unlikely case where the auxiliary process was
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terminated before calling execve as if it was successfully. The
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args.err is set to 0 as default and changed to a positive value
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only in case of failure, so in case of premature termination
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due a signal args.err will remain zeroed and it will be up to
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caller to actually collect it. */
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ec = args.err;
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if (ec > 0)
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/* There still an unlikely case where the child is cancelled after
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setting args.err, due to a positive error value. Also there is
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possible pid reuse race (where the kernel allocated the same pid
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to an unrelated process). Unfortunately due synchronization
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issues where the kernel might not have the process collected
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the waitpid below can not use WNOHANG. */
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__waitpid (new_pid, NULL, 0);
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}
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else
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ec = -new_pid;
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__munmap (stack, stack_size);
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if ((ec == 0) && (pid != NULL))
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*pid = new_pid;
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__libc_signal_restore_set (&args.oldmask);
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__libc_ptf_call (__pthread_setcancelstate, (state, NULL), 0);
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return ec;
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}
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/* Spawn a new process executing PATH with the attributes describes in *ATTRP.
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Before running the process perform the actions described in FILE-ACTIONS. */
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int
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__spawni (pid_t * pid, const char *file,
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const posix_spawn_file_actions_t * acts,
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const posix_spawnattr_t * attrp, char *const argv[],
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char *const envp[], int xflags)
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{
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return __spawnix (pid, file, acts, attrp, argv, envp, xflags,
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xflags & SPAWN_XFLAGS_USE_PATH ? __execvpe : __execve);
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}
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