1/* POSIX spawn interface. Linux version.
2 Copyright (C) 2016-2022 Free Software Foundation, Inc.
3 This file is part of the GNU C Library.
4
5 The GNU C Library is free software; you can redistribute it and/or
6 modify it under the terms of the GNU Lesser General Public
7 License as published by the Free Software Foundation; either
8 version 2.1 of the License, or (at your option) any later version.
9
10 The GNU C Library is distributed in the hope that it will be useful,
11 but WITHOUT ANY WARRANTY; without even the implied warranty of
12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
13 Lesser General Public License for more details.
14
15 You should have received a copy of the GNU Lesser General Public
16 License along with the GNU C Library; if not, see
17 <https://www.gnu.org/licenses/>. */
18
19#include <internal-signals.h>
20#include <ldsodefs.h>
21#include <local-setxid.h>
22#include <not-cancel.h>
23#include <paths.h>
24#include <shlib-compat.h>
25#include <spawn.h>
26#include <spawn_int.h>
27#include <sysdep.h>
28#include <sys/resource.h>
29#include <clone_internal.h>
30
31/* The Linux implementation of posix_spawn{p} uses the clone syscall directly
32 with CLONE_VM and CLONE_VFORK flags and an allocated stack. The new stack
33 and start function solves most the vfork limitation (possible parent
34 clobber due stack spilling). The remaining issue are:
35
36 1. That no signal handlers must run in child context, to avoid corrupting
37 parent's state.
38 2. The parent must ensure child's stack freeing.
39 3. Child must synchronize with parent to enforce 2. and to possible
40 return execv issues.
41
42 The first issue is solved by blocking all signals in child, even
43 the NPTL-internal ones (SIGCANCEL and SIGSETXID). The second and
44 third issue is done by a stack allocation in parent, and by using a
45 field in struct spawn_args where the child can write an error
46 code. CLONE_VFORK ensures that the parent does not run until the
47 child has either exec'ed successfully or exited. */
48
49
50/* The Unix standard contains a long explanation of the way to signal
51 an error after the fork() was successful. Since no new wait status
52 was wanted there is no way to signal an error using one of the
53 available methods. The committee chose to signal an error by a
54 normal program exit with the exit code 127. */
55#define SPAWN_ERROR 127
56
57
58struct posix_spawn_args
59{
60 sigset_t oldmask;
61 const char *file;
62 int (*exec) (const char *, char *const *, char *const *);
63 const posix_spawn_file_actions_t *fa;
64 const posix_spawnattr_t *restrict attr;
65 char *const *argv;
66 ptrdiff_t argc;
67 char *const *envp;
68 int xflags;
69 int err;
70};
71
72/* Older version requires that shell script without shebang definition
73 to be called explicitly using /bin/sh (_PATH_BSHELL). */
74static void
75maybe_script_execute (struct posix_spawn_args *args)
76{
77 if (SHLIB_COMPAT (libc, GLIBC_2_2, GLIBC_2_15)
78 && (args->xflags & SPAWN_XFLAGS_TRY_SHELL) && errno == ENOEXEC)
79 {
80 char *const *argv = args->argv;
81 ptrdiff_t argc = args->argc;
82
83 /* Construct an argument list for the shell. */
84 char *new_argv[argc + 2];
85 new_argv[0] = (char *) _PATH_BSHELL;
86 new_argv[1] = (char *) args->file;
87 if (argc > 1)
88 memcpy (new_argv + 2, argv + 1, argc * sizeof (char *));
89 else
90 new_argv[2] = NULL;
91
92 /* Execute the shell. */
93 args->exec (new_argv[0], new_argv, args->envp);
94 }
95}
96
97/* Function used in the clone call to setup the signals mask, posix_spawn
98 attributes, and file actions. It run on its own stack (provided by the
99 posix_spawn call). */
100static int
101__spawni_child (void *arguments)
102{
103 struct posix_spawn_args *args = arguments;
104 const posix_spawnattr_t *restrict attr = args->attr;
105 const posix_spawn_file_actions_t *file_actions = args->fa;
106
107 /* The child must ensure that no signal handler are enabled because it shared
108 memory with parent, so the signal disposition must be either SIG_DFL or
109 SIG_IGN. It does by iterating over all signals and although it could
110 possibly be more optimized (by tracking which signal potentially have a
111 signal handler), it might requires system specific solutions (since the
112 sigset_t data type can be very different on different architectures). */
113 struct sigaction sa;
114 memset (&sa, '\0', sizeof (sa));
115
116 sigset_t hset;
117 __sigprocmask (SIG_BLOCK, 0, &hset);
118 for (int sig = 1; sig < _NSIG; ++sig)
119 {
120 if ((attr->__flags & POSIX_SPAWN_SETSIGDEF)
121 && __sigismember (&attr->__sd, sig))
122 {
123 sa.sa_handler = SIG_DFL;
124 }
125 else if (__sigismember (&hset, sig))
126 {
127 if (__is_internal_signal (sig))
128 sa.sa_handler = SIG_IGN;
129 else
130 {
131 __libc_sigaction (sig, 0, &sa);
132 if (sa.sa_handler == SIG_IGN)
133 continue;
134 sa.sa_handler = SIG_DFL;
135 }
136 }
137 else
138 continue;
139
140 __libc_sigaction (sig, &sa, 0);
141 }
142
143#ifdef _POSIX_PRIORITY_SCHEDULING
144 /* Set the scheduling algorithm and parameters. */
145 if ((attr->__flags & (POSIX_SPAWN_SETSCHEDPARAM | POSIX_SPAWN_SETSCHEDULER))
146 == POSIX_SPAWN_SETSCHEDPARAM)
147 {
148 if (__sched_setparam (0, &attr->__sp) == -1)
149 goto fail;
150 }
151 else if ((attr->__flags & POSIX_SPAWN_SETSCHEDULER) != 0)
152 {
153 if (__sched_setscheduler (0, attr->__policy, &attr->__sp) == -1)
154 goto fail;
155 }
156#endif
157
158 if ((attr->__flags & POSIX_SPAWN_SETSID) != 0
159 && __setsid () < 0)
160 goto fail;
161
162 /* Set the process group ID. */
163 if ((attr->__flags & POSIX_SPAWN_SETPGROUP) != 0
164 && __setpgid (0, attr->__pgrp) != 0)
165 goto fail;
166
167 /* Set the effective user and group IDs. */
168 if ((attr->__flags & POSIX_SPAWN_RESETIDS) != 0
169 && (local_seteuid (__getuid ()) != 0
170 || local_setegid (__getgid ()) != 0))
171 goto fail;
172
173 /* Execute the file actions. */
174 if (file_actions != 0)
175 {
176 int cnt;
177 struct rlimit64 fdlimit;
178 bool have_fdlimit = false;
179
180 for (cnt = 0; cnt < file_actions->__used; ++cnt)
181 {
182 struct __spawn_action *action = &file_actions->__actions[cnt];
183
184 switch (action->tag)
185 {
186 case spawn_do_close:
187 if (__close_nocancel (action->action.close_action.fd) != 0)
188 {
189 if (!have_fdlimit)
190 {
191 __getrlimit64 (RLIMIT_NOFILE, &fdlimit);
192 have_fdlimit = true;
193 }
194
195 /* Signal errors only for file descriptors out of range. */
196 if (action->action.close_action.fd < 0
197 || action->action.close_action.fd >= fdlimit.rlim_cur)
198 goto fail;
199 }
200 break;
201
202 case spawn_do_open:
203 {
204 /* POSIX states that if fildes was already an open file descriptor,
205 it shall be closed before the new file is opened. This avoid
206 pontential issues when posix_spawn plus addopen action is called
207 with the process already at maximum number of file descriptor
208 opened and also for multiple actions on single-open special
209 paths (like /dev/watchdog). */
210 __close_nocancel (action->action.open_action.fd);
211
212 int ret = __open_nocancel (action->action.open_action.path,
213 action->action.
214 open_action.oflag | O_LARGEFILE,
215 action->action.open_action.mode);
216
217 if (ret == -1)
218 goto fail;
219
220 int new_fd = ret;
221
222 /* Make sure the desired file descriptor is used. */
223 if (ret != action->action.open_action.fd)
224 {
225 if (__dup2 (new_fd, action->action.open_action.fd)
226 != action->action.open_action.fd)
227 goto fail;
228
229 if (__close_nocancel (new_fd) != 0)
230 goto fail;
231 }
232 }
233 break;
234
235 case spawn_do_dup2:
236 /* Austin Group issue #411 requires adddup2 action with source
237 and destination being equal to remove close-on-exec flag. */
238 if (action->action.dup2_action.fd
239 == action->action.dup2_action.newfd)
240 {
241 int fd = action->action.dup2_action.newfd;
242 int flags = __fcntl (fd, F_GETFD, 0);
243 if (flags == -1)
244 goto fail;
245 if (__fcntl (fd, F_SETFD, flags & ~FD_CLOEXEC) == -1)
246 goto fail;
247 }
248 else if (__dup2 (action->action.dup2_action.fd,
249 action->action.dup2_action.newfd)
250 != action->action.dup2_action.newfd)
251 goto fail;
252 break;
253
254 case spawn_do_chdir:
255 if (__chdir (action->action.chdir_action.path) != 0)
256 goto fail;
257 break;
258
259 case spawn_do_fchdir:
260 if (__fchdir (action->action.fchdir_action.fd) != 0)
261 goto fail;
262 break;
263
264 case spawn_do_closefrom:
265 {
266 int lowfd = action->action.closefrom_action.from;
267 int r = INLINE_SYSCALL_CALL (close_range, lowfd, ~0U, 0);
268 if (r != 0 && !__closefrom_fallback (lowfd, false))
269 goto fail;
270 } break;
271
272 case spawn_do_tcsetpgrp:
273 {
274 /* Check if it is possible to avoid an extra syscall. */
275 pid_t pgrp = (attr->__flags & POSIX_SPAWN_SETPGROUP) != 0
276 && attr->__pgrp != 0
277 ? attr->__pgrp : __getpgid (0);
278 if (__tcsetpgrp (action->action.setpgrp_action.fd, pgrp) != 0)
279 goto fail;
280 }
281 }
282 }
283 }
284
285 /* Set the initial signal mask of the child if POSIX_SPAWN_SETSIGMASK
286 is set, otherwise restore the previous one. */
287 __sigprocmask (SIG_SETMASK, (attr->__flags & POSIX_SPAWN_SETSIGMASK)
288 ? &attr->__ss : &args->oldmask, 0);
289
290 args->exec (args->file, args->argv, args->envp);
291
292 /* This is compatibility function required to enable posix_spawn run
293 script without shebang definition for older posix_spawn versions
294 (2.15). */
295 maybe_script_execute (args);
296
297fail:
298 /* errno should have an appropriate non-zero value; otherwise,
299 there's a bug in glibc or the kernel. For lack of an error code
300 (EINTERNALBUG) describing that, use ECHILD. Another option would
301 be to set args->err to some negative sentinel and have the parent
302 abort(), but that seems needlessly harsh. */
303 args->err = errno ? : ECHILD;
304 _exit (SPAWN_ERROR);
305}
306
307/* Spawn a new process executing PATH with the attributes describes in *ATTRP.
308 Before running the process perform the actions described in FILE-ACTIONS. */
309static int
310__spawnix (pid_t * pid, const char *file,
311 const posix_spawn_file_actions_t * file_actions,
312 const posix_spawnattr_t * attrp, char *const argv[],
313 char *const envp[], int xflags,
314 int (*exec) (const char *, char *const *, char *const *))
315{
316 pid_t new_pid;
317 struct posix_spawn_args args;
318 int ec;
319
320 /* To avoid imposing hard limits on posix_spawn{p} the total number of
321 arguments is first calculated to allocate a mmap to hold all possible
322 values. */
323 ptrdiff_t argc = 0;
324 /* Linux allows at most max (0x7FFFFFFF, 1/4 stack size) arguments
325 to be used in a execve call. We limit to INT_MAX minus one due the
326 compatiblity code that may execute a shell script (maybe_script_execute)
327 where it will construct another argument list with an additional
328 argument. */
329 ptrdiff_t limit = INT_MAX - 1;
330 while (argv[argc++] != NULL)
331 if (argc == limit)
332 {
333 errno = E2BIG;
334 return errno;
335 }
336
337 int prot = (PROT_READ | PROT_WRITE
338 | ((GL (dl_stack_flags) & PF_X) ? PROT_EXEC : 0));
339
340 /* Add a slack area for child's stack. */
341 size_t argv_size = (argc * sizeof (void *)) + 512;
342 /* We need at least a few pages in case the compiler's stack checking is
343 enabled. In some configs, it is known to use at least 24KiB. We use
344 32KiB to be "safe" from anything the compiler might do. Besides, the
345 extra pages won't actually be allocated unless they get used.
346 It also acts the slack for spawn_closefrom (including MIPS64 getdents64
347 where it might use about 1k extra stack space). */
348 argv_size += (32 * 1024);
349 size_t stack_size = ALIGN_UP (argv_size, GLRO(dl_pagesize));
350 void *stack = __mmap (NULL, stack_size, prot,
351 MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK, -1, 0);
352 if (__glibc_unlikely (stack == MAP_FAILED))
353 return errno;
354
355 /* Disable asynchronous cancellation. */
356 int state;
357 __pthread_setcancelstate (PTHREAD_CANCEL_DISABLE, &state);
358
359 /* Child must set args.err to something non-negative - we rely on
360 the parent and child sharing VM. */
361 args.err = 0;
362 args.file = file;
363 args.exec = exec;
364 args.fa = file_actions;
365 args.attr = attrp ? attrp : &(const posix_spawnattr_t) { 0 };
366 args.argv = argv;
367 args.argc = argc;
368 args.envp = envp;
369 args.xflags = xflags;
370
371 __libc_signal_block_all (&args.oldmask);
372
373 /* The clone flags used will create a new child that will run in the same
374 memory space (CLONE_VM) and the execution of calling thread will be
375 suspend until the child calls execve or _exit.
376
377 Also since the calling thread execution will be suspend, there is not
378 need for CLONE_SETTLS. Although parent and child share the same TLS
379 namespace, there will be no concurrent access for TLS variables (errno
380 for instance). */
381 struct clone_args clone_args =
382 {
383 .flags = CLONE_VM | CLONE_VFORK,
384 .exit_signal = SIGCHLD,
385 .stack = (uintptr_t) stack,
386 .stack_size = stack_size,
387 };
388 new_pid = __clone_internal (&clone_args, __spawni_child, &args);
389
390 /* It needs to collect the case where the auxiliary process was created
391 but failed to execute the file (due either any preparation step or
392 for execve itself). */
393 if (new_pid > 0)
394 {
395 /* Also, it handles the unlikely case where the auxiliary process was
396 terminated before calling execve as if it was successfully. The
397 args.err is set to 0 as default and changed to a positive value
398 only in case of failure, so in case of premature termination
399 due a signal args.err will remain zeroed and it will be up to
400 caller to actually collect it. */
401 ec = args.err;
402 if (ec > 0)
403 /* There still an unlikely case where the child is cancelled after
404 setting args.err, due to a positive error value. Also there is
405 possible pid reuse race (where the kernel allocated the same pid
406 to an unrelated process). Unfortunately due synchronization
407 issues where the kernel might not have the process collected
408 the waitpid below can not use WNOHANG. */
409 __waitpid (new_pid, NULL, 0);
410 }
411 else
412 ec = -new_pid;
413
414 __munmap (stack, stack_size);
415
416 if ((ec == 0) && (pid != NULL))
417 *pid = new_pid;
418
419 __libc_signal_restore_set (&args.oldmask);
420
421 __pthread_setcancelstate (state, NULL);
422
423 return ec;
424}
425
426/* Spawn a new process executing PATH with the attributes describes in *ATTRP.
427 Before running the process perform the actions described in FILE-ACTIONS. */
428int
429__spawni (pid_t * pid, const char *file,
430 const posix_spawn_file_actions_t * acts,
431 const posix_spawnattr_t * attrp, char *const argv[],
432 char *const envp[], int xflags)
433{
434 /* It uses __execvpex to avoid run ENOEXEC in non compatibility mode (it
435 will be handled by maybe_script_execute). */
436 return __spawnix (pid, file, acts, attrp, argv, envp, xflags,
437 xflags & SPAWN_XFLAGS_USE_PATH ? __execvpex :__execve);
438}
439