sysdeps/unix/sysv/linux/spawni.c

   1 /* POSIX spawn interface.  Linux version.
   2    Copyright (C) 2016-2017 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    <http://www.gnu.org/licenses/>.  */
  18
  19 #include <spawn.h>
  20 #include <assert.h>
  21 #include <fcntl.h>
  22 #include <paths.h>
  23 #include <string.h>
  24 #include <sys/resource.h>
  25 #include <sys/wait.h>
  26 #include <sys/param.h>
  27 #include <sys/mman.h>
  28 #include <not-cancel.h>
  29 #include <local-setxid.h>
  30 #include <shlib-compat.h>
  31 #include <nptl/pthreadP.h>
  32 #include <dl-sysdep.h>
  33 #include <ldsodefs.h>
  34 #include <libc-internal.h>
  35 #include "spawn_int.h"
  36
  37 /* The Linux implementation of posix_spawn{p} uses the clone syscall directly
  38    with CLONE_VM and CLONE_VFORK flags and an allocated stack.  The new stack
  39    and start function solves most the vfork limitation (possible parent
  40    clobber due stack spilling). The remaining issue are:
  41
  42    1. That no signal handlers must run in child context, to avoid corrupting
  43       parent's state.
  44    2. The parent must ensure child's stack freeing.
  45    3. Child must synchronize with parent to enforce 2. and to possible
  46       return execv issues.
  47
  48    The first issue is solved by blocking all signals in child, even
  49    the NPTL-internal ones (SIGCANCEL and SIGSETXID).  The second and
  50    third issue is done by a stack allocation in parent, and by using a
  51    field in struct spawn_args where the child can write an error
  52    code. CLONE_VFORK ensures that the parent does not run until the
  53    child has either exec'ed successfully or exited.  */
  54
  55
  56 /* The Unix standard contains a long explanation of the way to signal
  57    an error after the fork() was successful.  Since no new wait status
  58    was wanted there is no way to signal an error using one of the
  59    available methods.  The committee chose to signal an error by a
  60    normal program exit with the exit code 127.  */
  61 #define SPAWN_ERROR     127
  62
  63 #ifdef __ia64__
  64 # define CLONE(__fn, __stack, __stacksize, __flags, __args) \
  65   __clone2 (__fn, __stack, __stacksize, __flags, __args, 0, 0, 0)
  66 #else
  67 # define CLONE(__fn, __stack, __stacksize, __flags, __args) \
  68   __clone (__fn, __stack, __flags, __args)
  69 #endif
  70
  71 #if _STACK_GROWS_DOWN
  72 # define STACK(__stack, __stack_size) (__stack + __stack_size)
  73 #elif _STACK_GROWS_UP
  74 # define STACK(__stack, __stack_size) (__stack)
  75 #endif
  76
  77
  78 struct posix_spawn_args
  79 {
  80   sigset_t oldmask;
  81   const char *file;
  82   int (*exec) (const char *, char *const *, char *const *);
  83   const posix_spawn_file_actions_t *fa;
  84   const posix_spawnattr_t *restrict attr;
  85   char *const *argv;
  86   ptrdiff_t argc;
  87   char *const *envp;
  88   int xflags;
  89   int err;
  90 };
  91
  92 /* Older version requires that shell script without shebang definition
  93    to be called explicitly using /bin/sh (_PATH_BSHELL).  */
  94 static void
  95 maybe_script_execute (struct posix_spawn_args *args)
  96 {
  97   if (SHLIB_COMPAT (libc, GLIBC_2_2, GLIBC_2_15)
  98       && (args->xflags & SPAWN_XFLAGS_TRY_SHELL) && errno == ENOEXEC)
  99     {
 100       char *const *argv = args->argv;
 101       ptrdiff_t argc = args->argc;
 102
 103       /* Construct an argument list for the shell.  */
 104       char *new_argv[argc + 1];
 105       new_argv[0] = (char *) _PATH_BSHELL;
 106       new_argv[1] = (char *) args->file;
 107       if (argc > 1)
 108         memcpy (new_argv + 2, argv + 1, argc * sizeof(char *));
 109       else
 110         new_argv[2] = NULL;
 111
 112       /* Execute the shell.  */
 113       args->exec (new_argv[0], new_argv, args->envp);
 114     }
 115 }
 116
 117 /* Function used in the clone call to setup the signals mask, posix_spawn
 118    attributes, and file actions.  It run on its own stack (provided by the
 119    posix_spawn call).  */
 120 static int
 121 __spawni_child (void *arguments)
 122 {
 123   struct posix_spawn_args *args = arguments;
 124   const posix_spawnattr_t *restrict attr = args->attr;
 125   const posix_spawn_file_actions_t *file_actions = args->fa;
 126   int ret;
 127
 128   /* The child must ensure that no signal handler are enabled because it shared
 129      memory with parent, so the signal disposition must be either SIG_DFL or
 130      SIG_IGN.  It does by iterating over all signals and although it could
 131      possibly be more optimized (by tracking which signal potentially have a
 132      signal handler), it might requires system specific solutions (since the
 133      sigset_t data type can be very different on different architectures).  */
 134   struct sigaction sa;
 135   memset (&sa, '\0', sizeof (sa));
 136
 137   sigset_t hset;
 138   __sigprocmask (SIG_BLOCK, 0, &hset);
 139   for (int sig = 1; sig < _NSIG; ++sig)
 140     {
 141       if ((attr->__flags & POSIX_SPAWN_SETSIGDEF)
 142           && sigismember (&attr->__sd, sig))
 143         {
 144           sa.sa_handler = SIG_DFL;
 145         }
 146       else if (sigismember (&hset, sig))
 147         {
 148           if (__nptl_is_internal_signal (sig))
 149             sa.sa_handler = SIG_IGN;
 150           else
 151             {
 152               __libc_sigaction (sig, 0, &sa);
 153               if (sa.sa_handler == SIG_IGN)
 154                 continue;
 155               sa.sa_handler = SIG_DFL;
 156             }
 157         }
 158       else
 159         continue;
 160
 161       __libc_sigaction (sig, &sa, 0);
 162     }
 163
 164 #ifdef _POSIX_PRIORITY_SCHEDULING
 165   /* Set the scheduling algorithm and parameters.  */
 166   if ((attr->__flags & (POSIX_SPAWN_SETSCHEDPARAM | POSIX_SPAWN_SETSCHEDULER))
 167       == POSIX_SPAWN_SETSCHEDPARAM)
 168     {
 169       if ((ret = __sched_setparam (0, &attr->__sp)) == -1)
 170         goto fail;
 171     }
 172   else if ((attr->__flags & POSIX_SPAWN_SETSCHEDULER) != 0)
 173     {
 174       if ((ret = __sched_setscheduler (0, attr->__policy, &attr->__sp)) == -1)
 175         goto fail;
 176     }
 177 #endif
 178
 179   /* Set the process group ID.  */
 180   if ((attr->__flags & POSIX_SPAWN_SETPGROUP) != 0
 181       && (ret = __setpgid (0, attr->__pgrp)) != 0)
 182     goto fail;
 183
 184   /* Set the effective user and group IDs.  */
 185   if ((attr->__flags & POSIX_SPAWN_RESETIDS) != 0
 186       && ((ret = local_seteuid (__getuid ())) != 0
 187           || (ret = local_setegid (__getgid ())) != 0))
 188     goto fail;
 189
 190   /* Execute the file actions.  */
 191   if (file_actions != 0)
 192     {
 193       int cnt;
 194       struct rlimit64 fdlimit;
 195       bool have_fdlimit = false;
 196
 197       for (cnt = 0; cnt < file_actions->__used; ++cnt)
 198         {
 199           struct __spawn_action *action = &file_actions->__actions[cnt];
 200
 201           switch (action->tag)
 202             {
 203             case spawn_do_close:
 204               if ((ret =
 205                    close_not_cancel (action->action.close_action.fd)) != 0)
 206                 {
 207                   if (!have_fdlimit)
 208                     {
 209                       __getrlimit64 (RLIMIT_NOFILE, &fdlimit);
 210                       have_fdlimit = true;
 211                     }
 212
 213                   /* Signal errors only for file descriptors out of range.  */
 214                   if (action->action.close_action.fd < 0
 215                       || action->action.close_action.fd >= fdlimit.rlim_cur)
 216                     goto fail;
 217                 }
 218               break;
 219
 220             case spawn_do_open:
 221               {
 222                 /* POSIX states that if fildes was already an open file descriptor,
 223                    it shall be closed before the new file is opened.  This avoid
 224                    pontential issues when posix_spawn plus addopen action is called
 225                    with the process already at maximum number of file descriptor
 226                    opened and also for multiple actions on single-open special
 227                    paths (like /dev/watchdog).  */
 228                 close_not_cancel (action->action.open_action.fd);
 229
 230                 ret = open_not_cancel (action->action.open_action.path,
 231                                        action->action.
 232                                        open_action.oflag | O_LARGEFILE,
 233                                        action->action.open_action.mode);
 234
 235                 if (ret == -1)
 236                   goto fail;
 237
 238                 int new_fd = ret;
 239
 240                 /* Make sure the desired file descriptor is used.  */
 241                 if (ret != action->action.open_action.fd)
 242                   {
 243                     if ((ret = __dup2 (new_fd, action->action.open_action.fd))
 244                         != action->action.open_action.fd)
 245                       goto fail;
 246
 247                     if ((ret = close_not_cancel (new_fd)) != 0)
 248                       goto fail;
 249                   }
 250               }
 251               break;
 252
 253             case spawn_do_dup2:
 254               if ((ret = __dup2 (action->action.dup2_action.fd,
 255                                  action->action.dup2_action.newfd))
 256                   != action->action.dup2_action.newfd)
 257                 goto fail;
 258               break;
 259             }
 260         }
 261     }
 262
 263   /* Set the initial signal mask of the child if POSIX_SPAWN_SETSIGMASK
 264      is set, otherwise restore the previous one.  */
 265   __sigprocmask (SIG_SETMASK, (attr->__flags & POSIX_SPAWN_SETSIGMASK)
 266                  ? &attr->__ss : &args->oldmask, 0);
 267
 268   args->err = 0;
 269   args->exec (args->file, args->argv, args->envp);
 270
 271   /* This is compatibility function required to enable posix_spawn run
 272      script without shebang definition for older posix_spawn versions
 273      (2.15).  */
 274   maybe_script_execute (args);
 275
 276 fail:
 277   /* errno should have an appropriate non-zero value; otherwise,
 278      there's a bug in glibc or the kernel.  For lack of an error code
 279      (EINTERNALBUG) describing that, use ECHILD.  Another option would
 280      be to set args->err to some negative sentinel and have the parent
 281      abort(), but that seems needlessly harsh.  */
 282   args->err = errno ? : ECHILD;
 283   _exit (SPAWN_ERROR);
 284 }
 285
 286 /* Spawn a new process executing PATH with the attributes describes in *ATTRP.
 287    Before running the process perform the actions described in FILE-ACTIONS. */
 288 static int
 289 __spawnix (pid_t * pid, const char *file,
 290            const posix_spawn_file_actions_t * file_actions,
 291            const posix_spawnattr_t * attrp, char *const argv[],
 292            char *const envp[], int xflags,
 293            int (*exec) (const char *, char *const *, char *const *))
 294 {
 295   pid_t new_pid;
 296   struct posix_spawn_args args;
 297   int ec;
 298
 299   /* To avoid imposing hard limits on posix_spawn{p} the total number of
 300      arguments is first calculated to allocate a mmap to hold all possible
 301      values.  */
 302   ptrdiff_t argc = 0;
 303   /* Linux allows at most max (0x7FFFFFFF, 1/4 stack size) arguments
 304      to be used in a execve call.  We limit to INT_MAX minus one due the
 305      compatiblity code that may execute a shell script (maybe_script_execute)
 306      where it will construct another argument list with an additional
 307      argument.  */
 308   ptrdiff_t limit = INT_MAX - 1;
 309   while (argv[argc++] != NULL)
 310     if (argc == limit)
 311       {
 312         errno = E2BIG;
 313         return errno;
 314       }
 315
 316   int prot = (PROT_READ | PROT_WRITE
 317              | ((GL (dl_stack_flags) & PF_X) ? PROT_EXEC : 0));
 318
 319   /* Add a slack area for child's stack.  */
 320   size_t argv_size = (argc * sizeof (void *)) + 512;
 321   size_t stack_size = ALIGN_UP (argv_size, GLRO(dl_pagesize));
 322   void *stack = __mmap (NULL, stack_size, prot,
 323                         MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK, -1, 0);
 324   if (__glibc_unlikely (stack == MAP_FAILED))
 325     return errno;
 326
 327   /* Disable asynchronous cancellation.  */
 328   int state;
 329   __libc_ptf_call (__pthread_setcancelstate,
 330                    (PTHREAD_CANCEL_DISABLE, &state), 0);
 331
 332   /* Child must set args.err to something non-negative - we rely on
 333      the parent and child sharing VM.  */
 334   args.err = -1;
 335   args.file = file;
 336   args.exec = exec;
 337   args.fa = file_actions;
 338   args.attr = attrp ? attrp : &(const posix_spawnattr_t) { 0 };
 339   args.argv = argv;
 340   args.argc = argc;
 341   args.envp = envp;
 342   args.xflags = xflags;
 343
 344   __libc_signal_block_all (&args.oldmask);
 345
 346   /* The clone flags used will create a new child that will run in the same
 347      memory space (CLONE_VM) and the execution of calling thread will be
 348      suspend until the child calls execve or _exit.
 349
 350      Also since the calling thread execution will be suspend, there is not
 351      need for CLONE_SETTLS.  Although parent and child share the same TLS
 352      namespace, there will be no concurrent access for TLS variables (errno
 353      for instance).  */
 354   new_pid = CLONE (__spawni_child, STACK (stack, stack_size), stack_size,
 355                    CLONE_VM | CLONE_VFORK | SIGCHLD, &args);
 356
 357   if (new_pid > 0)
 358     {
 359       ec = args.err;
 360       assert (ec >= 0);
 361       if (ec != 0)
 362           __waitpid (new_pid, NULL, 0);
 363     }
 364   else
 365     ec = -new_pid;
 366
 367   __munmap (stack, stack_size);
 368
 369   if ((ec == 0) && (pid != NULL))
 370     *pid = new_pid;
 371
 372   __libc_signal_restore_set (&args.oldmask);
 373
 374   __libc_ptf_call (__pthread_setcancelstate, (state, NULL), 0);
 375
 376   return ec;
 377 }
 378
 379 /* Spawn a new process executing PATH with the attributes describes in *ATTRP.
 380    Before running the process perform the actions described in FILE-ACTIONS. */
 381 int
 382 __spawni (pid_t * pid, const char *file,
 383           const posix_spawn_file_actions_t * acts,
 384           const posix_spawnattr_t * attrp, char *const argv[],
 385           char *const envp[], int xflags)
 386 {
 387   return __spawnix (pid, file, acts, attrp, argv, envp, xflags,
 388                     xflags & SPAWN_XFLAGS_USE_PATH ? __execvpe : __execve);
 389 }