lib/openat.c

   1 /* provide a replacement openat function
   2    Copyright (C) 2004-2024 Free Software Foundation, Inc.
   3
   4    This program is free software: you can redistribute it and/or modify
   5    it under the terms of the GNU General Public License as published by
   6    the Free Software Foundation, either version 3 of the License, or
   7    (at your option) any later version.
   8
   9    This program is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12    GNU General Public License for more details.
  13
  14    You should have received a copy of the GNU General Public License
  15    along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
  16
  17 /* written by Jim Meyering */
  18
  19 /* If the user's config.h happens to include <fcntl.h>, let it include only
  20    the system's <fcntl.h> here, so that orig_openat doesn't recurse to
  21    rpl_openat.  */
  22 #define __need_system_fcntl_h
  23 #include <config.h>
  24
  25 /* Get the original definition of open.  It might be defined as a macro.  */
  26 #include <fcntl.h>
  27 #include <sys/types.h>
  28 #undef __need_system_fcntl_h
  29
  30 #if HAVE_OPENAT
  31 static int
  32 orig_openat (int fd, char const *filename, int flags, mode_t mode)
  33 {
  34   return openat (fd, filename, flags, mode);
  35 }
  36 #endif
  37
  38 #ifdef __osf__
  39 /* Write "fcntl.h" here, not <fcntl.h>, otherwise OSF/1 5.1 DTK cc eliminates
  40    this include because of the preliminary #include <fcntl.h> above.  */
  41 # include "fcntl.h"
  42 #else
  43 # include <fcntl.h>
  44 #endif
  45
  46 #include "openat.h"
  47
  48 #include "cloexec.h"
  49
  50 #include <stdarg.h>
  51 #include <stddef.h>
  52 #include <stdlib.h>
  53 #include <string.h>
  54 #include <sys/stat.h>
  55 #include <errno.h>
  56
  57 #if HAVE_OPENAT
  58
  59 /* Like openat, but support O_CLOEXEC and work around Solaris 9 bugs
  60    with trailing slash.  */
  61 int
  62 rpl_openat (int dfd, char const *filename, int flags, ...)
  63 {
  64   /* 0 = unknown, 1 = yes, -1 = no.  */
  65 #if GNULIB_defined_O_CLOEXEC
  66   int have_cloexec = -1;
  67 #else
  68   static int have_cloexec;
  69 #endif
  70
  71   mode_t mode;
  72   int fd;
  73
  74   mode = 0;
  75   if (flags & O_CREAT)
  76     {
  77       va_list arg;
  78       va_start (arg, flags);
  79
  80       /* We have to use PROMOTED_MODE_T instead of mode_t, otherwise GCC 4
  81          creates crashing code when 'mode_t' is smaller than 'int'.  */
  82       mode = va_arg (arg, PROMOTED_MODE_T);
  83
  84       va_end (arg);
  85     }
  86
  87 # if OPEN_TRAILING_SLASH_BUG
  88   /* Fail if one of O_CREAT, O_WRONLY, O_RDWR is specified and the filename
  89      ends in a slash, as POSIX says such a filename must name a directory
  90      <https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_13>:
  91        "A pathname that contains at least one non-<slash> character and that
  92         ends with one or more trailing <slash> characters shall not be resolved
  93         successfully unless the last pathname component before the trailing
  94         <slash> characters names an existing directory"
  95      If the named file already exists as a directory, then
  96        - if O_CREAT is specified, open() must fail because of the semantics
  97          of O_CREAT,
  98        - if O_WRONLY or O_RDWR is specified, open() must fail because POSIX
  99          <https://pubs.opengroup.org/onlinepubs/9699919799/functions/openat.html>
 100          says that it fails with errno = EISDIR in this case.
 101      If the named file does not exist or does not name a directory, then
 102        - if O_CREAT is specified, open() must fail since open() cannot create
 103          directories,
 104        - if O_WRONLY or O_RDWR is specified, open() must fail because the
 105          file does not contain a '.' directory.  */
 106   if ((flags & O_CREAT)
 107       || (flags & O_ACCMODE) == O_RDWR
 108       || (flags & O_ACCMODE) == O_WRONLY)
 109     {
 110       size_t len = strlen (filename);
 111       if (len > 0 && filename[len - 1] == '/')
 112         {
 113           errno = EISDIR;
 114           return -1;
 115         }
 116     }
 117 # endif
 118
 119   fd = orig_openat (dfd, filename,
 120                     flags & ~(have_cloexec < 0 ? O_CLOEXEC : 0), mode);
 121
 122   if (flags & O_CLOEXEC)
 123     {
 124       if (! have_cloexec)
 125         {
 126           if (0 <= fd)
 127             have_cloexec = 1;
 128           else if (errno == EINVAL)
 129             {
 130               fd = orig_openat (dfd, filename, flags & ~O_CLOEXEC, mode);
 131               have_cloexec = -1;
 132             }
 133         }
 134       if (have_cloexec < 0 && 0 <= fd)
 135         set_cloexec_flag (fd, true);
 136     }
 137
 138
 139 # if OPEN_TRAILING_SLASH_BUG
 140   /* If the filename ends in a slash and fd does not refer to a directory,
 141      then fail.
 142      Rationale: POSIX says such a filename must name a directory
 143      <https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_13>:
 144        "A pathname that contains at least one non-<slash> character and that
 145         ends with one or more trailing <slash> characters shall not be resolved
 146         successfully unless the last pathname component before the trailing
 147         <slash> characters names an existing directory"
 148      If the named file without the slash is not a directory, open() must fail
 149      with ENOTDIR.  */
 150   if (fd >= 0)
 151     {
 152       /* We know len is positive, since open did not fail with ENOENT.  */
 153       size_t len = strlen (filename);
 154       if (filename[len - 1] == '/')
 155         {
 156           struct stat statbuf;
 157
 158           if (fstat (fd, &statbuf) >= 0 && !S_ISDIR (statbuf.st_mode))
 159             {
 160               close (fd);
 161               errno = ENOTDIR;
 162               return -1;
 163             }
 164         }
 165     }
 166 # endif
 167
 168   return fd;
 169 }
 170
 171 #else /* !HAVE_OPENAT */
 172
 173 # include "filename.h" /* solely for definition of IS_ABSOLUTE_FILE_NAME */
 174 # include "openat-priv.h"
 175 # include "save-cwd.h"
 176
 177 /* Replacement for Solaris' openat function.
 178    <https://www.google.com/search?q=openat+site:docs.oracle.com>
 179    First, try to simulate it via open ("/proc/self/fd/FD/FILE").
 180    Failing that, simulate it by doing save_cwd/fchdir/open/restore_cwd.
 181    If either the save_cwd or the restore_cwd fails (relatively unlikely),
 182    then give a diagnostic and exit nonzero.
 183    Otherwise, upon failure, set errno and return -1, as openat does.
 184    Upon successful completion, return a file descriptor.  */
 185 int
 186 openat (int fd, char const *file, int flags, ...)
 187 {
 188   mode_t mode = 0;
 189
 190   if (flags & O_CREAT)
 191     {
 192       va_list arg;
 193       va_start (arg, flags);
 194
 195       /* We have to use PROMOTED_MODE_T instead of mode_t, otherwise GCC 4
 196          creates crashing code when 'mode_t' is smaller than 'int'.  */
 197       mode = va_arg (arg, PROMOTED_MODE_T);
 198
 199       va_end (arg);
 200     }
 201
 202   return openat_permissive (fd, file, flags, mode, NULL);
 203 }
 204
 205 /* Like openat (FD, FILE, FLAGS, MODE), but if CWD_ERRNO is
 206    nonnull, set *CWD_ERRNO to an errno value if unable to save
 207    or restore the initial working directory.  This is needed only
 208    the first time remove.c's remove_dir opens a command-line
 209    directory argument.
 210
 211    If a previous attempt to restore the current working directory
 212    failed, then we must not even try to access a '.'-relative name.
 213    It is the caller's responsibility not to call this function
 214    in that case.  */
 215
 216 int
 217 openat_permissive (int fd, char const *file, int flags, mode_t mode,
 218                    int *cwd_errno)
 219 {
 220   struct saved_cwd saved_cwd;
 221   int saved_errno;
 222   int err;
 223   bool save_ok;
 224
 225   if (fd == AT_FDCWD || IS_ABSOLUTE_FILE_NAME (file))
 226     return open (file, flags, mode);
 227
 228   {
 229     char buf[OPENAT_BUFFER_SIZE];
 230     char *proc_file = openat_proc_name (buf, fd, file);
 231     if (proc_file)
 232       {
 233         int open_result = open (proc_file, flags, mode);
 234         int open_errno = errno;
 235         if (proc_file != buf)
 236           free (proc_file);
 237         /* If the syscall succeeds, or if it fails with an unexpected
 238            errno value, then return right away.  Otherwise, fall through
 239            and resort to using save_cwd/restore_cwd.  */
 240         if (0 <= open_result || ! EXPECTED_ERRNO (open_errno))
 241           {
 242             errno = open_errno;
 243             return open_result;
 244           }
 245       }
 246   }
 247
 248   save_ok = (save_cwd (&saved_cwd) == 0);
 249   if (! save_ok)
 250     {
 251       if (! cwd_errno)
 252         openat_save_fail (errno);
 253       *cwd_errno = errno;
 254     }
 255   if (0 <= fd && fd == saved_cwd.desc)
 256     {
 257       /* If saving the working directory collides with the user's
 258          requested fd, then the user's fd must have been closed to
 259          begin with.  */
 260       free_cwd (&saved_cwd);
 261       errno = EBADF;
 262       return -1;
 263     }
 264
 265   err = fchdir (fd);
 266   saved_errno = errno;
 267
 268   if (! err)
 269     {
 270       err = open (file, flags, mode);
 271       saved_errno = errno;
 272       if (save_ok && restore_cwd (&saved_cwd) != 0)
 273         {
 274           if (! cwd_errno)
 275             {
 276               /* Don't write a message to just-created fd 2.  */
 277               saved_errno = errno;
 278               if (err == STDERR_FILENO)
 279                 close (err);
 280               openat_restore_fail (saved_errno);
 281             }
 282           *cwd_errno = errno;
 283         }
 284     }
 285
 286   free_cwd (&saved_cwd);
 287   errno = saved_errno;
 288   return err;
 289 }
 290
 291 /* Return true if our openat implementation must resort to
 292    using save_cwd and restore_cwd.  */
 293 bool
 294 openat_needs_fchdir (void)
 295 {
 296   bool needs_fchdir = true;
 297   int fd = open ("/", O_SEARCH | O_CLOEXEC);
 298
 299   if (0 <= fd)
 300     {
 301       char buf[OPENAT_BUFFER_SIZE];
 302       char *proc_file = openat_proc_name (buf, fd, ".");
 303       if (proc_file)
 304         {
 305           needs_fchdir = false;
 306           if (proc_file != buf)
 307             free (proc_file);
 308         }
 309       close (fd);
 310     }
 311
 312   return needs_fchdir;
 313 }
 314
 315 #endif /* !HAVE_OPENAT */