lib/openat-proc.c

   1 /* Create /proc/self/fd-related names for subfiles of open directories.
   2
   3    Copyright (C) 2006, 2009-2014 Free Software Foundation, Inc.
   4
   5    This program is free software: you can redistribute it and/or modify
   6    it under the terms of the GNU General Public License as published by
   7    the Free Software Foundation; either version 3 of the License, or
   8    (at your option) any later version.
   9
  10    This program 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
  13    GNU General Public License for more details.
  14
  15    You should have received a copy of the GNU General Public License
  16    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
  17
  18 /* Written by Paul Eggert.  */
  19
  20 #include <config.h>
  21
  22 #include "openat-priv.h"
  23
  24 #include <sys/types.h>
  25 #include <sys/stat.h>
  26 #include <fcntl.h>
  27
  28 #include <stdio.h>
  29 #include <stdlib.h>
  30 #include <string.h>
  31 #include <unistd.h>
  32
  33 #include "intprops.h"
  34
  35 /* The results of open() in this file are not used with fchdir,
  36    and we do not leak fds to any single-threaded code that could use stdio,
  37    therefore save some unnecessary work in fchdir.c.
  38    FIXME - if the kernel ever adds support for multi-thread safety for
  39    avoiding standard fds, then we should use open_safer.  */
  40 #undef open
  41 #undef close
  42
  43 #define PROC_SELF_FD_FORMAT "/proc/self/fd/%d/%s"
  44
  45 #define PROC_SELF_FD_NAME_SIZE_BOUND(len) \
  46   (sizeof PROC_SELF_FD_FORMAT - sizeof "%d%s" \
  47    + INT_STRLEN_BOUND (int) + (len) + 1)
  48
  49
  50 /* Set BUF to the expansion of PROC_SELF_FD_FORMAT, using FD and FILE
  51    respectively for %d and %s.  If successful, return BUF if the
  52    result fits in BUF, dynamically allocated memory otherwise.  But
  53    return NULL if /proc is not reliable, either because the operating
  54    system support is lacking or because memory is low.  */
  55 char *
  56 openat_proc_name (char buf[OPENAT_BUFFER_SIZE], int fd, char const *file)
  57 {
  58   static int proc_status = 0;
  59
  60   /* Make sure the caller gets ENOENT when appropriate.  */
  61   if (!*file)
  62     {
  63       buf[0] = '\0';
  64       return buf;
  65     }
  66
  67   if (! proc_status)
  68     {
  69       /* Set PROC_STATUS to a positive value if /proc/self/fd is
  70          reliable, and a negative value otherwise.  Solaris 10
  71          /proc/self/fd mishandles "..", and any file name might expand
  72          to ".." after symbolic link expansion, so avoid /proc/self/fd
  73          if it mishandles "..".  Solaris 10 has openat, but this
  74          problem is exhibited on code that built on Solaris 8 and
  75          running on Solaris 10.  */
  76
  77       int proc_self_fd = open ("/proc/self/fd",
  78                                O_SEARCH | O_DIRECTORY | O_NOCTTY | O_NONBLOCK);
  79       if (proc_self_fd < 0)
  80         proc_status = -1;
  81       else
  82         {
  83           /* Detect whether /proc/self/fd/%i/../fd exists, where %i is the
  84              number of a file descriptor open on /proc/self/fd.  On Linux,
  85              that name resolves to /proc/self/fd, which was opened above.
  86              However, on Solaris, it may resolve to /proc/self/fd/fd, which
  87              cannot exist, since all names in /proc/self/fd are numeric.  */
  88           char dotdot_buf[PROC_SELF_FD_NAME_SIZE_BOUND (sizeof "../fd" - 1)];
  89           sprintf (dotdot_buf, PROC_SELF_FD_FORMAT, proc_self_fd, "../fd");
  90           proc_status = access (dotdot_buf, F_OK) ? -1 : 1;
  91           close (proc_self_fd);
  92         }
  93     }
  94
  95   if (proc_status < 0)
  96     return NULL;
  97   else
  98     {
  99       size_t bufsize = PROC_SELF_FD_NAME_SIZE_BOUND (strlen (file));
 100       char *result = buf;
 101       if (OPENAT_BUFFER_SIZE < bufsize)
 102         {
 103           result = malloc (bufsize);
 104           if (! result)
 105             return NULL;
 106         }
 107       sprintf (result, PROC_SELF_FD_FORMAT, fd, file);
 108       return result;
 109     }
 110 }