lib/openat-proc.c

   1 /* Create /proc/self/fd-related names for subfiles of open directories.
   2
   3    Copyright (C) 2006, 2009-2015 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 #define PROC_SELF_FD_FORMAT "/proc/self/fd/%d/%s"
  36
  37 #define PROC_SELF_FD_NAME_SIZE_BOUND(len) \
  38   (sizeof PROC_SELF_FD_FORMAT - sizeof "%d%s" \
  39    + INT_STRLEN_BOUND (int) + (len) + 1)
  40
  41
  42 /* Set BUF to the expansion of PROC_SELF_FD_FORMAT, using FD and FILE
  43    respectively for %d and %s.  If successful, return BUF if the
  44    result fits in BUF, dynamically allocated memory otherwise.  But
  45    return NULL if /proc is not reliable, either because the operating
  46    system support is lacking or because memory is low.  */
  47 char *
  48 openat_proc_name (char buf[OPENAT_BUFFER_SIZE], int fd, char const *file)
  49 {
  50   static int proc_status = 0;
  51
  52   /* Make sure the caller gets ENOENT when appropriate.  */
  53   if (!*file)
  54     {
  55       buf[0] = '\0';
  56       return buf;
  57     }
  58
  59   if (! proc_status)
  60     {
  61       /* Set PROC_STATUS to a positive value if /proc/self/fd is
  62          reliable, and a negative value otherwise.  Solaris 10
  63          /proc/self/fd mishandles "..", and any file name might expand
  64          to ".." after symbolic link expansion, so avoid /proc/self/fd
  65          if it mishandles "..".  Solaris 10 has openat, but this
  66          problem is exhibited on code that built on Solaris 8 and
  67          running on Solaris 10.  */
  68
  69       int proc_self_fd = open ("/proc/self/fd",
  70                                O_SEARCH | O_DIRECTORY | O_NOCTTY | O_NONBLOCK);
  71       if (proc_self_fd < 0)
  72         proc_status = -1;
  73       else
  74         {
  75           /* Detect whether /proc/self/fd/%i/../fd exists, where %i is the
  76              number of a file descriptor open on /proc/self/fd.  On Linux,
  77              that name resolves to /proc/self/fd, which was opened above.
  78              However, on Solaris, it may resolve to /proc/self/fd/fd, which
  79              cannot exist, since all names in /proc/self/fd are numeric.  */
  80           char dotdot_buf[PROC_SELF_FD_NAME_SIZE_BOUND (sizeof "../fd" - 1)];
  81           sprintf (dotdot_buf, PROC_SELF_FD_FORMAT, proc_self_fd, "../fd");
  82           proc_status = access (dotdot_buf, F_OK) ? -1 : 1;
  83           close (proc_self_fd);
  84         }
  85     }
  86
  87   if (proc_status < 0)
  88     return NULL;
  89   else
  90     {
  91       size_t bufsize = PROC_SELF_FD_NAME_SIZE_BOUND (strlen (file));
  92       char *result = buf;
  93       if (OPENAT_BUFFER_SIZE < bufsize)
  94         {
  95           result = malloc (bufsize);
  96           if (! result)
  97             return NULL;
  98         }
  99       sprintf (result, PROC_SELF_FD_FORMAT, fd, file);
 100       return result;
 101     }
 102 }