lib/openat-proc.c

   1 /* Create /proc/self/fd-related names for subfiles of open directories.
   2
   3    Copyright (C) 2006, 2009-2020 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 <https://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 #ifdef __KLIBC__
  34 # include <InnoTekLIBC/backend.h>
  35 #endif
  36
  37 #include "intprops.h"
  38
  39 /* Set BUF to the name of the subfile of the directory identified by
  40    FD, where the subfile is named FILE.  If successful, return BUF if
  41    the result fits in BUF, dynamically allocated memory otherwise.
  42    Return NULL (setting errno) on error.  */
  43 char *
  44 openat_proc_name (char buf[OPENAT_BUFFER_SIZE], int fd, char const *file)
  45 {
  46   char *result = buf;
  47   int dirlen;
  48
  49   /* Make sure the caller gets ENOENT when appropriate.  */
  50   if (!*file)
  51     {
  52       buf[0] = '\0';
  53       return buf;
  54     }
  55
  56 #ifndef __KLIBC__
  57 # define PROC_SELF_FD_FORMAT "/proc/self/fd/%d/"
  58   {
  59     enum {
  60       PROC_SELF_FD_DIR_SIZE_BOUND
  61         = (sizeof PROC_SELF_FD_FORMAT - (sizeof "%d" - 1)
  62            + INT_STRLEN_BOUND (int))
  63     };
  64
  65     static int proc_status = 0;
  66     if (! proc_status)
  67       {
  68         /* Set PROC_STATUS to a positive value if /proc/self/fd is
  69            reliable, and a negative value otherwise.  Solaris 10
  70            /proc/self/fd mishandles "..", and any file name might expand
  71            to ".." after symbolic link expansion, so avoid /proc/self/fd
  72            if it mishandles "..".  Solaris 10 has openat, but this
  73            problem is exhibited on code that built on Solaris 8 and
  74            running on Solaris 10.  */
  75
  76         int proc_self_fd =
  77           open ("/proc/self/fd",
  78                 O_SEARCH | O_DIRECTORY | O_NOCTTY | O_NONBLOCK | O_CLOEXEC);
  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_DIR_SIZE_BOUND + sizeof "../fd" - 1];
  89             sprintf (dotdot_buf, PROC_SELF_FD_FORMAT "../fd", proc_self_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_DIR_SIZE_BOUND + strlen (file);
 100         if (OPENAT_BUFFER_SIZE < bufsize)
 101           {
 102             result = malloc (bufsize);
 103             if (! result)
 104               return NULL;
 105           }
 106
 107         dirlen = sprintf (result, PROC_SELF_FD_FORMAT, fd);
 108       }
 109   }
 110 #else
 111   /* OS/2 kLIBC provides a function to retrieve a path from a fd.  */
 112   {
 113     char dir[_MAX_PATH];
 114     size_t bufsize;
 115
 116     if (__libc_Back_ioFHToPath (fd, dir, sizeof dir))
 117       return NULL;
 118
 119     dirlen = strlen (dir);
 120     bufsize = dirlen + 1 + strlen (file) + 1; /* 1 for '/', 1 for null */
 121     if (OPENAT_BUFFER_SIZE < bufsize)
 122       {
 123         result = malloc (bufsize);
 124         if (! result)
 125           return NULL;
 126       }
 127
 128     strcpy (result, dir);
 129     result[dirlen++] = '/';
 130   }
 131 #endif
 132
 133   strcpy (result + dirlen, file);
 134   return result;
 135 }