lib/careadlinkat.c

   1 /* Read symbolic links into a buffer without size limitation, relative to fd.
   2
   3    Copyright (C) 2001, 2003-2004, 2007, 2009-2020 Free Software Foundation,
   4    Inc.
   5
   6    This program is free software: you can redistribute it and/or modify
   7    it under the terms of the GNU General Public License as published by
   8    the Free Software Foundation; either version 3 of the License, or
   9    (at your option) any later version.
  10
  11    This program is distributed in the hope that it will be useful,
  12    but WITHOUT ANY WARRANTY; without even the implied warranty of
  13    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14    GNU General Public License for more details.
  15
  16    You should have received a copy of the GNU General Public License
  17    along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
  18
  19 /* Written by Paul Eggert, Bruno Haible, and Jim Meyering.  */
  20
  21 #include <config.h>
  22
  23 #include "careadlinkat.h"
  24
  25 #include <errno.h>
  26 #include <limits.h>
  27 #include <string.h>
  28 #include <unistd.h>
  29
  30 /* Define this independently so that stdint.h is not a prerequisite.  */
  31 #ifndef SIZE_MAX
  32 # define SIZE_MAX ((size_t) -1)
  33 #endif
  34
  35 #ifndef SSIZE_MAX
  36 # define SSIZE_MAX ((ssize_t) (SIZE_MAX / 2))
  37 #endif
  38
  39 #include "allocator.h"
  40
  41 /* Assuming the current directory is FD, get the symbolic link value
  42    of FILENAME as a null-terminated string and put it into a buffer.
  43    If FD is AT_FDCWD, FILENAME is interpreted relative to the current
  44    working directory, as in openat.
  45
  46    If the link is small enough to fit into BUFFER put it there.
  47    BUFFER's size is BUFFER_SIZE, and BUFFER can be null
  48    if BUFFER_SIZE is zero.
  49
  50    If the link is not small, put it into a dynamically allocated
  51    buffer managed by ALLOC.  It is the caller's responsibility to free
  52    the returned value if it is nonnull and is not BUFFER.  A null
  53    ALLOC stands for the standard allocator.
  54
  55    The PREADLINKAT function specifies how to read links.  It operates
  56    like POSIX readlinkat()
  57    <https://pubs.opengroup.org/onlinepubs/9699919799/functions/readlink.html>
  58    but can assume that its first argument is the same as FD.
  59
  60    If successful, return the buffer address; otherwise return NULL and
  61    set errno.  */
  62
  63 char *
  64 careadlinkat (int fd, char const *filename,
  65               char *buffer, size_t buffer_size,
  66               struct allocator const *alloc,
  67               ssize_t (*preadlinkat) (int, char const *, char *, size_t))
  68 {
  69   char *buf;
  70   size_t buf_size;
  71   size_t buf_size_max =
  72     SSIZE_MAX < SIZE_MAX ? (size_t) SSIZE_MAX + 1 : SIZE_MAX;
  73   char stack_buf[1024];
  74
  75 #if (defined GCC_LINT || defined lint) && _GL_GNUC_PREREQ (10, 1)
  76   /* Pacify preadlinkat without creating a pointer to the stack
  77      that a broken gcc -Wreturn-local-addr would cry wolf about.  See:
  78      https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95044
  79      This workaround differs from the mainline code, but
  80      no other way to pacify GCC 10.1.0 is known; even an explicit
  81      #pragma does not pacify GCC.  When the GCC bug is fixed this
  82      workaround should be limited to the broken GCC versions.  */
  83 # define WORK_AROUND_GCC_BUG_95044
  84 #endif
  85
  86   if (! alloc)
  87     alloc = &stdlib_allocator;
  88
  89   if (!buffer)
  90     {
  91 #ifdef WORK_AROUND_GCC_BUG_95044
  92       buffer = alloc->allocate (sizeof stack_buf);
  93 #else
  94       /* Allocate the initial buffer on the stack.  This way, in the
  95          common case of a symlink of small size, we get away with a
  96          single small malloc() instead of a big malloc() followed by a
  97          shrinking realloc().  */
  98       buffer = stack_buf;
  99 #endif
 100       buffer_size = sizeof stack_buf;
 101     }
 102
 103   buf = buffer;
 104   buf_size = buffer_size;
 105
 106   while (buf)
 107     {
 108       /* Attempt to read the link into the current buffer.  */
 109       ssize_t link_length = preadlinkat (fd, filename, buf, buf_size);
 110       size_t link_size;
 111       if (link_length < 0)
 112         {
 113           /* On AIX 5L v5.3 and HP-UX 11i v2 04/09, readlink returns -1
 114              with errno == ERANGE if the buffer is too small.  */
 115           int readlinkat_errno = errno;
 116           if (readlinkat_errno != ERANGE)
 117             {
 118               if (buf != buffer)
 119                 {
 120                   alloc->free (buf);
 121                   errno = readlinkat_errno;
 122                 }
 123               return NULL;
 124             }
 125         }
 126
 127       link_size = link_length;
 128
 129       if (link_size < buf_size)
 130         {
 131           buf[link_size++] = '\0';
 132
 133           if (buf == stack_buf)
 134             {
 135               char *b = alloc->allocate (link_size);
 136               buf_size = link_size;
 137               if (! b)
 138                 break;
 139               return memcpy (b, buf, link_size);
 140             }
 141
 142           if (link_size < buf_size && buf != buffer && alloc->reallocate)
 143             {
 144               /* Shrink BUF before returning it.  */
 145               char *b = alloc->reallocate (buf, link_size);
 146               if (b)
 147                 return b;
 148             }
 149
 150           return buf;
 151         }
 152
 153       if (buf != buffer)
 154         alloc->free (buf);
 155
 156       if (buf_size < buf_size_max / 2)
 157         buf_size = 2 * buf_size + 1;
 158       else if (buf_size < buf_size_max)
 159         buf_size = buf_size_max;
 160       else if (buf_size_max < SIZE_MAX)
 161         {
 162           errno = ENAMETOOLONG;
 163           return NULL;
 164         }
 165       else
 166         break;
 167       buf = alloc->allocate (buf_size);
 168     }
 169
 170   if (alloc->die)
 171     alloc->die (buf_size);
 172   errno = ENOMEM;
 173   return NULL;
 174 }