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 enum { STACK_BUF_SIZE = 1024 };
  42
  43 /* Act like careadlinkat (see below), with an additional argument
  44    STACK_BUF that can be used as temporary storage.
  45
  46    If GCC_LINT is defined, do not inline this function with GCC 10.1
  47    and later, to avoid creating a pointer to the stack that GCC
  48    -Wreturn-local-addr incorrectly complains about.  See:
  49    https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93644
  50    Although the noinline attribute can hurt performance a bit, no better way
  51    to pacify GCC is known; even an explicit #pragma does not pacify GCC.
  52    When the GCC bug is fixed this workaround should be limited to the
  53    broken GCC versions.  */
  54 #if _GL_GNUC_PREREQ (10, 1)
  55 # if defined GCC_LINT || defined lint
  56 __attribute__ ((__noinline__))
  57 # elif __OPTIMIZE__ && !__NO_INLINE__
  58 #  warning "GCC might issue a bogus -Wreturn-local-addr warning here."
  59 #  warning "See <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93644>."
  60 # endif
  61 #endif
  62 static char *
  63 readlink_stk (int fd, char const *filename,
  64               char *buffer, size_t buffer_size,
  65               struct allocator const *alloc,
  66               ssize_t (*preadlinkat) (int, char const *, char *, size_t),
  67               char stack_buf[STACK_BUF_SIZE])
  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
  74   if (! alloc)
  75     alloc = &stdlib_allocator;
  76
  77   if (!buffer)
  78     {
  79       buffer = stack_buf;
  80       buffer_size = STACK_BUF_SIZE;
  81     }
  82
  83   buf = buffer;
  84   buf_size = buffer_size;
  85
  86   while (buf)
  87     {
  88       /* Attempt to read the link into the current buffer.  */
  89       ssize_t link_length = preadlinkat (fd, filename, buf, buf_size);
  90       size_t link_size;
  91       if (link_length < 0)
  92         {
  93           /* On AIX 5L v5.3 and HP-UX 11i v2 04/09, readlink returns -1
  94              with errno == ERANGE if the buffer is too small.  */
  95           int readlinkat_errno = errno;
  96           if (readlinkat_errno != ERANGE)
  97             {
  98               if (buf != buffer)
  99                 {
 100                   alloc->free (buf);
 101                   errno = readlinkat_errno;
 102                 }
 103               return NULL;
 104             }
 105         }
 106
 107       link_size = link_length;
 108
 109       if (link_size < buf_size)
 110         {
 111           buf[link_size++] = '\0';
 112
 113           if (buf == stack_buf)
 114             {
 115               char *b = alloc->allocate (link_size);
 116               buf_size = link_size;
 117               if (! b)
 118                 break;
 119               return memcpy (b, buf, link_size);
 120             }
 121
 122           if (link_size < buf_size && buf != buffer && alloc->reallocate)
 123             {
 124               /* Shrink BUF before returning it.  */
 125               char *b = alloc->reallocate (buf, link_size);
 126               if (b)
 127                 return b;
 128             }
 129
 130           return buf;
 131         }
 132
 133       if (buf != buffer)
 134         alloc->free (buf);
 135
 136       if (buf_size < buf_size_max / 2)
 137         buf_size = 2 * buf_size + 1;
 138       else if (buf_size < buf_size_max)
 139         buf_size = buf_size_max;
 140       else if (buf_size_max < SIZE_MAX)
 141         {
 142           errno = ENAMETOOLONG;
 143           return NULL;
 144         }
 145       else
 146         break;
 147       buf = alloc->allocate (buf_size);
 148     }
 149
 150   if (alloc->die)
 151     alloc->die (buf_size);
 152   errno = ENOMEM;
 153   return NULL;
 154 }
 155
 156
 157 /* Assuming the current directory is FD, get the symbolic link value
 158    of FILENAME as a null-terminated string and put it into a buffer.
 159    If FD is AT_FDCWD, FILENAME is interpreted relative to the current
 160    working directory, as in openat.
 161
 162    If the link is small enough to fit into BUFFER put it there.
 163    BUFFER's size is BUFFER_SIZE, and BUFFER can be null
 164    if BUFFER_SIZE is zero.
 165
 166    If the link is not small, put it into a dynamically allocated
 167    buffer managed by ALLOC.  It is the caller's responsibility to free
 168    the returned value if it is nonnull and is not BUFFER.  A null
 169    ALLOC stands for the standard allocator.
 170
 171    The PREADLINKAT function specifies how to read links.  It operates
 172    like POSIX readlinkat()
 173    <https://pubs.opengroup.org/onlinepubs/9699919799/functions/readlink.html>
 174    but can assume that its first argument is the same as FD.
 175
 176    If successful, return the buffer address; otherwise return NULL and
 177    set errno.  */
 178
 179 char *
 180 careadlinkat (int fd, char const *filename,
 181               char *buffer, size_t buffer_size,
 182               struct allocator const *alloc,
 183               ssize_t (*preadlinkat) (int, char const *, char *, size_t))
 184 {
 185   /* Allocate the initial buffer on the stack.  This way, in the
 186      common case of a symlink of small size, we get away with a
 187      single small malloc instead of a big malloc followed by a
 188      shrinking realloc.
 189
 190      If GCC -Wreturn-local-addr warns about this buffer, the warning
 191      is bogus; see readlink_stk.  */
 192   char stack_buf[STACK_BUF_SIZE];
 193   return readlink_stk (fd, filename, buffer, buffer_size, alloc,
 194                        preadlinkat, stack_buf);
 195 }