maint/gnulib/lib/careadlinkat.c

/* */
This source file includes following definitions.
readlink_stk
careadlinkat
   1 /* Read symbolic links into a buffer without size limitation, relative to fd.
   2 
   3    Copyright (C) 2001, 2003-2004, 2007, 2009-2021 Free Software Foundation,
   4    Inc.
   5 
   6    This file is free software: you can redistribute it and/or modify
   7    it under the terms of the GNU Lesser General Public License as
   8    published by the Free Software Foundation; either version 2.1 of the
   9    License, or (at your option) any later version.
  10 
  11    This file 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 Lesser General Public License for more details.
  15 
  16    You should have received a copy of the GNU Lesser 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 "idx.h"
  26 #include "minmax.h"
  27 
  28 #include <errno.h>
  29 #include <limits.h>
  30 #include <string.h>
  31 #include <unistd.h>
  32 
  33 /* Define this independently so that stdint.h is not a prerequisite.  */
  34 #ifndef SIZE_MAX
  35 # define SIZE_MAX ((size_t) -1)
  36 #endif
  37 
  38 #ifndef SSIZE_MAX
  39 # define SSIZE_MAX ((ssize_t) (SIZE_MAX / 2))
  40 #endif
  41 
  42 #include "allocator.h"
  43 
  44 enum { STACK_BUF_SIZE = 1024 };
  45 
  46 /* Act like careadlinkat (see below), with an additional argument
  47    STACK_BUF that can be used as temporary storage.
  48 
  49    If GCC_LINT is defined, do not inline this function with GCC 10.1
  50    and later, to avoid creating a pointer to the stack that GCC
  51    -Wreturn-local-addr incorrectly complains about.  See:
  52    https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93644
  53    Although the noinline attribute can hurt performance a bit, no better way
  54    to pacify GCC is known; even an explicit #pragma does not pacify GCC.
  55    When the GCC bug is fixed this workaround should be limited to the
  56    broken GCC versions.  */
  57 #if _GL_GNUC_PREREQ (10, 1)
  58 # if defined GCC_LINT || defined lint
  59 __attribute__ ((__noinline__))
  60 # elif __OPTIMIZE__ && !__NO_INLINE__
  61 #  define GCC_BOGUS_WRETURN_LOCAL_ADDR
  62 # endif
  63 #endif
  64 static char *
  65 readlink_stk (int fd, char const *filename,
     /*  */
  66               char *buffer, size_t buffer_size,
  67               struct allocator const *alloc,
  68               ssize_t (*preadlinkat) (int, char const *, char *, size_t),
  69               char stack_buf[STACK_BUF_SIZE])
  70 {
  71   if (! alloc)
  72     alloc = &stdlib_allocator;
  73 
  74   if (!buffer)
  75     {
  76       buffer = stack_buf;
  77       buffer_size = STACK_BUF_SIZE;
  78     }
  79 
  80   char *buf = buffer;
  81   idx_t buf_size_max = MIN (IDX_MAX, MIN (SSIZE_MAX, SIZE_MAX));
  82   idx_t buf_size = MIN (buffer_size, buf_size_max);
  83 
  84   while (buf)
  85     {
  86       /* Attempt to read the link into the current buffer.  */
  87       idx_t link_length = preadlinkat (fd, filename, buf, buf_size);
  88       if (link_length < 0)
  89         {
  90           if (buf != buffer)
  91             {
  92               int readlinkat_errno = errno;
  93               alloc->free (buf);
  94               errno = readlinkat_errno;
  95             }
  96           return NULL;
  97         }
  98 
  99       idx_t link_size = link_length;
 100 
 101       if (link_size < buf_size)
 102         {
 103           buf[link_size++] = '\0';
 104 
 105           if (buf == stack_buf)
 106             {
 107               char *b = alloc->allocate (link_size);
 108               buf_size = link_size;
 109               if (! b)
 110                 break;
 111               return memcpy (b, buf, link_size);
 112             }
 113 
 114           if (link_size < buf_size && buf != buffer && alloc->reallocate)
 115             {
 116               /* Shrink BUF before returning it.  */
 117               char *b = alloc->reallocate (buf, link_size);
 118               if (b)
 119                 return b;
 120             }
 121 
 122           return buf;
 123         }
 124 
 125       if (buf != buffer)
 126         alloc->free (buf);
 127 
 128       if (buf_size_max / 2 <= buf_size)
 129         {
 130           errno = ENAMETOOLONG;
 131           return NULL;
 132         }
 133 
 134       buf_size = 2 * buf_size + 1;
 135       buf = alloc->allocate (buf_size);
 136     }
 137 
 138   if (alloc->die)
 139     alloc->die (buf_size);
 140   errno = ENOMEM;
 141   return NULL;
 142 }
 143 
 144 
 145 /* Assuming the current directory is FD, get the symbolic link value
 146    of FILENAME as a null-terminated string and put it into a buffer.
 147    If FD is AT_FDCWD, FILENAME is interpreted relative to the current
 148    working directory, as in openat.
 149 
 150    If the link is small enough to fit into BUFFER put it there.
 151    BUFFER's size is BUFFER_SIZE, and BUFFER can be null
 152    if BUFFER_SIZE is zero.
 153 
 154    If the link is not small, put it into a dynamically allocated
 155    buffer managed by ALLOC.  It is the caller's responsibility to free
 156    the returned value if it is nonnull and is not BUFFER.  A null
 157    ALLOC stands for the standard allocator.
 158 
 159    The PREADLINKAT function specifies how to read links.  It operates
 160    like POSIX readlinkat()
 161    <https://pubs.opengroup.org/onlinepubs/9699919799/functions/readlink.html>
 162    but can assume that its first argument is the same as FD.
 163 
 164    If successful, return the buffer address; otherwise return NULL and
 165    set errno.  */
 166 
 167 char *
 168 careadlinkat (int fd, char const *filename,
     /*  */
 169               char *buffer, size_t buffer_size,
 170               struct allocator const *alloc,
 171               ssize_t (*preadlinkat) (int, char const *, char *, size_t))
 172 {
 173   /* Allocate the initial buffer on the stack.  This way, in the
 174      common case of a symlink of small size, we get away with a
 175      single small malloc instead of a big malloc followed by a
 176      shrinking realloc.  */
 177   #ifdef GCC_BOGUS_WRETURN_LOCAL_ADDR
 178    #warning "GCC might issue a bogus -Wreturn-local-addr warning here."
 179    #warning "See <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93644>."
 180   #endif
 181   char stack_buf[STACK_BUF_SIZE];
 182   return readlink_stk (fd, filename, buffer, buffer_size, alloc,
 183                        preadlinkat, stack_buf);
 184 }
/* */
root/maint/gnulib/lib/careadlinkat.c

DEFINITIONS
