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/* readlinkat wrapper to return the link name in malloc'd storage.
Unlike xreadlinkat, only call exit on failure to change directory.
Copyright (C) 2001, 2003-2007, 2009-2022 Free Software Foundation, Inc.
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <https://www.gnu.org/licenses/>. */
/* Written by Jim Meyering <jim@meyering.net>
and Eric Blake <ebb9@byu.net>. */
#include <config.h>
#include "areadlink.h"
#include <errno.h>
#include <limits.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#if HAVE_READLINKAT
# ifndef SSIZE_MAX
# define SSIZE_MAX ((ssize_t) (SIZE_MAX / 2))
# endif
/* SYMLINK_MAX is used only for an initial memory-allocation sanity
check, so it's OK to guess too small on hosts where there is no
arbitrary limit to symbolic link length. */
# ifndef SYMLINK_MAX
# define SYMLINK_MAX 1024
# endif
# define MAXSIZE (SIZE_MAX < SSIZE_MAX ? SIZE_MAX : SSIZE_MAX)
/* Call readlinkat to get the symbolic link value of FILE, relative to FD.
SIZE is a hint as to how long the link is expected to be;
typically it is taken from st_size. It need not be correct.
Return a pointer to that NUL-terminated string in malloc'd storage.
If readlinkat fails, malloc fails, or if the link value is longer
than SSIZE_MAX, return NULL (caller may use errno to diagnose).
However, failure to change directory during readlinkat will issue
a diagnostic and exit. */
char *
areadlinkat_with_size (int fd, char const *file, size_t size)
{
/* Some buggy file systems report garbage in st_size. Defend
against them by ignoring outlandish st_size values in the initial
memory allocation. */
size_t symlink_max = SYMLINK_MAX;
size_t INITIAL_LIMIT_BOUND = 8 * 1024;
size_t initial_limit = (symlink_max < INITIAL_LIMIT_BOUND
? symlink_max + 1
: INITIAL_LIMIT_BOUND);
enum { stackbuf_size = 128 };
/* The initial buffer size for the link value. */
size_t buf_size = (size == 0 ? stackbuf_size
: size < initial_limit ? size + 1 : initial_limit);
while (1)
{
ssize_t r;
size_t link_length;
char stackbuf[stackbuf_size];
char *buf = stackbuf;
char *buffer = NULL;
if (! (size == 0 && buf_size == stackbuf_size))
{
buf = buffer = malloc (buf_size);
if (!buffer)
/* We can assume errno == ENOMEM here, since all platforms that have
readlinkat() have a POSIX compliant malloc(). */
return NULL;
}
r = readlinkat (fd, file, buf, buf_size);
link_length = r;
if (r < 0)
{
free (buffer);
return NULL;
}
if (link_length < buf_size)
{
buf[link_length] = 0;
if (!buffer)
{
buffer = malloc (link_length + 1);
if (buffer)
return memcpy (buffer, buf, link_length + 1);
}
else if (link_length + 1 < buf_size)
{
/* Shrink BUFFER before returning it. */
char *shrinked_buffer = realloc (buffer, link_length + 1);
if (shrinked_buffer != NULL)
buffer = shrinked_buffer;
}
return buffer;
}
free (buffer);
if (buf_size <= MAXSIZE / 2)
buf_size *= 2;
else if (buf_size < MAXSIZE)
buf_size = MAXSIZE;
else
{
errno = ENOMEM;
return NULL;
}
}
}
#else /* !HAVE_READLINKAT */
/* It is more efficient to change directories only once and call
areadlink_with_size, rather than repeatedly call the replacement
readlinkat. */
# define AT_FUNC_NAME areadlinkat_with_size
# define AT_FUNC_F1 areadlink_with_size
# define AT_FUNC_POST_FILE_PARAM_DECLS , size_t size
# define AT_FUNC_POST_FILE_ARGS , size
# define AT_FUNC_RESULT char *
# define AT_FUNC_FAIL NULL
# include "at-func.c"
# undef AT_FUNC_NAME
# undef AT_FUNC_F1
# undef AT_FUNC_POST_FILE_PARAM_DECLS
# undef AT_FUNC_POST_FILE_ARGS
# undef AT_FUNC_RESULT
# undef AT_FUNC_FAIL
#endif /* !HAVE_READLINKAT */
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