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