114 lines
3.6 KiB
C
114 lines
3.6 KiB
C
/* Read a symlink relative to an open directory.
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Copyright (C) 2009-2024 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 Eric Blake */
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#include <config.h>
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/* Specification. */
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#include <unistd.h>
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#include <errno.h>
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#include <stdlib.h>
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#include <string.h>
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#include <sys/stat.h>
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#if HAVE_READLINKAT
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# undef fstatat
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# undef readlinkat
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ssize_t
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rpl_readlinkat (int fd, char const *file, char *buf, size_t bufsize)
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{
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# if READLINK_TRAILING_SLASH_BUG
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size_t file_len = strlen (file);
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if (file_len && file[file_len - 1] == '/')
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{
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/* Even if FILE without the slash is a symlink to a directory,
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both lstat() and stat() must resolve the trailing slash to
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the directory rather than the symlink. We can therefore
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safely use fstatat(..., 0) to distinguish between EINVAL and
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ENOTDIR/ENOENT, avoiding extra overhead of rpl_fstatat(). */
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struct stat st;
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if (fstatat (fd, file, &st, 0) == 0 || errno == EOVERFLOW)
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errno = EINVAL;
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return -1;
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}
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# endif /* READLINK_TRAILING_SLASH_BUG */
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ssize_t r = readlinkat (fd, file, buf, bufsize);
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# if READLINK_TRUNCATE_BUG
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if (r < 0 && errno == ERANGE)
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{
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/* Try again with a bigger buffer. This is just for test cases;
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real code invariably discards short reads. */
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char stackbuf[4032];
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r = readlinkat (fd, file, stackbuf, sizeof stackbuf);
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if (r < 0)
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{
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if (errno == ERANGE)
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{
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/* Clear the buffer, which is good enough for real code.
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Thankfully, no test cases try short reads of enormous
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symlinks and what would be the point anyway? */
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r = bufsize;
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memset (buf, 0, r);
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}
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}
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else
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{
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if (bufsize < r)
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r = bufsize;
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memcpy (buf, stackbuf, r);
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}
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}
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# endif
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return r;
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}
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#else
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/* Gnulib provides a readlink stub for mingw; use it for distinction
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between EINVAL and ENOENT, rather than always failing with ENOSYS. */
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/* POSIX 2008 says that unlike readlink, readlinkat returns 0 for
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success instead of the buffer length. But this would render
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readlinkat worthless since readlink does not guarantee a
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NUL-terminated buffer. Assume this was a bug in POSIX. */
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/* Read the contents of symlink FILE into buffer BUF of size BUFSIZE, in the
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directory open on descriptor FD. If possible, do it without changing
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the working directory. Otherwise, resort to using save_cwd/fchdir,
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then readlink/restore_cwd. If either the save_cwd or the restore_cwd
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fails, then give a diagnostic and exit nonzero. */
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# define AT_FUNC_NAME readlinkat
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# define AT_FUNC_F1 readlink
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# define AT_FUNC_POST_FILE_PARAM_DECLS , char *buf, size_t bufsize
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# define AT_FUNC_POST_FILE_ARGS , buf, bufsize
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# define AT_FUNC_RESULT ssize_t
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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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#endif
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