fchmodat: AT_SYMLINK_NOFOLLOW fix for non-symlinks

Fix lchmod, and fchmodat with AT_SYMLINK_NOFOLLOW, so that
they act like chmod on non-symlinks.
* NEWS:
* doc/glibc-functions/lchmod.texi (lchmod):
* doc/posix-functions/fchmodat.texi (fchmodat):
Mention this.
* lib/fchmodat.c: Define __need_system_sys_stat_h before including
config.h, and undef it after including sys/stat.h the first time.
Include fcntl.h, stdio.h, unistd.h, intprops.h, and include
sys/stat.h a second time after defining orig_fchmodat.
(orig_fchmodat) [HAVE_FCHMODAT]: New function.
(fchmodat) [HAVE_FCHMODAT]: Work around the AT_SYMLINK_NOFOLLOW bug.
* lib/lchmod.c: New file.
* lib/sys_stat.in.h (fchmodat, lchmod):
Support replacing these functions.
* m4/fchmodat.m4 (gl_FUNC_FCHMODAT): If fchmodat exists,
test that AT_SYMLINK_NOFOLLOW works on non-symlinks.
* m4/lchmod.m4 (gl_FUNC_LCHMOD): Check for lstat.
Test that lchmod works on non-symlinks.
* m4/sys_stat_h.m4 (gl_SYS_STAT_H_DEFAULTS):
Default REPLACE_FCHMODAT and REPLACE_LCHMOD to 0.
* modules/fchmodat (Depends-on): Add fstatat, intprops, lchmod, unistd.
(Depends-on, configure.ac): Check REPLACE_FCHMODAT too.
* modules/lchmod (Files): Add lib/lchmod.c.
(Depends-on): Add errno, fcntl-h, fchmodat, intprops, lstat, unistd.
(configure.ac): Compile lchmod.c if needed.
(lib_SOURCES): Add lchmod.c.
* modules/sys_stat (sys/stat.h): Substitute REPLACE_FCHMODAT
and REPLACE_LCHMOD.
* tests/test-fchmodat.c: Include fcntl.h, sys/stat.h.
(main): Test fchmodat with AT_SYMLINK_NOFOLLOW on non-symlinks.

1 files changed, 72 insertions, 0 deletions

diff --git a/lib/lchmod.c b/lib/lchmod.c
new file mode 100644
index 0000000000..cc260ce4dc
--- /dev/null
+++ b/lib/lchmod.c
@@ -0,0 +1,72 @@
+/* Implement lchmod on platforms where it does not work correctly.
+
+   Copyright 2020 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 Paul Eggert */
+
+#include <config.h>
+
+#include <sys/stat.h>
+
+#include <errno.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <unistd.h>
+
+#include <intprops.h>
+
+/* Work like lchmod, except when FILE is a symbolic link.
+   In that case, set errno to EOPNOTSUPP and return -1.  */
+
+int
+lchmod (char const *file, mode_t mode)
+{
+#if HAVE_FCHMODAT
+  return fchmodat (AT_FDCWD, file, mode, AT_SYMLINK_NOFOLLOW);
+#elif defined O_PATH && defined AT_FDCWD
+  int fd = openat (AT_FDCWD, file, O_PATH | O_NOFOLLOW | O_CLOEXEC);
+  if (fd < 0)
+    return fd;
+  static char const fmt[] = "/proc/self/fd/%d";
+  char buf[sizeof fmt - sizeof "%d" + INT_BUFSIZE_BOUND (int)];
+  sprintf (buf, fmt, fd);
+  int chmod_result = chmod (buf, mode);
+  int chmod_errno = errno;
+  close (fd);
+  if (chmod_result == 0)
+    return chmod_result;
+  if (chmod_errno != ENOENT)
+    {
+      errno = chmod_errno;
+      return chmod_result;
+    }
+  /* /proc is not mounted; fall back on racy implementation.  */
+#endif
+
+#if HAVE_LSTAT
+  struct stat st;
+  int lstat_result = lstat (file, &st);
+  if (lstat_result != 0)
+    return lstat_result;
+  if (S_ISLNK (st.st_mode))
+    {
+      errno = EOPNOTSUPP;
+      return -1;
+    }
+#endif
+
+  return chmod (file, mode);
+}