From d682f8de7f9d384f4cfc482a3ba2960329a8db21 Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Sun, 12 Jun 2022 13:46:52 -0700
Subject: fchmodat: port better to MS-Windows etc.
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MS-Windows problem reported by Bruno Haible in:
https://lists.gnu.org/r/bug-gnulib/2022-06/msg00041.html
Although I don’t use MS-Windows I see some related fstatat etc.
problems and am trying to fix them with this further patch.
* lib/fchmodat.c (fchmodat):
* lib/lchmod.c (lchmod):
* lib/lchown.c (lchown)
[!HAVE_LCHOWN && HAVE_CHOWN && !CHOWN_MODIFIES_SYMLINK]:
* lib/renameatu.c (renameatu)
[HAVE_RENAME && RENAME_TRAILING_SLASH_SOURCE_BUG]:
Use readlinkat/readlink instead of fstatat/lstat to test merely
whether a string names a symlink, as this avoids problems
with EOVERFLOW.  Also, I hope it works around the MS-Windows
issues that Bruno noted.
* m4/fchmodat.m4 (gl_PREREQ_FCHMODAT):
Check for readlinkat, not lchmod.
* m4/lchmod.m4 (gl_FUNC_LCHMOD): Do not require AC_CANONICAL_HOST
or check for lstat.
(gl_PREREQ_LCHMOD): Check for readlink.
* modules/lchown (Depends-on): Add readlink.  Do not depend on
lstat merely because !HAVE_LCHOWN.
* modules/renameatu (Depends-on): Add fstatat, readlinkat.
---
 lib/lchmod.c | 17 +++++++----------
 1 file changed, 7 insertions(+), 10 deletions(-)

(limited to 'lib/lchmod.c')

diff --git a/lib/lchmod.c b/lib/lchmod.c
index b4cc0a8176..f21bf7f652 100644
--- a/lib/lchmod.c
+++ b/lib/lchmod.c
@@ -45,7 +45,10 @@
 int
 lchmod (char const *file, mode_t mode)
 {
-#ifdef O_PATH
+#if HAVE_READLINK
+  char readlink_buf[1];
+
+# ifdef O_PATH
   /* Open a file descriptor with O_NOFOLLOW, to make sure we don't
      follow symbolic links, if /proc is mounted.  O_PATH is used to
      avoid a failure if the file is not readable.
@@ -55,8 +58,7 @@ lchmod (char const *file, mode_t mode)
     return fd;
 
   int err;
-  char buf[1];
-  if (0 <= readlinkat (fd, "", buf, sizeof buf))
+  if (0 <= readlinkat (fd, "", readlink_buf, sizeof readlink_buf))
     err = EOPNOTSUPP;
   else if (errno == EINVAL)
     {
@@ -73,16 +75,11 @@ lchmod (char const *file, mode_t mode)
   errno = err;
   if (0 <= err)
     return err == 0 ? 0 : -1;
-#endif
+# endif
 
   /* O_PATH + /proc is not supported.  */
 
-#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))
+  if (0 <= readlink (file, readlink_buf, sizeof readlink_buf))
     {
       errno = EOPNOTSUPP;
       return -1;
-- 
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