Subject: [PATCH] chmod: fix symlink race condition
From: Paul Eggert <[email-masked]>
To: [email-masked]
Date: Tue, 27 Mar 2012 22:28:31 -0700
List-Id: <debbugs-submit.debbugs.gnu.org>

This fixes what I hope is an obvious race condition
that can occur if some other process substitutes a
symlink for a non-symlink while chmod is running.

=====
* src/chmod.c (process_file): Don't follow symlink if we
think the file is not a symlink.
---
 src/chmod.c |   10 +++++++++-
 1 files changed, 9 insertions(+), 1 deletions(-)

diff --git a/src/chmod.c b/src/chmod.c
index aa4ac77..2e1f1c7 100644
--- a/src/chmod.c
+++ b/src/chmod.c
@@ -268,7 +268,15 @@ process_file (FTS *fts, FTSENT *ent)
 
       if (! S_ISLNK (old_mode))
         {
-          if (chmodat (fts->fts_cwd_fd, file, new_mode) == 0)
+          /* Use any native support for AT_SYMLINK_NOFOLLOW, to avoid
+             following a symlink if there is a race.  */
+          #if HAVE_FCHMODAT || HAVE_LCHMOD
+          int follow_flag = AT_SYMLINK_NOFOLLOW;
+          #else
+          int follow_flag = 0;
+          #endif
+
+          if (fchmodat (fts->fts_cwd_fd, file, new_mode, follow_flag) == 0)
             chmod_succeeded = true;
           else
             {
-- 
1.7.6.5