GNU bug report logs - #11108
chmod: fix symlink race condition

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Paul Eggert <eggert <at> cs.ucla.edu>
Subject: bug#11108: closed (Re: bug#11108: [PATCH] chmod: fix symlink race
 condition)
Date: Wed, 20 Mar 2024 19:10:02 +0000

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Your bug report

#11108: chmod: fix symlink race condition

which was filed against the coreutils package, has been closed.

The explanation is attached below, along with your original report.
If you require more details, please reply to 11108 <at> debbugs.gnu.org.

-- 
11108: https://debbugs.gnu.org/cgi/bugreport.cgi?bug=11108
GNU Bug Tracking System
Contact help-debbugs <at> gnu.org with problems

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From: Pádraig Brady <P <at> draigBrady.com>
To: 11108-done <at> debbugs.gnu.org
Subject: Re: bug#11108: [PATCH] chmod: fix symlink race condition
Date: Wed, 20 Mar 2024 19:01:22 +0000

On 28/03/2012 21:28, Paul Eggert wrote:
> On 03/28/2012 01:13 PM, Jim Meyering wrote:
>>      $ ./chmod u+w f
>>      ./chmod: changing permissions of 'f': Operation not supported
> 
> Yeouch.  I undid the change for now.
> Hmm, why did "make check" work for me?
> I'll have to investigate later, alas.

Patch for this pushed at:
https://git.sv.gnu.org/cgit/coreutils.git/commit/?id=v9.4-163-g425b8a2f5

Marking this as done.

cheers,
Pádraig.

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From: Paul Eggert <eggert <at> cs.ucla.edu>
To: bug-coreutils <at> gnu.org
Subject: [PATCH] chmod: fix symlink race condition
Date: Tue, 27 Mar 2012 22:28:31 -0700

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

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