Re: [PATCH] unserialized task->files changing (v2)

[Eric Dumazet]

On Tuesday 08 August 2006 13:31, Kirill Korotaev wrote:
> Fixed race on put_files_struct on exec with proc.
> Restoring files on current on error path may lead
> to proc having a pointer to already kfree-d files_struct.
>
> ->files changing at exit.c and khtread.c are safe as
> exit_files() makes all things under lock.
>
> v2 patch changes:
> - introduced reset_files_struct() as Christoph Hellwig suggested
>
> Found during OpenVZ stress testing.

Sorry but there is something I dont understand. You ignored my point.

+void reset_files_struct(struct task_struct *tsk, struct files_struct *files)
+{
+ÂÂÂÂÂÂÂstruct files_struct *old;
+
+ÂÂÂÂÂÂÂold = tsk->files;
+ÂÂÂÂÂÂÂtask_lock(tsk);
+ÂÂÂÂÂÂÂtsk->files = files;
+ÂÂÂÂÂÂÂtask_unlock(tsk);
+ÂÂÂÂÂÂÂput_files_struct(old);
+}

Its seems very strange to protect tsk->files = files with a 
task_lock()/task_unlock(). What is it supposed to guard against ???

If this patch corrects the 'bug', then a simpler fix would  be to use a memory 
barrier between "tsk->files = files" and "put_files_struct(old);" 

No need to perform 2 atomics ops on the task lock.

old = tsk->files;
tsk->files = files;
smp_mb();
put_files_struct(old);

That would be enough to guard against proc code (because this code only needs 
to read tsk->files of course)

The same remark can be said for __exit_files() from kernel/exit.c

If this task_lock()/task_unlock() patch is really needed, then a comment in 
the source would be very fair.

Eric

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