Re: [Patch] vfs: increase scope of critical locked path in fget_light to avoid race

[Neil Horman <nhorman@redhat.com>]

On Fri, May 20, 2005 at 10:28:24PM +0100, Al Viro wrote:
> On Fri, May 20, 2005 at 11:25:50AM -0400, Neil Horman wrote:
> > I don't have the complete race scenario, just a stack that suggests that
> > files->fd was corrupted.  This problem isn't recreatable at will (yet), so this
> > is really based on a thought experiment more than anything else.  The conditions
> > that I was envisioning was a multithreaded application in which two threads
> > modified  the same file descriptor at the same time.  Its my understanding, from
> > the way I read the code that the ref count on a file_struct will still be one
> > for a multithreaded application, and as such it would be possible, using the
> > fget_light routine for one thread to be be preforming an operation on an
> > descriptor in the fd array, while another thread preformed another operation
> 
> Incorrect.  References to files_struct are held by task_struct.  Kernel
> stack is determined by task_struct.  So your two threads would have
> to share task_struct (i.e. be purely userland ones) and could not
> run in the kernel at the same time for very obvious reasons.
> 
> The rules are simple:
> 	* all access to files_struct is done from upper-half (i.e. is
> process-synchronous).
> 	* the only files_struct you can modify is *(current->files)
> 	* each task_struct that has ->files pointing to given files_struct
> contributes 1 to ->count of that files_struct.  There might be other holders
> of temporary references and they also contribute to ->count.
> 	* all changes of task->files itself are process-synchronous.  Only
> two kinds of changes are possible:
> 	1) current->files can be set to NULL.  That drops a reference to
> original files_struct.
> 	2) current->files can be replaced with a pointer to a new copy of
> previous files_struct.  This operations drops a reference to old one and
> sets the refcount on a copy to 1.  It could either be done explicitly (when
> unshare(2) gets merged into Linus' tree) or implicitly at the clone()/fork()
> time.  In the latter case that's done by parent to child before the child
> gets a chance to run.
> 	* at task creation time, child inherits ->files from parent; that
> acquires a new reference to it.  That might be followed by implicit unshare()
> (see above).  fork(2) always unshares ->files, clone(2) does that unless
> CLONE_FILES had been passed to it in flags.
> 
> IOW, the only way for two tasks to have ->files pointing to the same object
> is to have it unchanged all the way back to common ancestor.  In particular,
> if current->files->count is 1, we know that no other task has ->files pointing
> to our files_struct and that will remain true until we call clone().  It does
> *not* mean that current->files->count will remain 1; somebody might acquire
> a temporary reference to our files_struct.  However, we are guaranteed that
> all such references will be used only for read-only access (that happens,
> e.g., when somebody does ls /proc/<our_pid>/fd - they will grab a reference
> to our ->files and go looking at the descriptor table; they are not allowed
> to change it, though).
Ok, thanks for the through explination here, quite educational.  I'll go looking
for other causes for this. 

Thanks and regards
Neil

-- 
/***************************************************
 *Neil Horman
 *Software Engineer
 *Red Hat, Inc.
 *nhorman@redhat.com
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 *http://pgp.mit.edu
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