Bad file access on the rise

[Doug Ledford]

On 06/09/2013 11:42 AM, Matthew Garrett wrote:
> On Sun, Jun 09, 2013 at 11:05:44AM -0400, Doug Ledford wrote:
> 
>> And really, we've spent more time on this thread than it would take
>> Lennart to fix PA.  Just a quick stat and check of uid before trying to
>> remove the stale files and this would all go away.  Sure, your stat and
>> remove could race, but this is nothing more than a garbage collection
>> process anyway, so who cares?  We'll just get it next time.
> 
> "Stat and then open" is inherently open to race conditions.

Of course it is.

> In this 
> specific case it probably doesn't realistically matter,

Exactly my point.

> but it's not 
> behaviour that should be encouraged.

I would never encourage it in terms of suggesting people try to second
guess the kernel's rules and limitations.  However, you can use such a
technique to weed out otherwise known to fail cases, at least in
instances like this where it is a garbage collection operation and
failure to guess properly is obviously far from critical.

> The only way to know for certain 
> whether you're going to be able to open or unlink a file is to attempt 
> to open or unlink that file, and we shouldn't encourage people to think 
> otherwise.

Agreed.