Re: DoS with POSIX file locks?

[Miklos Szeredi]

> You'd have to ensure that none of the threads involved are able to grab
> new posix locks in the period between the unsharing of current->files to
> the moment when current->files->owner is swapped.
> 
> If not, one thread could in theory open a new file and grab a lock that
> can never be unlocked because its lockowner gets stolen away from it by
> another execing thread.

This race is already there.  Header comment on steal_locks() documents
it.

The patch does open this race window much wider, because pending locks
are also transfered to the task doing the exec.  The original
steal_locks() only stole already held locks.  But I don't think this
fundamentaly changes things.  It just shows more clearly how ugly the
current semantics are.

Miklos
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