Re: uselib() & 2.6.X?

[Marcelo Tosatti <marcelo.tosatti@cyclades.com>]

On Fri, Jan 07, 2005 at 04:15:28PM -0800, Linus Torvalds wrote:
> 
> 
> On Fri, 7 Jan 2005, Alan Cox wrote:
> >
> > Please don't use that for mainline - do_brk_locked doesn't follow kernel
> > convention
> 
> I agree, I also find the "do_brk_locked()" naming confusing. To me it 
> implies that we already _are_ locked, not that we're going to lock.
> 
> On the other hand, I think Alan's patch is equally confusing: the calling 
> rules for "do_brk()" and "do_mmap()" are the same, and they are "caller 
> takes mmap_sem". 
> 
> So I think you _both_ broke kernel conventions.
> 
> So I'd personally much prefer to just first fix the bug minimally (by just
> taking the lock in the two places that need it), and then _separately_ say
> "we should warn if anybody ever calls 'do_brk()' without the lock". That's 
> how we tend to verify locking in other cases, ie we have things like
> 
> 	if (!spin_is_locked(&t->sighand->siglock))
> 		BUG();
> 
> to verify the calling conventions. Same would go for mmap_sem (although we
> don't seem to have any "sem_is_writelocked()" test - although you can fake
> it with
> 
> 	if (down_read_trylock(&mm->mmap_sem))
> 		BUG();
> 
> instead.
> 
> Now _that_ is a non-silent failure mode. The machine doesn't just silently 
> deadlock: it tells you exactly what's wrong.

Only problem is that current do_brk() callers dont take the lock - you would
need a version of do_brk() that doesnt warn for them? 

But yes, the warning is better than silent failure or security problem for 
out-of-the tree users.