Re: + prctl-pr_set_mm-introduce-pr_set_mm_map-operation-v3.patch added to -mm tree

[Oleg Nesterov <oleg@redhat.com>]

On 08/23, Cyrill Gorcunov wrote:
>
> On Fri, Aug 22, 2014 at 09:22:41PM +0200, Oleg Nesterov wrote:
> > Hi Cyrill,
> >
> > I think the patch is fine but I can't understand the usage of mmap_sem
> > and alloc_lock,
> >
> > > +	stack_vma = find_vma(mm, (unsigned long)prctl_map->start_stack);
> >
> > OK, find_vma() needs mmap_sem. But otherwise, why this should be called
> > under down_read(&mm->mmap_sem) ? What this lock tries to protect?
>
> It should protect from allocation/devetion/mergin of another vma. IOW when
> I lookup for vma I need to be sure it exist and won't disappear at least
> while I validate it.

plus you need mmap_sem (at least for reading) when you update mm_struct,
this is clear.

My question was why the whole function should be called under mmap_sem?
It could take it only around find_vma() + check(RLIMIT_STACK) ?

In fact I do not think we need this vma_stack/RLIMIT_STACK check at all.
It buys nithing and looks strange. RLIMIT_STACK is mostly for self-debugging,
to catch the, say, unlimited recursion. An application can trivially
create a stack region of arbitrary size. I'd seriously suggest to remove it.

> > > +	if (prctl_map.auxv_size) {
> > > +		/* Last entry must be AT_NULL as specification requires */
> > > +		user_auxv[AT_VECTOR_SIZE - 2] = AT_NULL;
> > > +		user_auxv[AT_VECTOR_SIZE - 1] = AT_NULL;
> > > +
> > > +		task_lock(current);
> > > +		memcpy(mm->saved_auxv, user_auxv, sizeof(user_auxv));
> > > +		task_unlock(current);
> >
> > Again, could you explain this task_lock() ?
>
> It is used for serialization access to saved_auxv, ie when we fill it
> with new data the other reader (via procfs interface) should wait until
> we finish.

But proc_pid_auxv() doesn't take this lock? And even if it did, this lock
can't help. task_lock() is per-thread, and multiple threads (including
CLONE_VM tasks, vfork() for example) can share the same ->mm.

This certainly doesn't look right.

Oleg.