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

[Cyrill Gorcunov <gorcunov@gmail.com>]

On Sat, Aug 23, 2014 at 01:53:02PM +0200, Oleg Nesterov wrote:
> >
> > 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) ?

Stricktly speaking yes, but don't forget we might need to update
exe::file as well which requires lock to be taken. So it is simplier
to take the read-lock for the whole function.

> 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.

Look, allocate stack for self is not a problem (we do this for our parasite
code which executes inside dumpee address space) but RLIMIT_STACK check is
present in ipc shmem so I think we still need this check in a sake of
consistency. (note this code doesn't require any special caps so I need
to use as much checks/tests as possible).

> 
> > > > +	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.

It takes this lock but indeed this won't help much. Looks like I need
to use cred_guard_mutex instead of task_lock here, no?