Re: [PATCH RFC] mm: warn potential return NULL for kmalloc_array and kvmalloc_array with __GFP_NOFAIL

[Michal Hocko <mhocko@suse.com>]

On Thu 18-07-24 00:04:54, Christoph Hellwig wrote:
> On Thu, Jul 18, 2024 at 08:58:44AM +0200, Michal Hocko wrote:
> > WARN_ON is effectively BUG_ON with panic_on_warn so if this happens to
> > be in a user triggerable path then you would have an easy way to panic
> > the whole machine. It is likely true that the kernel could oops just
> > right after the failure but that could be recoverable at least.
> 
> If you set panic_on_warn you are either debugging in which case it's
> the right thing, or you are fucked.  So don't do it unless you
> expet frequent crashes.

I do agree and I wouldn't recommend running panic_on_warn on anything
even touching production setups. Reality check disagrees.

> > If anything I would just pr_warn with caller address or add dump_stack
> > to capture the full trace. That would give us the caller that needs
> > fixing without panicing the system with panic_on_warn.
> 
> The whole freaking point of __GFP_NOFAIL is that callers don't handle
> allocation failures.  So in fact a straight BUG is the right thing
> here.

OOPs from the NULL ptr could be just a safer option bacause it could be
recoverable.

Anyway, I am questioning that WARN/BUG/pr_warn on overflow check is
adding any actual value because GFP_NOFAIL large allocations could be
even more dangerous essentially rendering the system completely
unsuabale.

-- 
Michal Hocko
SUSE Labs