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Date:   Fri, 9 Nov 2018 11:25:36 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc:     Kyungtae Kim <kt0755@gmail.com>, akpm@linux-foundation.org,
        pavel.tatashin@microsoft.com, vbabka@suse.cz, osalvador@suse.de,
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        iamjoonsoo.kim@lge.com, alexander.h.duyck@linux.intel.com,
        mgorman@techsingularity.net, lifeasageek@gmail.com,
        threeearcat@gmail.com, syzkaller@googlegroups.com,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Subject: Re: UBSAN: Undefined behaviour in mm/page_alloc.c
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On Fri 09-11-18 19:07:49, Tetsuo Handa wrote:
> On 2018/11/09 18:56, Michal Hocko wrote:
> > Does this following look better?
> 
> Yes.
> 
> >> Also, why not to add BUG_ON(gfp_mask & __GFP_NOFAIL); here?
> > 
> > Because we do not want to blow up the kernel just because of a stupid
> > usage of the allocator. Can you think of an example where it would
> > actually make any sense?
> > 
> > I would argue that such a theoretical abuse would blow up on an
> > unchecked NULL ptr access. Isn't that enough?
> 
> We after all can't avoid blowing up the kernel even if we don't add BUG_ON().
> Stopping with BUG_ON() is saner than NULL pointer dereference messages.

I disagree (strongly to be more explicit). You never know the context
the allocator is called from. We do not want to oops with a random state
(locks heled etc). If the access blows up in the user then be it, the
bug will be clear and to be fixed but BUG_ON on an invalid core kernel
function is just a bad idea. I believe Linus was quite explicit about it
and I fully agree with him.

Besides that this is really off-topic to the issue at hands. Don't you
think?
-- 
Michal Hocko
SUSE Labs