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Subject: Re: UBSAN: Undefined behaviour in mm/page_alloc.c
To:     Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@kernel.org>
Cc:     Kyungtae Kim <kt0755@gmail.com>, pavel.tatashin@microsoft.com,
        osalvador@suse.de, rppt@linux.vnet.ibm.com, aaron.lu@intel.com,
        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>
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From:   Vlastimil Babka <vbabka@suse.cz>
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Message-ID: <d88fae5c-e12d-ca35-d200-587a2ff02ec9@suse.cz>
Date:   Wed, 14 Nov 2018 00:23:28 +0100
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Sender: linux-kernel-owner@vger.kernel.org
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On 11/14/18 12:15 AM, Andrew Morton wrote:
> On Tue, 13 Nov 2018 10:43:05 +0100 Michal Hocko <mhocko@kernel.org> wrote:
> 
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -4364,6 +4353,15 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order, int preferred_nid,
>>  	gfp_t alloc_mask; /* The gfp_t that was actually used for allocation */
>>  	struct alloc_context ac = { };
>>  
>> +	/*
>> +	 * There are several places where we assume that the order value is sane
>> +	 * so bail out early if the request is out of bound.
>> +	 */
>> +	if (unlikely(order >= MAX_ORDER)) {
>> +		WARN_ON_ONCE(!(gfp_mask & __GFP_NOWARN));
>> +		return NULL;
>> +	}
>> +
> 
> I know "everybody enables CONFIG_DEBUG_VM", but given this is fastpath,
> we could help those who choose not to enable it by using
> 
> #ifdef CONFIG_DEBUG_VM
> 	if (WARN_ON_ONCE(order >= MAX_ORDER && !(gfp_mask & __GFP_NOWARN)))
> 		return NULL;
> #endif

Hmm, but that would mean there's still potential undefined behavior for
!CONFIG_DEBUG_VM, so I would prefer not to do it like that.

> 
> (Again curses 91241681c62 ("include/linux/mmdebug.h: make VM_WARN* non-rvals"))
>