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SubjectRe: UBSAN: Undefined behaviour in mm/page_alloc.c
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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"))
>

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