Re: [PATCH] mm: fix invalid node in alloc_migrate_target()

[Vlastimil Babka]

On 03/29/2016 03:06 PM, Vlastimil Babka wrote:
> On 03/25/2016 08:22 PM, Andrew Morton wrote:
> > Also, mm/mempolicy.c:offset_il_node() worries me:
> >
> > 	do {
> > 		nid = next_node(nid, pol->v.nodes);
> > 		c++;
> > 	} while (c <= target);
> >
> > Can't `nid' hit MAX_NUMNODES?
>
> AFAICS it can. interleave_nid() uses this and the nid is then used e.g.
> in node_zonelist() where it's used for NODE_DATA(nid). That's quite
> scary. It also predates git. Why don't we see crashes or KASAN finding this?

Ah, I see. In offset_il_node(), nid is initialized to -1, and the number 
of do-while iterations calling next_node() is up to the number of bits 
set in the pol->v.nodes bitmap, so it can't reach past the last set bit 
and return MAX_NUMNODES.