Re: [PATCH] mm: don't allow empty relative nodemask in mpol_relative_nodemask()

["David Hildenbrand (Arm)" <david@kernel.org>]

On 6/2/26 10:44, Gregory Price wrote:
> On Mon, Jun 01, 2026 at 04:32:25PM +0200, David Hildenbrand (Arm) wrote:
>>>
>>> Hi Joshua.
>>>
>>> Indeed, quick and dirty shot.
>>>
>>> The problem is that nodes_fold() can't work with the sz == 0. In
>>> other words, folding to a 0-bit bitmap is an error. We don't check
>>> that on bitmaps level because it's an internal helper, and it's a
>>> caller's responsibility to validate the parameters.
>>>
>>> nodes_onto(), or more specifically bitmap_onto(), is a different
>>> story. In case of empty relmap, the function actually clears all the
>>> bits in dst and returns.
>>
>> It's very weird that mpol_new_nodemask() (->create() callback) disallows empty
>> nodemasks, but mpol_rebind_nodemask() (->rebind() callback) would allow empty
>> nodemasks.
>>
> 
> Was this actually observed?
> 
> mpol_rebind_nodemask() happens when cgroup.cpuset changes, and
> cgroup.cpuset cannot be empty.
> 
> cpuset only changes with sysfs twiddles or offlining.  In either case,
> cpuset *guarantees* that cpuset.mems will never be empty.
> 
> So... is this an observed bug or just a statically discovered
> "bug" that can't actually be reached?

According to the report [1] syzkaller can trigger it. There is no reproducer,
though.


[1]
https://lore.kernel.org/all/CA+0ovCgxbZkXa+OU8w3s84R3KNPNxxRfmsNR-udh+afQBbGNmw@mail.gmail.com/

-- 
Cheers,

David