Re: [PATCH bpf-next v2 5/7] mm: introduce BPF kfunc to access memory events

[Roman Gushchin <roman.gushchin@linux.dev>]

Chris Mason <clm@meta.com> writes:

> On 12/19/25 11:41 PM, Roman Gushchin wrote:
>> bot+bpf-ci@kernel.org writes:
>> 
>>>> diff --git a/mm/bpf_memcontrol.c b/mm/bpf_memcontrol.c
>>>> index d84fe6f3ed43..858eb43766ce 100644
>>>> --- a/mm/bpf_memcontrol.c
>>>> +++ b/mm/bpf_memcontrol.c
>>>> @@ -103,6 +103,22 @@ __bpf_kfunc unsigned long bpf_mem_cgroup_usage(struct mem_cgroup *memcg)
>>>>  	return mem_cgroup_usage(memcg, false) * PAGE_SIZE;
>>>>  }
>>>>
>>>> +/**
>>>> + * bpf_mem_cgroup_memory_events - Read memory cgroup's memory event value
>>>> + * @memcg: memory cgroup
>>>> + * @event: memory event id
>>>> + *
>>>> + * Returns current memory event count.
>>>> + */
>>>> +__bpf_kfunc unsigned long bpf_mem_cgroup_memory_events(struct mem_cgroup *memcg,
>>>> +						enum memcg_memory_event event)
>>>> +{
>>>> +	if (event >= MEMCG_NR_MEMORY_EVENTS)
>>>> +		return (unsigned long)-1;
>>>
>>> Should this also check for negative values?  The enum type is typically
>>> signed, so a BPF program passing a negative event value would pass this
>>> check but cause an out-of-bounds read from memcg->memory_events[].
>>>
>>> The similar bpf_mem_cgroup_page_state() in this same file uses:
>>>
>>>     if (idx < 0 || idx >= MEMCG_NR_STAT)
>>>
>>> which checks both bounds.  Could the check here be changed to:
>>>
>>>     if (event < 0 || event >= MEMCG_NR_MEMORY_EVENTS)
>>>
>>> to match that pattern?
>> 
>> No, the bpf verifier enforces event > 0.
>> It's a false positive.
>
> I'll add some words here to the bpf prompts, thanks Roman.

I'll try to play with it too, hopefully we can fix it.

Thanks!