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

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Message-ID: <dfc73fd6-9e5f-4b62-ac3f-7c9a327dd7fc@meta.com>
Date: Sat, 20 Dec 2025 08:19:48 -0500
From: Chris Mason <clm@...a.com>
To: Roman Gushchin <roman.gushchin@...ux.dev>, bot+bpf-ci@...nel.org
Cc: bpf@...r.kernel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        inwardvessel@...il.com, ast@...nel.org, daniel@...earbox.net,
        shakeel.butt@...ux.dev, mhocko@...nel.org, hannes@...xchg.org,
        andrii@...nel.org, martin.lau@...nel.org, eddyz87@...il.com,
        yonghong.song@...ux.dev, ihor.solodrai@...ux.dev
Subject: Re: [PATCH bpf-next v2 5/7] mm: introduce BPF kfunc to access memory
 events

On 12/19/25 11:41 PM, Roman Gushchin wrote:
> bot+bpf-ci@...nel.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.

-chris