linux-kernel - Re: [PATCH v2 08/23] mm: introduce BPF kfuncs to deal with memcg pointers

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Message-ID: <f5d71202-188d-45ff-a5e8-387d060fca47@meta.com>
Date: Tue, 28 Oct 2025 14:03:18 -0400
From: Chris Mason <clm@...a.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>
Cc: bot+bpf-ci@...nel.org, Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Michal Hocko <mhocko@...nel.org>,
        Shakeel Butt <shakeel.butt@...ux.dev>,
        Johannes Weiner <hannes@...xchg.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        inwardvessel <inwardvessel@...il.com>, linux-mm <linux-mm@...ck.org>,
        "open list:CONTROL GROUP (CGROUP)" <cgroups@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, Martin KaFai Lau <martin.lau@...nel.org>,
        Song Liu <song@...nel.org>, Kumar Kartikeya Dwivedi <memxor@...il.com>,
        Tejun Heo <tj@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
        Eduard <eddyz87@...il.com>, Yonghong Song <yonghong.song@...ux.dev>,
        Ihor Solodrai <ihor.solodrai@...ux.dev>
Subject: Re: [PATCH v2 08/23] mm: introduce BPF kfuncs to deal with memcg
 pointers

On 10/28/25 1:12 PM, Alexei Starovoitov wrote:
> On Tue, Oct 28, 2025 at 9:11 AM Roman Gushchin <roman.gushchin@...ux.dev> wrote:
>>
>> bot+bpf-ci@...nel.org writes:
>>
>>> ```
>>> commit ec8e2e1769704b4f69261f36e6aaf035d47c5f0f
>>> Author: Roman Gushchin <roman.gushchin@...ux.dev>
>>> Can this dereference a NULL css pointer?  The function checks css for
>>> NULL at line 42 with "if (css && css_tryget(css))", which suggests css
>>> can be NULL.  If a BPF program calls this function with a NULL css,
>>> wouldn't it crash here before reaching that check?
>>
>> Here css passed as an argument to bpf_get_mem_cgroup() can't be NULL,
>> because the BPF verifier guarantees that it's a valid css pointer.
>>
>> However the result of rcu_dereference_raw(cgroup->subsys[ssid]) can be
>> NULL, this is why the "if (css && css_tryget(css))" check is required
>> down below.
> 
> Yeah. Not sure how feasible it is to teach AI about KF_RCU semantics.

I pulled it down locally to try and w/semcode it is properly catching this:

False Positives Eliminated

1. EH-001 NULL dereference - css parameter dereferenced without check

- Why false positive: BPF verifier ensures pointer parameters are
non-NULL. All kernel kfuncs follow the same pattern of not checking
parameters for NULL (css_rstat_updated, css_rstat_flush,
bpf_put_mem_cgroup, etc.). The KF_RET_NULL flag controls return value,
not parameter nullability.

My plan is to just have the prompt read Documentation/bpf/kfuncs.rst,
which Eduard suggested.  I'll make a bpf kfuncs pattern and do that.

-chris