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

[Roman Gushchin]

Chris Mason <clm@xxxxxxxx> writes:

> On 10/28/25 1:12 PM, Alexei Starovoitov wrote:
>> On Tue, Oct 28, 2025 at 9:11 AM Roman Gushchin <roman.gushchin@xxxxxxxxx> wrote:
>>>
>>> bot+bpf-ci@xxxxxxxxxx writes:
>>>
>>>> ```
>>>> commit ec8e2e1769704b4f69261f36e6aaf035d47c5f0f
>>>> Author: Roman Gushchin <roman.gushchin@xxxxxxxxx>
>>>> 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.

Awesome, thank you!