Re: [PATCH] mm/bpf_memcontrol: mark BPF memcg kfuncs static

[Roman Gushchin]

Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> writes:

> On Wed, Jun 17, 2026 at 1:22 PM JP Kobryn <jp.kobryn@xxxxxxxxx> wrote:
>>
>> The kfuncs in bpf_memcontrol.c are not called by in-kernel C code. They are
>> only referenced by BPF programs and resolved through BTF.
>>
>> Since no external linkage is needed, mark these functions static.
>>
>> Fixes: 5904db9891f8 ("mm: introduce BPF kfuncs to deal with memcg pointers")
>> Fixes: 5c7db3239c9f ("mm: introduce bpf_get_root_mem_cgroup() BPF kfunc")
>> Fixes: 99430ab8b804 ("mm: introduce BPF kfuncs to access memcg statistics and events")
>> Reported-by: kernel test robot <lkp@xxxxxxxxx>
>> Closes: https://lore.kernel.org/oe-kbuild-all/202606150332.Xy4Egd9s-lkp@xxxxxxxxx/
>> Signed-off-by: JP Kobryn <jp.kobryn@xxxxxxxxx>
>> ---
>>  mm/bpf_memcontrol.c | 21 +++++++++++----------
>>  1 file changed, 11 insertions(+), 10 deletions(-)
>>
>> diff --git a/mm/bpf_memcontrol.c b/mm/bpf_memcontrol.c
>> index 716df49d7647..eff79bb2c758 100644
>> --- a/mm/bpf_memcontrol.c
>> +++ b/mm/bpf_memcontrol.c
>> @@ -20,7 +20,7 @@ __bpf_kfunc_start_defs();
>>   *
>>   * Return: A pointer to the root memory cgroup.
>>   */
>> -__bpf_kfunc struct mem_cgroup *bpf_get_root_mem_cgroup(void)
>> +__bpf_kfunc static struct mem_cgroup *bpf_get_root_mem_cgroup(void)
>
> sashiko disappoints. It couldn't notice that kfuncs should not be static.
> We have few "__bpf_kfunc static" in net/ipv4/
> that work by sort-of "luck", since their addresses are taken.
>
> In general kfuncs cannot be declared static.

It seems like it thought hard on it but failed based on the existing
code and a bit vague documentation in Documentation/bpf/kfuncs.rst:

  kfunc definitions should also always be annotated with the ``__bpf_kfunc``
  macro. This prevents issues such as the compiler inlining the kfunc if it's a
  static kernel function, or the function ...

So it reads like bpf kfuncs can be static.

This is the reasoning from the logs (a dismissed concern, to be precise):
{
      "type": "Dead Code / Linker Error",

      "description": "Marking kfuncs as static could cause the compiler
      to optimize them away, breaking BPF verifier BTF resolution.",

      "reasoning": "The patch adds `static` to multiple `__bpf_kfunc`
      annotated functions that are not called anywhere in C
      code. Normally, a compiler would remove unused static functions or
      fail to expose them in object files, which could prevent the BTF
      ID generation logic (`resolve_btfids`) from finding them. However,
      the `__bpf_kfunc` macro expands to include `__used` and
      `__retain`, which strictly instructs the compiler and linker to
      preserve the functions in the object file regardless of
      usage. Additionally, `resolve_btfids` and `BTF_ID_FLAGS`
      successfully parse and resolve `STB_LOCAL` static symbols, making
      this change completely safe and correct.",

      "locations": [
        {
          "file": "mm/bpf_memcontrol.c",
          "function_or_symbol": null,
          "line_range": null,
          "why_this_location_matters": "This applies to all the
          functions modified by the patch."
        }
      ]
    }


Thanks