Re: [PATCH bpf-next v5 2/2] bpf: fix stackmap overflow check in __bpf_get_stackid()

["Lecomte, Arnaud" <contact@arnaud-lcm.com>]

On 30/08/2025 02:28, Alexei Starovoitov wrote:
> On Fri, Aug 29, 2025 at 11:50 AM Song Liu <song@kernel.org> wrote:
>> On Fri, Aug 29, 2025 at 10:29 AM Alexei Starovoitov
>> <alexei.starovoitov@gmail.com> wrote:
>> [...]
>>>>   static long __bpf_get_stackid(struct bpf_map *map,
>>>> -                             struct perf_callchain_entry *trace, u64 flags)
>>>> +                             struct perf_callchain_entry *trace, u64 flags, u32 max_depth)
>>>>   {
>>>>          struct bpf_stack_map *smap = container_of(map, struct bpf_stack_map, map);
>>>>          struct stack_map_bucket *bucket, *new_bucket, *old_bucket;
>>>> @@ -263,6 +263,8 @@ static long __bpf_get_stackid(struct bpf_map *map,
>>>>
>>>>          trace_nr = trace->nr - skip;
>>>>          trace_len = trace_nr * sizeof(u64);
>>>> +       trace_nr = min(trace_nr, max_depth - skip);
>>>> +
>>> The patch might have fixed this particular syzbot repro
>>> with OOB in stackmap-with-buildid case,
>>> but above two line looks wrong.
>>> trace_len is computed before being capped by max_depth.
>>> So non-buildid case below is using
>>> memcpy(new_bucket->data, ips, trace_len);
>>>
>>> so OOB is still there?
>> +1 for this observation.
>>
>> We are calling __bpf_get_stackid() from two functions: bpf_get_stackid
>> and bpf_get_stackid_pe. The check against max_depth is only needed
>> from bpf_get_stackid_pe, so it is better to just check here.
> Good point.
Nice catch, thanks !
>
>> I have got the following on top of patch 1/2. This makes more sense to
>> me.
>>
>> PS: The following also includes some clean up in __bpf_get_stack.
>> I include those because it also uses stack_map_calculate_max_depth.
>>
>> Does this look better?
> yeah. It's certainly cleaner to avoid adding extra arg to
> __bpf_get_stackid()
>
Are Song patches going to be applied then ?  Or should I raise a new 
revision
  of the patch with Song's modifications with a Co-developped tag ?
Thanks for your guidance in advance,
Arnaud