Re: [PATCH bpf-next v4 2/3] bpf: Report arena faults to BPF stderr

[Puranjay Mohan <puranjay@kernel.org>]

Alexei Starovoitov <alexei.starovoitov@gmail.com> writes:

> On Mon, Sep 1, 2025 at 6:34 AM Puranjay Mohan <puranjay@kernel.org> wrote:
>>
>> Alexei Starovoitov <alexei.starovoitov@gmail.com> writes:
>>
>> > On Fri, Aug 29, 2025 at 3:30 AM Xu Kuohai <xukuohai@huaweicloud.com> wrote:
>> >>
>> >> > +
>> >> > +void bpf_prog_report_arena_violation(bool write, unsigned long addr)
>> >> > +{
>> >> > +     struct bpf_stream_stage ss;
>> >> > +     struct bpf_prog *prog;
>> >> > +     u64 user_vm_start;
>> >> > +
>> >> > +     prog = bpf_prog_find_from_stack();
>> >>
>> >> bpf_prog_find_from_stack depends on arch_bpf_stack_walk, which isn't available
>> >> on all archs. How about switching to bpf_prog_ksym_find with the fault pc?
>> >
>> > Out of archs that support bpf arena only riscv doesn't
>> > support arch_bpf_stack_walk(), which is probably fixable.
>> > But I agree that direct bpf_prog_ksym_find() is cleaner here.
>> > We need to make sure it works for subprogs, since streams[2] are
>> > valid only for main prog.
>> > I think we can add:
>> > struct bpf_prog_aux {
>> >   ...
>> >   struct bpf_prog_aux *main_prog;
>> > };
>> > init it during jit_subprogs() and use it for stream access.
>> > We can also remove skipping of subprogs in find_from_stack_cb() then.
>> >
>> > Kumar, wdyt?
>>
>> So, IIUC, after adding struct bpf_prog_aux *main_prog_aux in struct
>> bpf_prog_aux,
>>
>> We can do in bpf_prog_alloc_no_stats():
>>    fp->aux->main_prog_aux = aux;
>>
>> and in jit_subprogs():
>>     func[i]->aux->main_prog_aux = prog->aux;
>>
>> and then all users of bpf_stream_get() can do
>>     bpf_stream_get(stream_id, prog->aux->main_prog_aux);
>>
>> with above we can allow find_from_stack_cb() to return subprogs.
>> and bpf_prog_ksym_find() can be used in
>> bpf_prog_report_arena_violation() without any other changes.
>
> Yes. That's exactly the proposal.

I think we should go ahead with this approach but also divide
bpf_prog_aux into two as you suggested. I will send a follow-up set for
that.

Thanks,
Puranjay