Re: [PATCH bpf-next v3 3/6] bpf: Disallow !kprobe_write_ctx progs tail-calling kprobe_write_ctx progs

[Leon Hwang]

On 2026/3/11 23:44, Alexei Starovoitov wrote:
> On Wed, Mar 11, 2026 at 2:22 AM Kumar Kartikeya Dwivedi
> <memxor@xxxxxxxxx> wrote:
>>

[...]

>>
>> I agree but the main question is whether such a case is realistic, are
>> we going to have write_ctx programs tail calling this way?
>> Tail calls are already pretty rare, thinking more about it extension
>> programs are probably also broken wrt checks in this set.
>> bpf_check_attach_target is doing none of these things for
>> prog_extension = true. Nobody reported a problem, so I doubt anyone is
>> hitting this.
>> It probably also needs to be fixed.
>> Since you noticed it, we should close the gap conservatively for now,
>> and wait for a real use case to pop up before enabling this one-way.
> 
> +1
> tail_calls in general hopefully will be deprecated soon.
> As soon as we have support for indirect calls there won't be any reason
> for tail calls to exist. (other than not breaking current users).
> We definitely don't want to open it up further.
> So the simplest fix.

Got it.

Will follow the both-ways check approach in the next revision.

Will apply the conservative check to extension programs using another
patch series, after verifying the potential issues for them.

Thanks,
Leon