Re: [PATCH bpf-next v6 6/7] riscv, bpf: Mixing bpf2bpf and tailcalls

[Kumar Kartikeya Dwivedi]

On Fri Jul 17, 2026 at 9:37 AM CEST, Pu Lehui wrote:
>
>
> On 2026/7/10 17:43, Daniel Borkmann wrote:
>> On 7/10/26 2:53 AM, Pu Lehui wrote:
>>> On 2026/7/10 3:51, Björn Töpel wrote:
>>>> On Thu, 9 Jul 2026 at 17:09, Pu Lehui <pulehui@xxxxxxxxxxxxxxx> wrote:
>>>>> On 2026/7/9 19:37, Björn Töpel wrote:
>>>>>> Sorry for the delay here -- the bot had me thinking a bit.
>>>>>>
>>>>>> On Wed, 8 Jul 2026 at 10:54, Pu Lehui <pulehui@xxxxxxxxxxxxxxx> wrote:
>>>>>> ...
>>>>>>
>>>>>>>> This assumes a fixed number of instructions before the tailcall
>>>>>>>> entry
>>>>>>>> point. When is_subprog is true, the rv_addi() instruction is not
>>>>>>>> emitted,
>>>>>>>> which means the tailcall entry point moves forward by 4 bytes
>>>>>>>> relative to
>>>>>>>> where RV_TAILCALL_OFFSET expects it to be.
>>>>>>>>
>>>>>>>> The tailcall entry is used in emit_bpf_tail_call() when
>>>>>>>> calculating the
>>>>>>>> jump target for the tail call. If RV_TAILCALL_OFFSET doesn't
>>>>>>>> account for
>>>>>>>> the conditional emission, could this cause the wrong entry point
>>>>>>>> to be
>>>>>>>> used when tail calling into subprograms?
>>>>>>>
>>>>>>> This is not an issue, subprog can not be the tailcall callee.
>>>>>>
>>>>>> Say, that we have an entry function that does bpf_for_each_map_array()
>>>>>> into a callback cb(). cb() is a subprogram, so no init of TCC. Now,
>>>>>> cb() calls another subprogram that does a tailcall.
>>>>>>
>>>>>> The callback to cb() is coming from the kernel, so a6 could have been
>>>>>> clobbered, no? We're entering a subprogram coming from the C ABI. Now,
>>>>>> if the callback calls a subprog that's tail-call reachable, the TCC
>>>>>> can be garbage?
>>>>>
>>>>> Sashiko reported the same issue yesterday. I verified that the verifier
>>>>> rejects cases where a tail call is invoked within a callback;
>>>>> specifically, while the callback's return value range is [0, 1], the
>>>>> simulation of the exit path in `check_helper_call` (handling the tail
>>>>> call helper) marks R0 as UNKNOWN, causing the verifier to reject
>>>>> it—so I
>>>>> didn't investigate further. However, I just verified that the verifier
>>>>> *does* accept the scenario where a callback calls a subprogram, and
>>>>> that
>>>>> subprogram subsequently calls a tail call. I believe this scenario
>>>>> ought
>>>>> to be rejected; perhaps some changes are needed in the verifier,
>>>>> otherwise tail calls on other architectures (like x86) would also be
>>>>> susceptible to infinite loop issues.
>>>>
>>>> Thanks for trying it out!
>>>>
>>>> Ok, so seems like we need a verifier fix pre-landing?
>
> Hi Björn,
>
> Just a quick note that the verifier fix we discussed has now landed in
> bpf-next [0]. Looking forward to your feedback on v6 whenever you get a
> chance. :)

Yep, if there are no more concerns, it would make sense to pull this in.

Björn, what do you think?

> [...]