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

[Pu Lehui]

On 2026/7/10 3:51, Björn Töpel wrote:
> On Thu, 9 Jul 2026 at 17:09, Pu Lehui <pulehui at huaweicloud.com> 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 at huaweicloud.com> 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?

make sense to me, will try it.