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

[Björn Töpel]

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?