Re: [PATCH v2 3/5] powerpc64/bpf: use consistent tailcall offset in trampoline

Hari Bathini
2026-03-03T13:43:37Z

On 22/02/26 6:37 pm, adubey wrote:
> On 2026-02-20 12:09, Hari Bathini wrote:
>> Ideally, the offset used to load the tail call info field and to find
>> the pass by reference address for tail call field should be the same.
>> But while setting up the tail call info in the trampoline, this was
>> not followed. This can be misleading and can lead to unpredictable
>> results if and when bpf_has_stack_frame() ends up returning true
>> for trampoline frame. Since commit 15513beeb673 ("powerpc64/bpf:
>> Moving tail_call_cnt to bottom of frame") and commit 2ed2d8f6fb38
>> ("powerpc64/bpf: Support tailcalls with subprogs") ensured tail call
>> field is at the bottom of the stack frame for BPF programs as well as
>> BPF trampoline, avoid relying on bpf_jit_stack_tailcallinfo_offset()
>> and bpf_has_stack_frame() for trampoline frame and always calculate
>> tail call field offset with reference to older frame.
>

> It's good to add comment about padding field placed after tailcall_info
> in the trampoline stack layout. Visibly padding is following tailcall_info
> but tailcall_info is bottom-most field. Clear comment around this
> will be really helpful.

"Generated stack layout:" does capture that clearly. Don't find it
relevant to explain it again in this context..

- Hari