Re: [PATCH v2 3/6] powerpc64/bpf: Tailcall handling with trampolines

bot+bpf-ci
2026-01-14T12:25:30Z

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> diff --git a/arch/powerpc/net/bpf_jit_comp.c b/arch/powerpc/net/bpf_jit_comp.c
> index 069a8822c30d..e3088cf089d1 100644
> --- a/arch/powerpc/net/bpf_jit_comp.c
> +++ b/arch/powerpc/net/bpf_jit_comp.c

[ ... ]

> +static void bpf_trampoline_setup_tail_call_info(u32 *image, struct codegen_context *ctx,
> + int func_frame_offset,
> + int bpf_dummy_frame_size, int r4_off)
> +{
> + if (IS_ENABLED(CONFIG_PPC64)) {
> + /* See bpf_jit_stack_tailcallinfo_offset() */
> + int tailcallinfo_offset = BPF_PPC_TAILCALL;
> + /*
> + * func_frame_offset = ...(1)
> + * bpf_dummy_frame_size + trampoline_frame_size
> + */
> + EMIT(PPC_RAW_LD(_R4, _R1, func_frame_offset));
> + EMIT(PPC_RAW_LD(_R3, _R4, -tailcallinfo_offset));
^^^^

Should this LD instruction be an ADDI instead? Looking at the stack frame
setup, after the two STLU instructions in __arch_prepare_bpf_trampoline():

STLU _R1, _R1, -bpf_dummy_frame_size -> stores original _R1 at dummy frame
STLU _R1, _R1, -bpf_frame_size -> stores dummy frame ptr at tramp frame

From the trampoline frame, the back chain stored in the dummy frame at offset
bpf_frame_size points to the caller's frame. But func_frame_offset equals
(bpf_dummy_frame_size + bpf_frame_size), which points into the caller's frame
itself. Loading from that address returns the caller's back chain (pointing
to the grandparent frame), not a pointer to the caller's frame.

This appears to cause the subsequent load at [_R4 - 8] to read the grandparent's
tail_call_info rather than the caller's. Would loading from [_R1 + bpf_frame_size]
give the correct pointer to the caller's frame?

For comparison, the subprogram prologue in bpf_jit_build_prologue() uses
[_R1 + 0] to get the back chain pointing to its caller, which seems like
the right pattern.

> +
> + /*
> + * Setting the tail_call_info in trampoline's frame
> + * depending on if previous frame had value or reference.
> + */
> + EMIT(PPC_RAW_CMPLWI(_R3, MAX_TAIL_CALL_CNT));
> + PPC_COND_BRANCH(COND_GT, CTX_NIA(ctx) + 8);
> + EMIT(PPC_RAW_ADDI(_R3, _R4, bpf_jit_stack_tailcallinfo_offset(ctx)));

[ ... ]

---
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CI run summary: https://github.com/kernel-patches/bpf/actions/runs/20993216190