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

From: adubey <[email-masked]>
Date: Sun, 22 Feb 2026 18:37:32 +0530

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.

> 
> Fixes: 2ed2d8f6fb38 ("powerpc64/bpf: Support tailcalls with subprogs")
> Signed-off-by: Hari Bathini <[email-masked]>
> ---
> 
> Changes since v1:
> * Fixed spelling error in changelog.
> * Fixed a comment in bpf_trampoline_setup_tail_call_info().
> 
> 
>  arch/powerpc/net/bpf_jit.h        |  5 -----
>  arch/powerpc/net/bpf_jit_comp.c   | 12 +++++-------
>  arch/powerpc/net/bpf_jit_comp64.c |  5 ++++-
>  3 files changed, 9 insertions(+), 13 deletions(-)
> 
> diff --git a/arch/powerpc/net/bpf_jit.h b/arch/powerpc/net/bpf_jit.h
> index 82bbf63f0e57..7354e1d72f79 100644
> --- a/arch/powerpc/net/bpf_jit.h
> +++ b/arch/powerpc/net/bpf_jit.h
> @@ -81,9 +81,6 @@
> 
>  #ifdef CONFIG_PPC64
> 
> -/* for gpr non volatile registers BPG_REG_6 to 10 */
> -#define BPF_PPC_STACK_SAVE	(6 * 8)
> -
>  /* If dummy pass (!image), account for maximum possible instructions 
> */
>  #define PPC_LI64(d, i)		do {					      \
>  	if (!image)							      \
> @@ -219,8 +216,6 @@ int bpf_jit_emit_exit_insn(u32 *image, struct
> codegen_context *ctx, int tmp_reg,
>  int bpf_add_extable_entry(struct bpf_prog *fp, u32 *image, u32
> *fimage, int pass,
>  			  struct codegen_context *ctx, int insn_idx,
>  			  int jmp_off, int dst_reg, u32 code);
> -
> -int bpf_jit_stack_tailcallinfo_offset(struct codegen_context *ctx);
>  #endif
> 
>  #endif
> diff --git a/arch/powerpc/net/bpf_jit_comp.c 
> b/arch/powerpc/net/bpf_jit_comp.c
> index fb6cc1f832a8..860b118391ed 100644
> --- a/arch/powerpc/net/bpf_jit_comp.c
> +++ b/arch/powerpc/net/bpf_jit_comp.c
> @@ -642,15 +642,13 @@ static void
> bpf_trampoline_setup_tail_call_info(u32 *image, struct codegen_conte
>  						int bpf_dummy_frame_size, int r4_off)
>  {
>  	if (IS_ENABLED(CONFIG_PPC64)) {
> -		/* See Generated stack layout */
> -		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));
> +		/* Refer to trampoline's Generated stack layout */
> +		EMIT(PPC_RAW_LD(_R3, _R4, -BPF_PPC_TAILCALL));
> 
>  		/*
>  		 * Setting the tail_call_info in trampoline's frame
> @@ -658,7 +656,7 @@ static void
> bpf_trampoline_setup_tail_call_info(u32 *image, struct codegen_conte
>  		 */
>  		EMIT(PPC_RAW_CMPLWI(_R3, MAX_TAIL_CALL_CNT));
>  		PPC_BCC_CONST_SHORT(COND_GT, 8);
> -		EMIT(PPC_RAW_ADDI(_R3, _R4, 
> bpf_jit_stack_tailcallinfo_offset(ctx)));
> +		EMIT(PPC_RAW_ADDI(_R3, _R4, -BPF_PPC_TAILCALL));
>  		/*
>  		 * From ...(1) above:
>  		 * trampoline_frame_bottom =                            ...(2)
> @@ -666,14 +664,14 @@ static void
> bpf_trampoline_setup_tail_call_info(u32 *image, struct codegen_conte
>  		 *
>  		 * Using ...(2) derived above:
>  		 * trampoline_tail_call_info_offset =                  ...(3)
> -		 *      trampoline_frame_bottom - tailcallinfo_offset
> +		 *      trampoline_frame_bottom - BPF_PPC_TAILCALL
>  		 *
>  		 * From ...(3):
>  		 * Use trampoline_tail_call_info_offset to write reference of main's
>  		 * tail_call_info in trampoline frame.
>  		 */
>  		EMIT(PPC_RAW_STL(_R3, _R1, (func_frame_offset - 
> bpf_dummy_frame_size)
> -								- tailcallinfo_offset));
> +								- BPF_PPC_TAILCALL));
>  	} else {
>  		/* See bpf_jit_stack_offsetof() and BPF_PPC_TC */
>  		EMIT(PPC_RAW_LL(_R4, _R1, r4_off));
> diff --git a/arch/powerpc/net/bpf_jit_comp64.c
> b/arch/powerpc/net/bpf_jit_comp64.c
> index 44ce8a8783f9..5d4d2bb23cef 100644
> --- a/arch/powerpc/net/bpf_jit_comp64.c
> +++ b/arch/powerpc/net/bpf_jit_comp64.c
> @@ -42,6 +42,9 @@
>   * exception boundary.
>   */
> 
> +/* BPF non-volatile registers save area size */
> +#define BPF_PPC_STACK_SAVE	(6*8)
Please maintain space beside *, suggested by Christophe in tailcall 
reviews
> +
>  /* for bpf JIT code internal usage */
>  #define BPF_PPC_STACK_LOCALS	24
>  /*
> @@ -148,7 +151,7 @@ static int bpf_jit_stack_local(struct 
> codegen_context *ctx)
>  	}
>  }
> 
> -int bpf_jit_stack_tailcallinfo_offset(struct codegen_context *ctx)
> +static int bpf_jit_stack_tailcallinfo_offset(struct codegen_context 
> *ctx)
>  {
>  	return bpf_jit_stack_local(ctx) + BPF_PPC_STACK_LOCALS + 
> BPF_PPC_STACK_SAVE;
>  }
-Abhishek