Re: [PATCH v2 5/5] powerpc64/bpf: fix handling of BPF stack in exception callback

[adubey <adubey@linux.ibm.com>]

On 2026-02-20 12:09, Hari Bathini wrote:
> Exception callback reuses the stack frame of exception boundary. When
> exception boundary and exception callback programs have different BPF
> stack depth, the current stack unwind in exception callback will fail.
> Adjust the stack frame size of exception callback, in its prologue,
> if its BPF stack depth is different from that of exception boundary.
> 
> Reported-by: bot+bpf-ci@kernel.org
> Closes:
> https://lore.kernel.org/bpf/2a310e86a59eb4c44c3ac9e5647814469d9c955580c9c0f1b3d9ca4a44717a34@mail.kernel.org/
> Fixes: 11d45eee9f42 ("powerpc64/bpf: Additional NVR handling for 
> bpf_throw")
> Signed-off-by: Hari Bathini <hbathini@linux.ibm.com>
> ---
> 
> Changes since v1:
> * Fixed incorrect use of PPC_RAW_SUB() as pointed out by
>   bot+bpf-ci@kernel.org.
> 
> 
>  arch/powerpc/net/bpf_jit_comp64.c | 30 ++++++++++++++++++++++--------
>  1 file changed, 22 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/powerpc/net/bpf_jit_comp64.c
> b/arch/powerpc/net/bpf_jit_comp64.c
> index 5d4d2bb23cef..33b2a7fd9067 100644
> --- a/arch/powerpc/net/bpf_jit_comp64.c
> +++ b/arch/powerpc/net/bpf_jit_comp64.c
> @@ -32,14 +32,15 @@
>   *
>   *		[	prev sp		] <-------------
>   *		[    tail_call_info	] 8		|
> - *		[   nv gpr save area	] 6*8 + (12*8)	|
> + *		[   nv gpr save area	] 6*8		|
> + *		[ addl. nv gpr save area] (12*8)	| <--- exception 
> boundary/callback program
>   *		[    local_tmp_var	] 24		|
>   * fp (r31) -->	[   ebpf stack space	] upto 512	|
>   *		[     frame header	] 32/112	|
>   * sp (r1) --->	[    stack pointer	] --------------
>   *
>   * Additional (12*8) in 'nv gpr save area' only in case of
> - * exception boundary.
> + * exception boundary/callback.
>   */
> 
>  /* BPF non-volatile registers save area size */
> @@ -128,12 +129,13 @@ static inline bool bpf_has_stack_frame(struct
> codegen_context *ctx)
>   *		[	  ...       	] 		|
>   * sp (r1) --->	[    stack pointer	] --------------
>   *		[    tail_call_info	] 8
> - *		[   nv gpr save area	] 6*8 + (12*8)
> + *		[   nv gpr save area	] 6*8
> + *		[ addl. nv gpr save area] (12*8) <--- exception boundary/callback 
> program
>   *		[    local_tmp_var	] 24
>   *		[   unused red zone	] 224
>   *
>   * Additional (12*8) in 'nv gpr save area' only in case of
> - * exception boundary.
> + * exception boundary/callback.
>   */
>  static int bpf_jit_stack_local(struct codegen_context *ctx)
>  {
> @@ -240,10 +242,6 @@ void bpf_jit_build_prologue(u32 *image, struct
> codegen_context *ctx)
> 
>  	if (bpf_has_stack_frame(ctx) && !ctx->exception_cb) {
>  		/*
> -		 * exception_cb uses boundary frame after stack walk.
> -		 * It can simply use redzone, this optimization reduces
> -		 * stack walk loop by one level.
> -		 *
>  		 * We need a stack frame, but we don't necessarily need to
>  		 * save/restore LR unless we call other functions
>  		 */
> @@ -287,6 +285,22 @@ void bpf_jit_build_prologue(u32 *image, struct
> codegen_context *ctx)
>  		 * program(main prog) as third arg
>  		 */
>  		EMIT(PPC_RAW_MR(_R1, _R5));
> +		/*
> +		 * Exception callback reuses the stack frame of exception boundary.
> +		 * But BPF stack depth of exception callback and exception boundary
> +		 * don't have to be same. If BPF stack depth is different, adjust 
> the
> +		 * stack frame size considering BPF stack depth of exception 
> callback.
> +		 * The non-volatile register save area remains unchanged. These non-
> +		 * volatile registers are restored in exception callback's epilogue.
> +		 */
> +		EMIT(PPC_RAW_LD(bpf_to_ppc(TMP_REG_1), _R5, 0));
> +		EMIT(PPC_RAW_SUB(bpf_to_ppc(TMP_REG_2), bpf_to_ppc(TMP_REG_1), 
> _R1));
> +		EMIT(PPC_RAW_ADDI(bpf_to_ppc(TMP_REG_2), bpf_to_ppc(TMP_REG_2),
> +			-BPF_PPC_EXC_STACKFRAME));
> +		EMIT(PPC_RAW_CMPLDI(bpf_to_ppc(TMP_REG_2), ctx->stack_size));
> +		PPC_BCC_CONST_SHORT(COND_EQ, 12);
> +		EMIT(PPC_RAW_MR(_R1, bpf_to_ppc(TMP_REG_1)));
> +		EMIT(PPC_RAW_STDU(_R1, _R1, -(BPF_PPC_EXC_STACKFRAME + 
> ctx->stack_size)));
For the last comment here:
"Do we need to setup FP again? If I get it right, still the FP is 
pointing to older frame, any reference in
callback will resolve to old frame."

Following inst takes care of above; it was missing for me in rebasing, 
my bad.
EMIT(PPC_RAW_ADDI(bpf_to_ppc(BPF_REG_FP), _R1,
                                 STACK_FRAME_MIN_SIZE + 
ctx->stack_size));

>  	}
> 
>  	/*
-Abhishek