Re: [PATCH v2 5/6] powerpc64/bpf: Support exceptions

[Hari Bathini <hbathini@linux.ibm.com>]

On 14/01/26 5:14 pm, adubey@linux.ibm.com wrote:
> From: Abhishek Dubey <adubey@linux.ibm.com>
> 
> The modified prologue/epilogue generation code now
> enables exception-callback to use the stack frame of
> the program marked as exception boundary, where callee
> saved registers are stored.
> 
> As per ppc64 ABIv2 documentation[1], r14-r31 are callee
> saved registers. BPF programs on ppc64 already saves
> r26-r31 registers. Saving the remaining set of callee
> saved registers(r14-r25) is handled in the next patch.
> 
> [1] https://ftp.rtems.org/pub/rtems/people/sebh/ABI64BitOpenPOWERv1.1_16July2015_pub.pdf
> 
> Signed-off-by: Abhishek Dubey <adubey@linux.ibm.com>
> ---
>   arch/powerpc/net/bpf_jit.h        |  2 ++
>   arch/powerpc/net/bpf_jit_comp.c   |  7 ++++
>   arch/powerpc/net/bpf_jit_comp64.c | 53 +++++++++++++++++++++----------
>   3 files changed, 45 insertions(+), 17 deletions(-)
> 
> diff --git a/arch/powerpc/net/bpf_jit.h b/arch/powerpc/net/bpf_jit.h
> index 5d735bc5e6bd..fb548ae5d143 100644
> --- a/arch/powerpc/net/bpf_jit.h
> +++ b/arch/powerpc/net/bpf_jit.h
> @@ -179,6 +179,8 @@ struct codegen_context {
>   	u64 arena_vm_start;
>   	u64 user_vm_start;
>   	bool is_subprog;
> +	bool exception_boundary;
> +	bool exception_cb;
>   };
>   
>   #define bpf_to_ppc(r)	(ctx->b2p[r])
> diff --git a/arch/powerpc/net/bpf_jit_comp.c b/arch/powerpc/net/bpf_jit_comp.c
> index e3088cf089d1..26991940d36e 100644
> --- a/arch/powerpc/net/bpf_jit_comp.c
> +++ b/arch/powerpc/net/bpf_jit_comp.c
> @@ -207,6 +207,8 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *fp)
>   	cgctx.arena_vm_start = bpf_arena_get_kern_vm_start(fp->aux->arena);
>   	cgctx.user_vm_start = bpf_arena_get_user_vm_start(fp->aux->arena);
>   	cgctx.is_subprog = bpf_is_subprog(fp);
> +	cgctx.exception_boundary = fp->aux->exception_boundary;
> +	cgctx.exception_cb = fp->aux->exception_cb;
>   
>   	/* Scouting faux-generate pass 0 */
>   	if (bpf_jit_build_body(fp, NULL, NULL, &cgctx, addrs, 0, false)) {
> @@ -436,6 +438,11 @@ void bpf_jit_free(struct bpf_prog *fp)
>   	bpf_prog_unlock_free(fp);
>   }
>   
> +bool bpf_jit_supports_exceptions(void)
> +{
> +	return IS_ENABLED(CONFIG_PPC64);
> +}
> +
>   bool bpf_jit_supports_subprog_tailcalls(void)
>   {
>   	return IS_ENABLED(CONFIG_PPC64);
> diff --git a/arch/powerpc/net/bpf_jit_comp64.c b/arch/powerpc/net/bpf_jit_comp64.c
> index ec58395f74f7..a6083dd9786c 100644
> --- a/arch/powerpc/net/bpf_jit_comp64.c
> +++ b/arch/powerpc/net/bpf_jit_comp64.c
> @@ -89,7 +89,9 @@ static inline bool bpf_has_stack_frame(struct codegen_context *ctx)
>   	 * - the bpf program uses its stack area
>   	 * The latter condition is deduced from the usage of BPF_REG_FP
>   	 */
> -	return ctx->seen & SEEN_FUNC || bpf_is_seen_register(ctx, bpf_to_ppc(BPF_REG_FP));
> +	return ctx->seen & SEEN_FUNC ||
> +	       bpf_is_seen_register(ctx, bpf_to_ppc(BPF_REG_FP)) ||
> +	       ctx->exception_cb;
>   }
>   
>   /*
> @@ -190,23 +192,32 @@ void bpf_jit_build_prologue(u32 *image, struct codegen_context *ctx)
>   		EMIT(PPC_RAW_STDU(_R1, _R1, -(BPF_PPC_STACKFRAME + ctx->stack_size)));
>   	}
>   
> -	/*
> -	 * Back up non-volatile regs -- BPF registers 6-10
> -	 * If we haven't created our own stack frame, we save these
> -	 * in the protected zone below the previous stack frame
> -	 */
> -	for (i = BPF_REG_6; i <= BPF_REG_10; i++)
> -		if (bpf_is_seen_register(ctx, bpf_to_ppc(i)))
> -			EMIT(PPC_RAW_STD(bpf_to_ppc(i), _R1, bpf_jit_stack_offsetof(ctx, bpf_to_ppc(i))));
> +	if (!ctx->exception_cb) {
> +		/*
> +		 * Back up non-volatile regs -- BPF registers 6-10
> +		 * If we haven't created our own stack frame, we save these
> +		 * in the protected zone below the previous stack frame
> +		 */
> +		for (i = BPF_REG_6; i <= BPF_REG_10; i++)
> +			if (ctx->exception_boundary || bpf_is_seen_register(ctx, bpf_to_ppc(i)))
> +				EMIT(PPC_RAW_STD(bpf_to_ppc(i), _R1,
> +					bpf_jit_stack_offsetof(ctx, bpf_to_ppc(i))));
>   
> -	if (ctx->arena_vm_start)
> -		EMIT(PPC_RAW_STD(bpf_to_ppc(ARENA_VM_START), _R1,
> +		if (ctx->exception_boundary || ctx->arena_vm_start)
> +			EMIT(PPC_RAW_STD(bpf_to_ppc(ARENA_VM_START), _R1,
>   				 bpf_jit_stack_offsetof(ctx, bpf_to_ppc(ARENA_VM_START))));
>   
> -	/* Setup frame pointer to point to the bpf stack area */
> -	if (bpf_is_seen_register(ctx, bpf_to_ppc(BPF_REG_FP)))
> -		EMIT(PPC_RAW_ADDI(bpf_to_ppc(BPF_REG_FP), _R1,
> +		/* Setup frame pointer to point to the bpf stack area */
> +		if (bpf_is_seen_register(ctx, bpf_to_ppc(BPF_REG_FP)))
> +			EMIT(PPC_RAW_ADDI(bpf_to_ppc(BPF_REG_FP), _R1,
>   				STACK_FRAME_MIN_SIZE + ctx->stack_size));
> +	} else {
> +		/*
> +		 * Exception callback receives Frame Pointer of main
> +		 * program as third arg
> +		 */
> +		EMIT(PPC_RAW_MR(_R1, _R5));
> +	}
>   
>   	if (ctx->arena_vm_start)
>   		PPC_LI64(bpf_to_ppc(ARENA_VM_START), ctx->arena_vm_start);
> @@ -218,17 +229,25 @@ static void bpf_jit_emit_common_epilogue(u32 *image, struct codegen_context *ctx
>   
>   	/* Restore NVRs */
>   	for (i = BPF_REG_6; i <= BPF_REG_10; i++)
> -		if (bpf_is_seen_register(ctx, bpf_to_ppc(i)))
> +		if (ctx->exception_cb || bpf_is_seen_register(ctx, bpf_to_ppc(i)))
>   			EMIT(PPC_RAW_LD(bpf_to_ppc(i), _R1, bpf_jit_stack_offsetof(ctx, bpf_to_ppc(i))));
>   
> -	if (ctx->arena_vm_start)
> +	if (ctx->exception_cb || ctx->arena_vm_start)
>   		EMIT(PPC_RAW_LD(bpf_to_ppc(ARENA_VM_START), _R1,
>   				bpf_jit_stack_offsetof(ctx, bpf_to_ppc(ARENA_VM_START))));
>   

> +	if (ctx->exception_cb) {
> +		/*
> +		 * LR value from boundary-frame is received as second parameter
> +		 * in exception callback.
> +		 */
> +		EMIT(PPC_RAW_MTLR(_R4));
> +	}
> +

No. Both second and third parameter of exception_cb() are stack pointer
(or frame pointer, if you prefer that) and not LR.
The above hunk is wrong. It still worked because of mtlr
instruction below that restored LR from PPC_LR_STKOFF offset
on the stack. Please drop the above hunk.

>   	/* Tear down our stack frame */
>   	if (bpf_has_stack_frame(ctx)) {
>   		EMIT(PPC_RAW_ADDI(_R1, _R1, BPF_PPC_STACKFRAME + ctx->stack_size));
> -		if (ctx->seen & SEEN_FUNC) {
> +		if (ctx->seen & SEEN_FUNC || ctx->exception_cb) {
>   			EMIT(PPC_RAW_LD(_R0, _R1, PPC_LR_STKOFF));
>   			EMIT(PPC_RAW_MTLR(_R0));
>   		}

- Hari