Re: [PATCH v13 5/8] powerpc/bpf: Move out dummy_tramp_addr after Long branch stub

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

On 31/08/26 12:46 pm, Saket Kumar Bhaskar wrote:
> From: Abhishek Dubey <adubey@linux.ibm.com>
> 
> Move the long branch address field to the bottom of the long
> branch stub. This allows uninterrupted disassembly until the
> last 4/8 bytes. The last bytes exclusion is logically necessary to
> prevent disassembly failure, otherwise the actual program layout
> is never altered. Hence no effect on overall program size.
> Also, align dummy_tramp_addr field with 8-byte boundary.
> 
> Following is disassembler output for test program with moved down
> dummy_tramp_addr field:
> .....
> .....
> pc:68    left:44     a6 03 08 7c  :  mtlr 0
> pc:72    left:40     bc ff ff 4b  :  b .-68
> pc:76    left:36     a6 02 68 7d  :  mflr 11
> pc:80    left:32     05 00 9f 42  :  bcl 20, 31, .+4
> pc:84    left:28     a6 02 88 7d  :  mflr 12
> pc:88    left:24     14 00 8c e9  :  ld 12, 20(12)
> pc:92    left:20     a6 03 89 7d  :  mtctr 12
> pc:96    left:16     a6 03 68 7d  :  mtlr 11
> pc:100   left:12     20 04 80 4e  :  bctr
> pc:104   left:8      c0 34 1d 00  :
> 
> Failure log:
> Can't disasm instruction at offset 104: c0 34 1d 00 00 00 00 c0
> Disassembly logic can truncate at 104, ignoring last 8 bytes.
> 
> Update the dummy_tramp_addr field offset calculation from the end
> of the program to reflect its new location, for bpf_arch_text_poke()
> to update the actual trampoline's address in this field.
> 
> All BPF trampoline selftests continue to pass with this patch applied.
> 

This makes it easy to disassemble the bpf programs. Looks good.

Reviewed-by: Hari Bathini <hbathini@linux.ibm.com>

> Signed-off-by: Abhishek Dubey <adubey@linux.ibm.com>
> Signed-off-by: Saket Kumar Bhaskar <skb99@linux.ibm.com>
> Tested-by: Yeswanth Krishna Tellakula <yeswanth@linux.ibm.com>
> ---
>   arch/powerpc/net/bpf_jit_comp.c | 44 ++++++++++++++++++---------------
>   1 file changed, 24 insertions(+), 20 deletions(-)
> 
> diff --git a/arch/powerpc/net/bpf_jit_comp.c b/arch/powerpc/net/bpf_jit_comp.c
> index 8ca36a933c7a..c2717f0d9cdd 100644
> --- a/arch/powerpc/net/bpf_jit_comp.c
> +++ b/arch/powerpc/net/bpf_jit_comp.c
> @@ -52,19 +52,19 @@ asm (
>   void bpf_jit_build_fentry_stubs(u32 *image, u32 *fimage, struct codegen_context *ctx)
>   {
>   	int ool_stub_idx, long_branch_stub_idx;
> -	int ool_stub_sz;
> +	int stub_sz;
>   
>   	/*
> +	 * The dummy_tramp_addr field is placed at bottom of Long branch stub.
>   	 * Align the mis-aligned dummy_tramp_addr field in the fimage.
>   	 * The alignment NOP must appear before OOL stub, to make
>   	 * ool_stub_idx & long_branch_stub_idx constant from end.
>   	 *
>   	 * The fimage can be non 8-byte aligned, so final alignment depends
> -	 * on start of fimage and the stub's instruction count offset. The
> -	 * OOL stub size is 4 instructions (with CONFIG_PPC_FTRACE_OUT_OF_LINE)
> -	 * or 3 instructions (without) before dummy_tramp_addr.
> -	 *
> -	 * Emit a NOP here if address is not SZL aligned.
> +	 * on start of fimage and the stub's instruction count. The
> +	 * stubs block has 11 instructions (with CONFIG_PPC_FTRACE_OUT_OF_LINE)
> +	 * or 10 instructions (without) before dummy_tramp_addr field. Emit a
> +	 * NOP if the address of dummy_tramp_addr is non aligned.
>   	 *
>   	 * In pass=0 when image==NULL, conservatively account for space
>   	 * required to accommodate alignment NOP. In case final pass skips
> @@ -72,8 +72,8 @@ void bpf_jit_build_fentry_stubs(u32 *image, u32 *fimage, struct codegen_context
>   	 * jited_len signifies correct program size.
>   	 */
>   
> -	ool_stub_sz = IS_ENABLED(CONFIG_PPC_FTRACE_OUT_OF_LINE) ? 16 : 12;
> -	if (!image || !IS_ALIGNED((unsigned long)fimage + ctx->idx*4 + ool_stub_sz, SZL))
> +	stub_sz = IS_ENABLED(CONFIG_PPC_FTRACE_OUT_OF_LINE) ? 44 : 40;
> +	if (!image || !IS_ALIGNED((unsigned long)fimage + ctx->idx*4 + stub_sz, SZL))
>   		EMIT(PPC_RAW_NOP());
>   
>   	/*
> @@ -94,28 +94,29 @@ void bpf_jit_build_fentry_stubs(u32 *image, u32 *fimage, struct codegen_context
>   
>   	/*
>   	 * Long branch stub:
> -	 *	.long	<dummy_tramp_addr>  // 8-byte aligned
>   	 *	mflr	r11
>   	 *	bcl	20,31,$+4
> -	 *	mflr	r12
> -	 *	ld	r12, -8-SZL(r12)
> +	 *	mflr	r12	// lr/r12 stores pc of current(this) inst.
> +	 *	ld	r12, 20(r12) // offset(dummy_tramp_addr) from prev inst. is 20
>   	 *	mtctr	r12
> -	 *	mtlr	r11 // needed to retain ftrace ABI
> +	 *	mtlr	r11	// needed to retain ftrace ABI
>   	 *	bctr
> +	 *	.long	<dummy_tramp_addr>  // SZL bytes aligned
>   	 */
> -	if (image)
> -		*((unsigned long *)&image[ctx->idx]) = (unsigned long)dummy_tramp;
> -
> -	ctx->idx += SZL / 4;
>   	long_branch_stub_idx = ctx->idx;
>   	EMIT(PPC_RAW_MFLR(_R11));
>   	EMIT(PPC_RAW_BCL4());
>   	EMIT(PPC_RAW_MFLR(_R12));
> -	EMIT(PPC_RAW_LL(_R12, _R12, -8-SZL));
> +	EMIT(PPC_RAW_LL(_R12, _R12, 20));
>   	EMIT(PPC_RAW_MTCTR(_R12));
>   	EMIT(PPC_RAW_MTLR(_R11));
>   	EMIT(PPC_RAW_BCTR());
>   
> +	if (image)
> +		*((unsigned long *)&image[ctx->idx]) = (unsigned long)dummy_tramp;
> +
> +	ctx->idx += SZL / 4;
> +
>   	if (!bpf_jit_ool_stub) {
>   		bpf_jit_ool_stub = (ctx->idx - ool_stub_idx) * 4;
>   		bpf_jit_long_branch_stub = (ctx->idx - long_branch_stub_idx) * 4;
> @@ -1309,6 +1310,7 @@ static void do_isync(void *info __maybe_unused)
>    * bpf_func:
>    *	[nop|b]	ool_stub
>    * 2. Out-of-line stub:
> + *	nop	// optional nop for alignment
>    * ool_stub:
>    *	mflr	r0
>    *	[b|bl]	<bpf_prog>/<long_branch_stub>
> @@ -1316,14 +1318,14 @@ static void do_isync(void *info __maybe_unused)
>    *	b	bpf_func + 4
>    * 3. Long branch stub:
>    * long_branch_stub:
> - *	.long	<branch_addr>/<dummy_tramp>
>    *	mflr	r11
>    *	bcl	20,31,$+4
>    *	mflr	r12
> - *	ld	r12, -16(r12)
> + *	ld	r12, 20(r12)
>    *	mtctr	r12
>    *	mtlr	r11 // needed to retain ftrace ABI
>    *	bctr
> + *	.long	<branch_addr>/<dummy_tramp>
>    *
>    * dummy_tramp is used to reduce synchronization requirements.
>    *
> @@ -1425,10 +1427,12 @@ int bpf_arch_text_poke(void *ip, enum bpf_text_poke_type old_t,
>   	 * 1. Update the address in the long branch stub:
>   	 * If new_addr is out of range, we will have to use the long branch stub, so patch new_addr
>   	 * here. Otherwise, revert to dummy_tramp, but only if we had patched old_addr here.
> +	 *
> +	 * dummy_tramp_addr moved to bottom of long branch stub.
>   	 */
>   	if ((new_addr && !is_offset_in_branch_range(new_addr - ip)) ||
>   	    (old_addr && !is_offset_in_branch_range(old_addr - ip)))
> -		ret = patch_ulong((void *)(bpf_func_end - bpf_jit_long_branch_stub - SZL),
> +		ret = patch_ulong((void *)(bpf_func_end - SZL), /* SZL: dummy_tramp_addr offset */
>   				  (new_addr && !is_offset_in_branch_range(new_addr - ip)) ?
>   				  (unsigned long)new_addr : (unsigned long)dummy_tramp);
>   	if (ret)