Re: [PATCH v3 1/5] powerpc/bpf: fix alignment of long branch trampoline address

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

On 12/04/26 3:44 am, adubey@linux.ibm.com wrote:
> From: Abhishek Dubey <adubey@linux.ibm.com>
> 
> Ensure the dummy trampoline address field present between the OOL stub
> and the long branch stub is 8-byte aligned, for memory compatibility
> when content loaded to a register.
> 
> Reported-by: Hari Bathini <hbathini@linux.ibm.com>
> Fixes: d243b62b7bd3 ("powerpc64/bpf: Add support for bpf trampolines")
> Cc: stable@vger.kernel.org
> Signed-off-by: Abhishek Dubey <adubey@linux.ibm.com>
> ---
>   arch/powerpc/net/bpf_jit.h        |  4 ++--
>   arch/powerpc/net/bpf_jit_comp.c   | 34 ++++++++++++++++++++++++++-----
>   arch/powerpc/net/bpf_jit_comp64.c |  4 ++--
>   3 files changed, 33 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/powerpc/net/bpf_jit.h b/arch/powerpc/net/bpf_jit.h
> index 7354e1d72f79..1184ad15d5a4 100644
> --- a/arch/powerpc/net/bpf_jit.h
> +++ b/arch/powerpc/net/bpf_jit.h
> @@ -208,8 +208,8 @@ int bpf_jit_emit_func_call_rel(u32 *image, u32 *fimage, struct codegen_context *
>   int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, u32 *fimage, struct codegen_context *ctx,
>   		       u32 *addrs, int pass, bool extra_pass);
>   void bpf_jit_build_prologue(u32 *image, struct codegen_context *ctx);
> -void bpf_jit_build_epilogue(u32 *image, struct codegen_context *ctx);
> -void bpf_jit_build_fentry_stubs(u32 *image, struct codegen_context *ctx);
> +void bpf_jit_build_epilogue(u32 *image, u32 *fimage, struct codegen_context *ctx);
> +void bpf_jit_build_fentry_stubs(u32 *image, u32 *fimage, struct codegen_context *ctx);
>   void bpf_jit_realloc_regs(struct codegen_context *ctx);
>   int bpf_jit_emit_exit_insn(u32 *image, struct codegen_context *ctx, int tmp_reg, long exit_addr);
>   
> diff --git a/arch/powerpc/net/bpf_jit_comp.c b/arch/powerpc/net/bpf_jit_comp.c
> index a62a9a92b7b5..c255b30a37b0 100644
> --- a/arch/powerpc/net/bpf_jit_comp.c
> +++ b/arch/powerpc/net/bpf_jit_comp.c
> @@ -49,11 +49,34 @@ asm (
>   "	.popsection				;"
>   );
>   
> -void bpf_jit_build_fentry_stubs(u32 *image, struct codegen_context *ctx)
> +void bpf_jit_build_fentry_stubs(u32 *image, u32 *fimage, struct codegen_context *ctx)
>   {
>   	int ool_stub_idx, long_branch_stub_idx;
>   
>   	/*
> +	 * In the final pass, 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.
> +	 *
> +	 * Need alignment NOP in following conditions:
> +	 *

I don't think I follow this table..

> +	 * OOL stub aligned	CONFIG_PPC_FTRACE_OUT_OF_LINE	Alignment NOP
> +	 *      Y                               Y                     N

> +	 *      Y                               N                     Y

Please help me understand why a NOP is needed here

> +	 *      N                               Y                     Y

> +	 *      N                               N                     N

and also, why a NOP isn't needed here..

- Hari