Re: [PATCH bpf-next 1/5] arm64: ftrace: Add ftrace direct call support

[Song Liu]

> On Apr 12, 2022, at 10:49 PM, Xu Kuohai <xukuohai@xxxxxxxxxx> wrote:
> 
> Add ftrace direct support for arm64.
> 
> 1. When there is custom trampoline only, replace the fentry nop to a
>   jump instruction that jumps directly to the custom trampoline.
> 
> 2. When ftrace trampoline and custome coexist, jump from fentry to
>   ftrace trampoline first, then jump to custom trampoline when ftrace
>   trampoline exits. The currently unused register pt_regs->x0 is used
>   as an intermediary for jumping from ftrace trampoline to custom
>   trampoline.
> 
> Signed-off-by: Xu Kuohai <xukuohai@xxxxxxxxxx>

For the series: 

Acked-by: Song Liu <songliubraving@xxxxxx>

(Pardon my laziness, I somehow only got 1/5 and 3/5 in one of my 
inboxes, and nothing in my other inbox. :( ) 

Just one nitpick for 2/5: as we move is_valid_bpf_tramp_flags to
trampoline.c, we should change the multi-line comment into net
style:

	/* BPF_TRAMP_F_RET_FENTRY_RET is only used by bpf_struct_ops,
	 * and it must be used alone.
	 */

Thanks,
Song

> ---
> arch/arm64/Kconfig               |  2 ++
> arch/arm64/include/asm/ftrace.h  | 10 ++++++++++
> arch/arm64/kernel/asm-offsets.c  |  1 +
> arch/arm64/kernel/entry-ftrace.S | 18 +++++++++++++++---
> 4 files changed, 28 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 57c4c995965f..81cc330daafc 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -177,6 +177,8 @@ config ARM64
> 	select HAVE_DYNAMIC_FTRACE
> 	select HAVE_DYNAMIC_FTRACE_WITH_REGS \
> 		if $(cc-option,-fpatchable-function-entry=2)
> +	select HAVE_DYNAMIC_FTRACE_WITH_DIRECT_CALLS \
> +		if DYNAMIC_FTRACE_WITH_REGS
> 	select FTRACE_MCOUNT_USE_PATCHABLE_FUNCTION_ENTRY \
> 		if DYNAMIC_FTRACE_WITH_REGS
> 	select HAVE_EFFICIENT_UNALIGNED_ACCESS
> diff --git a/arch/arm64/include/asm/ftrace.h b/arch/arm64/include/asm/ftrace.h
> index 1494cfa8639b..3a363d6a3bd0 100644
> --- a/arch/arm64/include/asm/ftrace.h
> +++ b/arch/arm64/include/asm/ftrace.h
> @@ -78,6 +78,16 @@ static inline unsigned long ftrace_call_adjust(unsigned long addr)
> 	return addr;
> }
> 
> +static inline void arch_ftrace_set_direct_caller(struct pt_regs *regs,
> +						 unsigned long addr)
> +{
> +	/*
> +	 * Place custom trampoline address in regs->orig_x0 to let ftrace
> +	 * trampoline jump to it.
> +	 */
> +	regs->orig_x0 = addr;
> +}
> +
> #ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
> struct dyn_ftrace;
> int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec);
> diff --git a/arch/arm64/kernel/asm-offsets.c b/arch/arm64/kernel/asm-offsets.c
> index 1197e7679882..b1ed0bf01c59 100644
> --- a/arch/arm64/kernel/asm-offsets.c
> +++ b/arch/arm64/kernel/asm-offsets.c
> @@ -80,6 +80,7 @@ int main(void)
>   DEFINE(S_SDEI_TTBR1,		offsetof(struct pt_regs, sdei_ttbr1));
>   DEFINE(S_PMR_SAVE,		offsetof(struct pt_regs, pmr_save));
>   DEFINE(S_STACKFRAME,		offsetof(struct pt_regs, stackframe));
> +  DEFINE(S_ORIG_X0,		offsetof(struct pt_regs, orig_x0));
>   DEFINE(PT_REGS_SIZE,		sizeof(struct pt_regs));
>   BLANK();
> #ifdef CONFIG_COMPAT
> diff --git a/arch/arm64/kernel/entry-ftrace.S b/arch/arm64/kernel/entry-ftrace.S
> index e535480a4069..b1bd6576f205 100644
> --- a/arch/arm64/kernel/entry-ftrace.S
> +++ b/arch/arm64/kernel/entry-ftrace.S
> @@ -60,6 +60,9 @@
> 	str	x29, [sp, #S_FP]
> 	.endif
> 
> +	/* Set orig_x0 to zero  */
> +	str     xzr, [sp, #S_ORIG_X0]
> +
> 	/* Save the callsite's SP and LR */
> 	add	x10, sp, #(PT_REGS_SIZE + 16)
> 	stp	x9, x10, [sp, #S_LR]
> @@ -119,12 +122,21 @@ ftrace_common_return:
> 	/* Restore the callsite's FP, LR, PC */
> 	ldr	x29, [sp, #S_FP]
> 	ldr	x30, [sp, #S_LR]
> -	ldr	x9, [sp, #S_PC]
> -
> +	ldr	x10, [sp, #S_PC]
> +
> +	ldr	x11, [sp, #S_ORIG_X0]
> +	cbz	x11, 1f
> +	/* Set x9 to parent ip before jump to bpf trampoline */
> +	mov	x9,  x30
> +	/* Set lr to self ip */
> +	ldr	x30, [sp, #S_PC]
> +	/* Set x10 (used for return address) to bpf trampoline */
> +	mov	x10, x11
> +1:
> 	/* Restore the callsite's SP */
> 	add	sp, sp, #PT_REGS_SIZE + 16
> 
> -	ret	x9
> +	ret	x10
> SYM_CODE_END(ftrace_common)
> 
> #ifdef CONFIG_FUNCTION_GRAPH_TRACER
> -- 
> 2.30.2
>