Re: [PATCH 3/5] LoongArch: ftrace: Add direct call support

[Youling Tang <tangyouling@loongson.cn>]

Hi, Mark

On 2023/4/26 21:43, Mark Rutland wrote:
> Hi,
>
> On Wed, Apr 26, 2023 at 09:49:12AM +0800, Youling Tang wrote:
>> select the DYNAMIC_FTRACE_WITH_DIRECT_CALLS to provide the
>> register_ftrace_direct[_multi] interfaces allowing users to register
>> the customed trampoline (direct_caller) as the mcount for one or
>> more target functions. And modify_ftrace_direct[_multi] are also
>> provided for modifying direct_caller.
>>
>> There are a few cases to distinguish:
>> - If a direct call ops is the only one tracing a function:
>>    - If the direct called trampoline is within the reach of a 'bl'
>>      instruction
>>       -> the ftrace patchsite jumps to the trampoline
>>    - Else
>>       -> the ftrace patchsite jumps to the ftrace_regs_caller trampoline
>> 	which reads the ops pointer in the patchsite and jumps to the
>>   	direct call address stored in the ops
> I think you forgot to update this wording; there's no ops pointer in the
> patchsite as you don't implement DYNAMIC_FTRACE_WITH_CALL_OPS.
>
> I think you can delete the "Else" case here, and replace the above with:
>
> - If a direct call ops is the only one tracing a function AND the direct called
>    trampoline is within the reach of a 'bl' instruction
>    -> the ftrace patchsite jumps to the trampoline
>
>> - Else
>>    -> the ftrace patchsite jumps to the ftrace_regs_caller trampoline and
>>       its ops literal points to ftrace_list_ops so it iterates over all
>>       registered ftrace ops, including the direct call ops and calls its
>>       call_direct_funcs handler which stores the direct called trampoline's
>>       address in the ftrace_regs and the ftrace_regs_caller trampoline will
> Likewise here, there's no "ops literal" associated with the patchsite.

Yes, thank you for pointing out, I will fix this description.


Thanks,

Youling.

>
> Otherwise, this looks sane to me.
>
> Mark.
>
>>       return to that address instead of returning to the traced function
>>
>> Signed-off-by: Qing Zhang <zhangqing@loongson.cn>
>> Signed-off-by: Youling Tang <tangyouling@loongson.cn>
>> ---
>>   arch/loongarch/Kconfig              |  1 +
>>   arch/loongarch/include/asm/ftrace.h | 12 ++++++++++++
>>   arch/loongarch/kernel/ftrace_dyn.c  |  8 ++++++++
>>   arch/loongarch/kernel/mcount_dyn.S  |  7 ++++++-
>>   4 files changed, 27 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/loongarch/Kconfig b/arch/loongarch/Kconfig
>> index 3ddde336e6a5..d6068a88d53f 100644
>> --- a/arch/loongarch/Kconfig
>> +++ b/arch/loongarch/Kconfig
>> @@ -92,6 +92,7 @@ config LOONGARCH
>>   	select HAVE_DMA_CONTIGUOUS
>>   	select HAVE_DYNAMIC_FTRACE
>>   	select HAVE_DYNAMIC_FTRACE_WITH_ARGS
>> +	select HAVE_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
>>   	select HAVE_DYNAMIC_FTRACE_WITH_REGS
>>   	select HAVE_EBPF_JIT
>>   	select HAVE_EFFICIENT_UNALIGNED_ACCESS if !ARCH_STRICT_ALIGN
>> diff --git a/arch/loongarch/include/asm/ftrace.h b/arch/loongarch/include/asm/ftrace.h
>> index 3418d32d4fc7..f789e680f633 100644
>> --- a/arch/loongarch/include/asm/ftrace.h
>> +++ b/arch/loongarch/include/asm/ftrace.h
>> @@ -57,6 +57,18 @@ static __always_inline struct pt_regs *arch_ftrace_get_regs(struct ftrace_regs *
>>   #define ftrace_graph_func ftrace_graph_func
>>   void ftrace_graph_func(unsigned long ip, unsigned long parent_ip,
>>   		       struct ftrace_ops *op, struct ftrace_regs *fregs);
>> +
>> +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
>> +static inline void
>> +__arch_ftrace_set_direct_caller(struct pt_regs *regs, unsigned long addr)
>> +{
>> +	regs->regs[13] = addr;	/* t1 */
>> +}
>> +
>> +#define arch_ftrace_set_direct_caller(fregs, addr) \
>> +	__arch_ftrace_set_direct_caller(&(fregs)->regs, addr)
>> +#endif /* CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS */
>> +
>>   #endif
>>   
>>   #endif /* __ASSEMBLY__ */
>> diff --git a/arch/loongarch/kernel/ftrace_dyn.c b/arch/loongarch/kernel/ftrace_dyn.c
>> index 3cc4f8159f48..4062a7e63137 100644
>> --- a/arch/loongarch/kernel/ftrace_dyn.c
>> +++ b/arch/loongarch/kernel/ftrace_dyn.c
>> @@ -66,6 +66,14 @@ static bool ftrace_find_callable_addr(struct dyn_ftrace *rec, struct module *mod
>>   	unsigned long pc = rec->ip + LOONGARCH_INSN_SIZE;
>>   	struct plt_entry *plt;
>>   
>> +	/*
>> +	 * If a custom trampoline is unreachable, rely on the ftrace_regs_caller
>> +	 * trampoline which knows how to indirectly reach that trampoline
>> +	 * through ops->direct_call.
>> +	 */
>> +	if (*addr != FTRACE_ADDR && *addr != FTRACE_REGS_ADDR && !reachable_by_bl(*addr, pc))
>> +		*addr = FTRACE_REGS_ADDR;
>> +
>>   	/*
>>   	 * When the target is within range of the 'bl' instruction, use 'addr'
>>   	 * as-is and branch to that directly.
>> diff --git a/arch/loongarch/kernel/mcount_dyn.S b/arch/loongarch/kernel/mcount_dyn.S
>> index bbabf06244c2..1008fbc3cabc 100644
>> --- a/arch/loongarch/kernel/mcount_dyn.S
>> +++ b/arch/loongarch/kernel/mcount_dyn.S
>> @@ -42,7 +42,6 @@
>>   	.if \allregs
>>   	PTR_S		tp, sp, PT_R2
>>   	PTR_S		t0, sp, PT_R12
>> -	PTR_S		t1, sp, PT_R13
>>   	PTR_S		t2, sp, PT_R14
>>   	PTR_S		t3, sp, PT_R15
>>   	PTR_S		t4, sp, PT_R16
>> @@ -64,6 +63,8 @@
>>   	PTR_S		zero, sp, PT_R0
>>   	.endif
>>   	PTR_S		ra, sp, PT_ERA /* Save trace function ra at PT_ERA */
>> +	move		t1, zero
>> +	PTR_S		t1, sp, PT_R13
>>   	PTR_ADDI	t8, sp, PT_SIZE
>>   	PTR_S		t8, sp, PT_R3
>>   	.endm
>> @@ -104,8 +105,12 @@ ftrace_common_return:
>>   	PTR_L		a7, sp, PT_R11
>>   	PTR_L		fp, sp, PT_R22
>>   	PTR_L		t0, sp, PT_ERA
>> +	PTR_L		t1, sp, PT_R13
>>   	PTR_ADDI	sp, sp, PT_SIZE
>> +	bnez		t1,.Ldirect
>>   	jr		t0
>> +.Ldirect:
>> +	jr		t1
>>   SYM_CODE_END(ftrace_common)
>>   
>>   SYM_CODE_START(ftrace_caller)
>> -- 
>> 2.37.1
>>