Re: [PATCH v6 5/7] arm64: ftrace: Add dynamic ftrace support

[AKASHI Takahiro <takahiro.akashi@linaro.org>]

On 03/14/2014 03:10 AM, Will Deacon wrote:
> On Thu, Mar 13, 2014 at 10:13:48AM +0000, AKASHI Takahiro wrote:
>> This patch allows "dynamic ftrace" if CONFIG_DYNAMIC_FTRACE is enabled.
>> Here we can turn on and off tracing dynamically per-function base.
>>
>> On arm64, this is done by patching single branch instruction to _mcount()
>> inserted by gcc -pg option. The branch is replaced to NOP initially at
>> kernel start up, and later on, NOP to branch to ftrace_caller() when
>> enabled or branch to NOP when disabled.
>> Please note that ftrace_caller() is a counterpart of _mcount() in case of
>> 'static' ftrace.
>>
>> More details on architecture specific requirements are described in
>> Documentation/trace/ftrace-design.txt.
>>
>> Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
>> ---
>>   arch/arm64/Kconfig               |    1 +
>>   arch/arm64/include/asm/ftrace.h  |   15 +++++
>>   arch/arm64/kernel/entry-ftrace.S |   43 +++++++++++++++
>>   arch/arm64/kernel/ftrace.c       |  113 ++++++++++++++++++++++++++++++++++++++
>>   4 files changed, 172 insertions(+)
>>
>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>> index 6b3fef6..6954959 100644
>> --- a/arch/arm64/Kconfig
>> +++ b/arch/arm64/Kconfig
>> @@ -33,6 +33,7 @@ config ARM64
>>   	select HAVE_DMA_API_DEBUG
>>   	select HAVE_DMA_ATTRS
>>   	select HAVE_DMA_CONTIGUOUS
>> +	select HAVE_DYNAMIC_FTRACE
>>   	select HAVE_EFFICIENT_UNALIGNED_ACCESS
>>   	select HAVE_FTRACE_MCOUNT_RECORD
>>   	select HAVE_FUNCTION_TRACER
>> diff --git a/arch/arm64/include/asm/ftrace.h b/arch/arm64/include/asm/ftrace.h
>> index 58ea595..ed5c448 100644
>> --- a/arch/arm64/include/asm/ftrace.h
>> +++ b/arch/arm64/include/asm/ftrace.h
>> @@ -18,6 +18,21 @@
>>
>>   #ifndef __ASSEMBLY__
>>   extern void _mcount(unsigned long);
>> +
>> +struct dyn_arch_ftrace {
>> +	/* No extra data needed for arm64 */
>> +};
>> +
>> +extern unsigned long ftrace_graph_call;
>> +
>> +static inline unsigned long ftrace_call_adjust(unsigned long addr)
>> +{
>> +	/*
>> +	 * addr is the address of the mcount call instruction.
>> +	 * recordmcount does the necessary offset calculation.
>> +	 */
>> +	return addr;
>> +}
>
> You could just as easily implement this as a dummy macro, but I guess it
> doesn't matter either way.

FYI, all archs define this as an inline function.
Leave it as it is.

>>   #endif /* __ASSEMBLY__ */
>>
>>   #endif /* __ASM_FTRACE_H */
>> diff --git a/arch/arm64/kernel/entry-ftrace.S b/arch/arm64/kernel/entry-ftrace.S
>> index 0ac31c8..c0fbe10 100644
>> --- a/arch/arm64/kernel/entry-ftrace.S
>> +++ b/arch/arm64/kernel/entry-ftrace.S
>> @@ -86,6 +86,7 @@
>>   	add	\reg, \reg, #8
>>   	.endm
>>
>> +#ifndef CONFIG_DYNAMIC_FTRACE
>>   /*
>>    * void _mcount(unsigned long return_address)
>>    * @return_address: return address to instrumented function
>> @@ -134,6 +135,48 @@ skip_ftrace_call:
>>   #endif /* CONFIG_FUNCTION_GRAPH_TRACER */
>>   ENDPROC(_mcount)
>>
>> +#else /* CONFIG_DYNAMIC_FTRACE */
>> +/*
>> + * _mcount() is used to build the kernel with -pg option, but all the branch
>> + * instructions to _mcount() are replaced to NOP initially at kernel start up,
>> + * and later on, NOP to branch to ftrace_caller() when enabled or branch to
>> + * NOP when disabled per-function base.
>> + */
>> +ENTRY(_mcount)
>> +	ret
>> +ENDPROC(_mcount)
>
> Judging by your comment then, this should never be called. Is that right? If
> so, we could add a BUG-equivalent so we know if we missed an mcount during
> patching.

Steven explained this.

>> +/*
>> + * void ftrace_caller(unsigned long return_address)
>> + * @return_address: return address to instrumented function
>> + *
>> + * This function is a counterpart of _mcount() in 'static' ftrace, and
>> + * makes calls to:
>> + *     - tracer function to probe instrumented function's entry,
>> + *     - ftrace_graph_caller to set up an exit hook
>> + */
>> +ENTRY(ftrace_caller)
>> +	mcount_enter
>> +
>> +	mcount_get_pc0	x0		//     function's pc
>> +	mcount_get_lr	x1		//     function's lr
>> +
>> +	.global ftrace_call
>> +ftrace_call:				// tracer(pc, lr);
>> +	nop				// This will be replaced with "bl xxx"
>> +					// where xxx can be any kind of tracer.
>> +
>> +#ifdef CONFIG_FUNCTION_GRAPH_TRACER
>> +	.global ftrace_graph_call
>> +ftrace_graph_call:			// ftrace_graph_caller();
>> +	nop				// If enabled, this will be replaced
>> +					// "b ftrace_graph_caller"
>> +#endif
>> +
>> +	mcount_exit
>> +ENDPROC(ftrace_caller)
>> +#endif /* CONFIG_DYNAMIC_FTRACE */
>> +
>>   ENTRY(ftrace_stub)
>>   	ret
>>   ENDPROC(ftrace_stub)
>> diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c
>> index a559ab8..8c26476 100644
>> --- a/arch/arm64/kernel/ftrace.c
>> +++ b/arch/arm64/kernel/ftrace.c
>> @@ -17,6 +17,89 @@
>>   #include <asm/ftrace.h>
>>   #include <asm/insn.h>
>>
>> +#ifdef CONFIG_DYNAMIC_FTRACE
>> +/*
>> + * Replace a single instruction, which may be a branch or NOP.
>> + * If @validate == true, a replaced instruction is checked against 'old'.
>> + */
>> +static int ftrace_modify_code(unsigned long pc, unsigned int old,
>> +			      unsigned int new, bool validate)
>> +{
>> +	unsigned int replaced;
>
> u32 is a bit clearer for instructions.

Fix it.

>> +	/*
>> +	 * Note:
>> +	 * Due to modules and __init, code can disappear and change,
>> +	 * we need to protect against faulting as well as code changing.
>> +	 * We do this by aarch64_insn_*() which use the probe_kernel_*().
>> +	 *
>> +	 * No lock is held here because all the modifications are run
>> +	 * through stop_machine().
>> +	 */
>> +	if (validate) {
>> +		if (aarch64_insn_read((void *)pc, &replaced))
>> +			return -EFAULT;
>> +
>> +		if (replaced != old)
>> +			return -EINVAL;
>> +	}
>> +	if (aarch64_insn_patch_text_nosync((void *)pc, new))
>> +		return -EPERM;
>
> I think you're better off propagating the errors here, rather than
> overriding them with EFAULT/EINVAL/EPERM.

Steven explained this.

>> +
>> +	return 0;
>> +}
>> +
>> +/*
>> + * Replace tracer function in ftrace_caller()
>> + */
>> +int ftrace_update_ftrace_func(ftrace_func_t func)
>> +{
>> +	unsigned long pc;
>> +	unsigned int new;
>> +
>> +	pc = (unsigned long)&ftrace_call;
>> +	new = aarch64_insn_gen_branch_imm(pc, (unsigned long)func, true);
>> +
>> +	return ftrace_modify_code(pc, 0, new, false);
>> +}
>> +
>> +/*
>> + * Turn on the call to ftrace_caller() in instrumented function
>> + */
>> +int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
>> +{
>> +	unsigned long pc = rec->ip;
>> +	unsigned int old, new;
>> +
>> +	old = aarch64_insn_gen_nop();
>> +	new = aarch64_insn_gen_branch_imm(pc, addr, true);
>> +
>> +	return ftrace_modify_code(pc, old, new, true);
>> +}
>> +
>> +/*
>> + * Turn off the call to ftrace_caller() in instrumented function
>> + */
>> +int ftrace_make_nop(struct module *mod,
>> +		    struct dyn_ftrace *rec, unsigned long addr)
>> +{
>> +	unsigned long pc = rec->ip;
>> +	unsigned int old, new;
>> +
>> +	old = aarch64_insn_gen_branch_imm(pc, addr, true);
>> +	new = aarch64_insn_gen_nop();
>> +
>> +	return ftrace_modify_code(pc, old, new, true);
>> +}
>> +
>> +int __init ftrace_dyn_arch_init(void *data)
>> +{
>> +	*(unsigned long *)data = 0;
>> +
>> +	return 0;
>> +}
>> +#endif /* CONFIG_DYNAMIC_FTRACE */
>> +
>>   #ifdef CONFIG_FUNCTION_GRAPH_TRACER
>>   /*
>>    * function_graph tracer expects ftrace_return_to_handler() to be called
>> @@ -61,4 +144,34 @@ void prepare_ftrace_return(unsigned long *parent, unsigned long self_addr,
>>   		return;
>>   	}
>>   }
>> +
>> +#ifdef CONFIG_DYNAMIC_FTRACE
>> +/*
>> + * Turn on/off the call to ftrace_graph_caller() in ftrace_caller()
>> + * depending on @enable.
>> + */
>> +static int ftrace_modify_graph_caller(bool enable)
>> +{
>> +	unsigned long pc = (unsigned long)&ftrace_graph_call;
>> +	unsigned int branch, nop, old, new;
>> +
>> +	branch = aarch64_insn_gen_branch_imm(pc,
>> +			(unsigned long)ftrace_graph_caller, false);
>> +	nop = aarch64_insn_gen_nop();
>> +	old = enable ? nop : branch;
>> +	new = enable ? branch : nop;
>> +
>> +	return ftrace_modify_code(pc, old, new, true);
>
> You could rewrite this as:
>
> 	if (enable)
> 		return ftrace_modify_code(pc, nop, branch, true);
> 	else
> 		return ftrace_modify_code(pc, branch, nop, true);
>
> which I find easier to read.

Yep, fix it.

-Takahiro AKASHI

> Will
>