Re: [PATCHv4] arm: ftrace: Adds support for CONFIG_DYNAMIC_FTRACE_WITH_REGS

[Nicolai Stange <nicstange@gmail.com>]

Hi Abel,

On Tue, Feb 28 2017, Abel Vesa wrote:

> On Mon, Feb 27, 2017 at 04:52:06PM +0100, Nicolai Stange wrote:
>> On Fri, Feb 24 2017, Abel Vesa wrote:
>> 
>> <snip>
>> 
>> > diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
>> > index fda6a46..877df5b 100644
>> > --- a/arch/arm/Kconfig
>> > +++ b/arch/arm/Kconfig
>> > @@ -55,6 +55,7 @@ config ARM
>> >  	select HAVE_DMA_API_DEBUG
>> >  	select HAVE_DMA_CONTIGUOUS if MMU
>> >  	select HAVE_DYNAMIC_FTRACE if (!XIP_KERNEL) && !CPU_ENDIAN_BE32 && MMU
>> > +	select HAVE_DYNAMIC_FTRACE_WITH_REGS if HAVE_DYNAMIC_FTRACE && OLD_MCOUNT
>> 
>> 
>> AFAICS, your code depends on the __gnu_mcount_nc calling conventions,
>> i.e. on gcc pushing the original lr on the stack. In particular, there's
>> no implementation of a ftrace_regs_caller_old or so.
>> 
>> So shouldn't this read as !OLD_MCOUNT instead?
> The implementation of __ftrace_modify_code which sets the kernel text to rw
> needs OLD_MCOUNT (that is, the arch specific one, not the generic one).

You're right that ARM's implementation of __ftrace_modify_code() is hidden
within an #ifdef CONFIG_OLD_MCOUNT.

But,
 - its implementation doesn't "need" or depend on OLD_MCOUNT
 - and it's true in general that the kernel text must be made writable
   before ftrace_modify_all_code() attempts to write to it.

So, I bet that the set_kernel_text_rw()-calling ARM implementations of
arch_ftrace_update_code() and __ftrace_modify_code() resp. have been
inserted under that CONFIG_OLD_MCOUNT #ifdef by mistake with commit
80d6b0c2eed2 ("ARM: mm: allow text and rodata sections to be
read-only").

In conclusion, I claim that DYNAMIC_FTRACE w/o OLD_MCOUNT had been
broken before your patch already. I didn't explicitly test that though.

I think that should be fixed rather than your DYNAMIC_FTRACE_WITH_REGS
pulling in OLD_MCOUNT in order to repair DYNAMIC_FTRACE.

Especially since your implementation seems to require !OLD_MCOUNT...

>> Also, at least the ldmia ..., {..., sp, ...} insn needs a !THUMB2_KERNEL.
>> 
>> 
>> >  	select HAVE_EFFICIENT_UNALIGNED_ACCESS if (CPU_V6 || CPU_V6K || CPU_V7) && MMU
>> >  	select HAVE_EXIT_THREAD
>> >  	select HAVE_FTRACE_MCOUNT_RECORD if (!XIP_KERNEL)
>> 
>> <snip>
>> 
>> > diff --git a/arch/arm/kernel/entry-ftrace.S b/arch/arm/kernel/entry-ftrace.S
>> > index c73c403..3916dd6 100644
>> > --- a/arch/arm/kernel/entry-ftrace.S
>> > +++ b/arch/arm/kernel/entry-ftrace.S
>> > @@ -92,12 +92,78 @@
>> >  2:	mcount_exit
>> >  .endm
>> >  
>> > +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
>> > +
>> > +.macro __ftrace_regs_caller
>> > +
>> > +	sub	sp, sp, #8	@ space for CPSR and OLD_R0 (not used)
>> > +
>> > +	add 	ip, sp, #12	@ move in IP the value of SP as it was
>> > +				@ before the push {lr} of the mcount mechanism
>> > +	stmdb	sp!, {ip,lr,pc}
>> > +	stmdb	sp!, {r0-r11,lr}
>> > +
>> > +	@ stack content at this point:
>> > +	@ 0  4          48   52       56   60   64    68       72
>> > +	@ R0 | R1 | ... | LR | SP + 4 | LR | PC | PSR | OLD_R0 | previous LR |
>> 
>> Being a constant, the saved pc is not very useful, I think.
> So you're saying skip it ? But you still need to leave space for it. So why not
> just save it even if the value is not useful?

No, no, I don't want to skip it. I'd just prefer to have the pt_regs'
->ARM_lr and ->ARM_pc slots filled with different, perhaps more useful
values:

>> 
>> Wouldn't it be better (and more consistent with other archs) to have
>> 
>>   pt_regs->ARM_lr = original lr
>>   pt_refs->ARM_pc = current lr
>> 
>> instead?

The stack would look like this then

@ ...           | ARM_ip | ARM_sp | ARM_lr      | ARM_pc      | ...          |
@ 0  4          48       52       56            60            64    68       72
@ R0 | R1 | ... | LR     | SP + 4 | original LR | original PC | PSR | OLD_R0 | original LR |

I.e. the pt_regs would capture almost the full context of the
instrumented function (except for ip).

Thanks,

Nicolai