[PATCHv4] arm: ftrace: Adds support for CONFIG_DYNAMIC_FTRACE_WITH_REGS

Abel Vesa
2017-02-28T00:56:36Z

On Mon, Feb 27, 2017 at 04:52:06PM +0100, Nicolai Stange wrote:
> Hi Abel,
>
> 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).
>
> 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?
>
> 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?
>
> A (hypothetical) klp_arch_set_pc(struct pt_regs *regs, unsigned long ip)
> would do the more intuitive
>
> regs->ARM_pc = ip;
>
> rather than a
>
> regs->ARM_lr = ip
>
> then.
You are right. There is a subsequent patch I'm currently working on that will
enable livepatch and will bring an implementation for klp_arch_set_pc as you
described it. I still don't get what is wrong with the code though?

>
> In addition, the original lr register would be made available to ftrace
> handlers this way.
>
>
> > + mov r3, sp @ struct pt_regs*
> > + ldr r2, =function_trace_op
> > + ldr r2, [r2] @ pointer to the current
> > + @ function tracing op
> > + ldr r1, [sp, #PT_REGS_SIZE] @ lr of instrumented func
> > + mcount_adjust_addr r0, lr @ instrumented function
> > +
> > + .globl ftrace_regs_call
> > +ftrace_regs_call:
> > + bl ftrace_stub
> > +
> > +#ifdef CONFIG_FUNCTION_GRAPH_TRACER
> > + .globl ftrace_graph_regs_call
> > +ftrace_graph_regs_call:
> > + mov r0, r0
> > +#endif
> > + @ pop saved regs
> > + @ first, get the previous LR value from stack
> > + ldr lr, [sp, #PT_REGS_SIZE]
> > + @ now pop the rest of the saved registers INCLUDING stack pointer
> > + ldmia sp, {r0-r11, ip, sp, pc}
> > +.endm
> > +
>
>
> > +#ifdef CONFIG_FUNCTION_GRAPH_TRACER
> > +.macro __ftrace_graph_regs_caller
> > +
> > + sub r0, fp, #4 @ lr of instrumented routine (parent)
> > +
> > + @ called from __ftrace_regs_caller
> > + ldr r1, [sp, #S_LR] @ instrumented routine (func)
> > + mcount_adjust_addr r1, r1
> > +
> > + sub r2, r0, #4 @ frame pointer
>
> Given that r2 is prepare_ftrace_return()'s frame_pointer argument, is
>
> r2 = fp - 4 - 4 = fp - 8
>
> really correct / what is wanted here?
>
You are right.
- sub r2, r0, #4 @ frame pointer
+ mov r2, fp @ frame pointer

> > + bl prepare_ftrace_return
> > +
> > + @ pop registers saved in ftrace_regs_caller
> > + @ first, get the previous LR value from stack
> > + ldr lr, [sp, #PT_REGS_SIZE]
> > + @ now pop the rest of the saved registers INCLUDING stack pointer
> > + ldmia sp, {r0-r11, ip, sp, pc}
> > +.endm
> > +#endif
> > +#endif
> > +
>
> <snip>
>
>
> Thanks,
>
> Nicolai

Thanks