Re: [PATCH -v5mkII 13/17] arm/ftrace: Use __patch_text()

[Steven Rostedt]

On Wed, 8 Jan 2020 13:22:52 +0100
Arnd Bergmann <arnd@xxxxxxxx> wrote:

> On Wed, Nov 13, 2019 at 10:29 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > @@ -49,8 +49,8 @@ obj-$(CONFIG_HAVE_ARM_SCU)    += smp_scu.o
> >  obj-$(CONFIG_HAVE_ARM_TWD)     += smp_twd.o
> >  obj-$(CONFIG_ARM_ARCH_TIMER)   += arch_timer.o
> >  obj-$(CONFIG_FUNCTION_TRACER)  += entry-ftrace.o
> > -obj-$(CONFIG_DYNAMIC_FTRACE)   += ftrace.o insn.o
> > -obj-$(CONFIG_FUNCTION_GRAPH_TRACER)    += ftrace.o insn.o
> > +obj-$(CONFIG_DYNAMIC_FTRACE)   += ftrace.o insn.o patch.o
> > +obj-$(CONFIG_FUNCTION_GRAPH_TRACER)    += ftrace.o insn.o patch.o
> >  obj-$(CONFIG_JUMP_LABEL)       += jump_label.o insn.o patch.o
> >  obj-$(CONFIG_KEXEC)            += machine_kexec.o relocate_kernel.o  
> 
> This broke randconfig builds with big-endian ARMv5:
> 
> ../arch/arm/kernel/patch.c: In function '__patch_text_real':
> ../arch/arm/kernel/patch.c:94:11: error: implicit declaration of
> function '__opcode_to_mem_thumb32'
> [-Werror=implicit-function-declaration]
>     insn = __opcode_to_mem_thumb32(insn);
>            ^~~~~~~~~~~~~~~~~~~~~~~
> 
> The problem is that we don't have a BE32 definition of
> __opcode_to_mem_thumb32, mostly because no hardware
> supports that.
> 
> One possible workaround is a big ugly:
> 
> diff --git a/arch/arm/kernel/patch.c b/arch/arm/kernel/patch.c
> index d0a05a3bdb96..1067fd122897 100644
> --- a/arch/arm/kernel/patch.c
> +++ b/arch/arm/kernel/patch.c
> @@ -90,9 +90,11 @@ void __kprobes __patch_text_real(void *addr,
> unsigned int insn, bool remap)
> 
>                 size = sizeof(u32);
>         } else {
> +#ifdef CONFIG_THUMB2_KERNEL
>                 if (thumb2)

If we change the above to:

		if (IS_ENABLED(CONFIG_THUMB2_KERNEL) && thumb2)

Would the compiler optmize out __opcode_to_mem_thumb32(). We would need
to add a declaration for it, but wont need to define it. At least we
wont have nasty #ifdef logic in the code.

-- Steve

>                         insn = __opcode_to_mem_thumb32(insn);
>                 else
> +#endif
>                         insn = __opcode_to_mem_arm(insn);
> 
>                 *(u32 *)waddr = insn;
> 
> 
> Any other suggestions?
> 
>        Arnd