linux-kernel - Re: [PATCH -v5mkII 13/17] arm/ftrace: Use __patch

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Message-ID: <20200108091628.7e548401@gandalf.local.home>
Date:   Wed, 8 Jan 2020 09:16:28 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>, bristot@...hat.com,
        Jason Baron <jbaron@...mai.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, namit@...are.com,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jessica Yu <jeyu@...nel.org>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Mark Rutland <mark.rutland@....com>,
        James Morse <james.morse@....com>,
        Russell King <rmk+kernel@...linux.org.uk>,
        Rabin Vincent <rabin@....in>, Nicolas Pitre <nico@...xnic.net>
Subject: Re: [PATCH -v5mkII 13/17] arm/ftrace: Use __patch_text()

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

> On Wed, Nov 13, 2019 at 10:29 AM Peter Zijlstra <peterz@...radead.org> 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