linux-kernel - Re: [PATCH v7 2/3] arm64: implement ftrace with regs

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Date:   Wed, 6 Feb 2019 16:05:24 +0100
From:   Torsten Duwe <duwe@....de>
To:     Julien Thierry <julien.thierry@....com>
Cc:     Mark Rutland <mark.rutland@....com>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Arnd Bergmann <arnd@...db.de>,
        AKASHI Takahiro <takahiro.akashi@...aro.org>,
        Amit Daniel Kachhap <amit.kachhap@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        live-patching@...r.kernel.org
Subject: Re: [PATCH v7 2/3] arm64: implement ftrace with regs

On Wed, Feb 06, 2019 at 08:59:44AM +0000, Julien Thierry wrote:
> Hi Torsten,
> 
> On 18/01/2019 16:39, Torsten Duwe wrote:
> 
> > --- a/arch/arm64/kernel/ftrace.c
> > +++ b/arch/arm64/kernel/ftrace.c
> > @@ -133,17 +163,45 @@ int ftrace_make_call(struct dyn_ftrace *
> >  	return ftrace_modify_code(pc, old, new, true);
> >  }
> >  
> > +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
> > +int ftrace_modify_call(struct dyn_ftrace *rec, unsigned long old_addr,
> > +			unsigned long addr)
> > +{
> > +	unsigned long pc = rec->ip + REC_IP_BRANCH_OFFSET;
> > +	u32 old, new;
> > +
> > +	old = aarch64_insn_gen_branch_imm(pc, old_addr, true);
> > +	new = aarch64_insn_gen_branch_imm(pc, addr, true);
> > +
> > +	return ftrace_modify_code(pc, old, new, true);
> > +}
> > +#endif
> > +
> >  /*
> >   * 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 long pc = rec->ip + REC_IP_BRANCH_OFFSET;
> 
> Sorry to come back on this patch again, but I was looking at the ftrace
> code a bit, and I see that when processing the ftrace call locations,
> ftrace calls ftrace_call_adjust() on every ip registered as mcount
> caller (or in our case patchable entries). This ftrace_call_adjust() is
> arch specific, so I was thinking we could place the offset in here once
> and for all so we don't have to worry about it in the future.

Now that you mention it - yes indeed that's the correct facility to fix
the deviating address, as Steve has also confirmed. I had totally forgotten
about this hook.

> Also, I'm unsure whether it would be safe, but we could patch the "mov
> x9, lr" there as well. In theory, this would be called at init time
> (before secondary CPUs are brought up) and when loading a module (so I'd
> expect no-one is executing that code *yet*.
> 
> If this is possible, I think it would make things a bit cleaner.

This is in fact very tempting, but it will introduce a nasty side effect
to ftrace_call_adjust. Is there any obvious documentation that specifies
guarantees about ftrace_call_adjust being called exactly once for each site?

	Torsten