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

From: Torsten Duwe <duwe@lst.de>
To: Julien Thierry <julien.thierry@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
	Arnd Bergmann <arnd@arndb.de>,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	linux-kernel@vger.kernel.org,
	Steven Rostedt <rostedt@goodmis.org>,
	AKASHI Takahiro <takahiro.akashi@linaro.org>,
	Ingo Molnar <mingo@redhat.com>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Amit Daniel Kachhap <amit.kachhap@arm.com>,
	live-patching@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v7 2/3] arm64: implement ftrace with regs
Date: Wed, 6 Feb 2019 16:05:24 +0100	[thread overview]
Message-ID: <20190206150524.GA28892@lst.de> (raw)
In-Reply-To: <e206e229-7813-2d37-d5c6-7a9785e311c9@arm.com>

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

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