linux-kernel - Re: [PATCH v2] objtool,ftrace: Implement UNWIND_HINT_RET

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Date:   Wed, 1 Apr 2020 17:38:10 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     tglx@...utronix.de, linux-kernel@...r.kernel.org, x86@...nel.org,
        mhiramat@...nel.org, mbenes@...e.cz,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH v2] objtool,ftrace: Implement UNWIND_HINT_RET_OFFSET

On Wed, Apr 01, 2020 at 09:39:16AM -0500, Josh Poimboeuf wrote:
> On Wed, Apr 01, 2020 at 04:22:26PM +0200, Peter Zijlstra wrote:

> > > > @@ -366,6 +367,13 @@ create_trampoline(struct ftrace_ops *ops
> > > >  	if (WARN_ON(ret < 0))
> > > >  		goto fail;
> > > > 
> > > > +	if (ops->flags & FTRACE_OPS_FL_SAVE_REGS) {
> > > > +		ip = ftrace_regs_caller_ret;
> > > > +		ret = probe_kernel_read(ip, (void *)retq, RET_SIZE);
> > > > +		if (WARN_ON(ret < 0))
> > > > +			goto fail;
> > > > +	}
> > > > +

> Right, but 'ip' needs to point to the trampoline's version of
> 'ftrace_regs_caller_ret', not the original ftrace_64 version.

Duh,

		ip = trampline + (ftrace_regs_caller_ret - ftrace_regs_caller);

it is.

Luckily this would've triggerd a splat the first time I'd have actually
booted this, because the regular text is RO, while the trampoline is
still RW here. Still, good spotting.