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

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Date:   Wed, 1 Apr 2020 11:39:11 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>, tglx@...utronix.de,
        linux-kernel@...r.kernel.org, x86@...nel.org, mhiramat@...nel.org,
        mbenes@...e.cz
Subject: Re: [PATCH v2] objtool,ftrace: Implement UNWIND_HINT_RET_OFFSET

On Wed, 1 Apr 2020 09:39:16 -0500
Josh Poimboeuf <jpoimboe@...hat.com> wrote:

> > > > +	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;
> > > > +	}
> > > > +  
> > > 
> > > Hm?  This function creates a trampoline but it looks like this change is
> > > overwriting the original ftrace_64 code itself?  
> > 
> > Ahh. So if you look at what the trampoline copies, you'll note we'll
> > copy until -- but *NOT* including -- the jmp ftrace_epilogue. Instead
> > we'll write a RET at the end.
> > 
> > However, due to splitting the return path, such that each instruction
> > has a unique stack offset, we now have a second jmp ftrace_epilogue in
> > the middle of the function. That too needs to be overwritten by a RET.  
> 
> Right, but 'ip' needs to point to the trampoline's version of
> 'ftrace_regs_caller_ret', not the original ftrace_64 version.

I noticed the same thing. And after applying just this patch (with tweaking
the obtool mark ups to make it compile) it crashed.

-- Steve