linux-kernel - Re: [PATCH v3 18/39] x86/ibt,ftrace: Make function-graph play nice

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Date:   Fri, 4 Mar 2022 14:05:19 -0800
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, joao@...rdrivepizza.com, hjl.tools@...il.com,
        andrew.cooper3@...rix.com, linux-kernel@...r.kernel.org,
        ndesaulniers@...gle.com, keescook@...omium.org,
        samitolvanen@...gle.com, mark.rutland@....com,
        alyssa.milburn@...el.com, mbenes@...e.cz, rostedt@...dmis.org,
        mhiramat@...nel.org, alexei.starovoitov@...il.com
Subject: Re: [PATCH v3 18/39] x86/ibt,ftrace: Make function-graph play nice

On Fri, Mar 04, 2022 at 10:44:54PM +0100, Peter Zijlstra wrote:
> On Fri, Mar 04, 2022 at 01:03:34PM -0800, Josh Poimboeuf wrote:
> > On Fri, Mar 04, 2022 at 08:48:53PM +0100, Peter Zijlstra wrote:
> > > On Fri, Mar 04, 2022 at 09:51:54AM -0800, Josh Poimboeuf wrote:
> > > > On Thu, Mar 03, 2022 at 12:23:39PM +0100, Peter Zijlstra wrote:
> > > > > +
> > > > > +	addq $16, %rsp
> > > > > +	ANNOTATE_INTRA_FUNCTION_CALL
> > > > > +	call .Ldo_rop
> > > > > +	int3
> > > > > +.Ldo_rop:
> > > > > +	mov %rdi, (%rsp)
> > > > > +	UNWIND_HINT_FUNC
> > > > > +	RET
> > > > 
> > > > Why the int3?
> > > 
> > > Speculation trap :-) Either I'm too paranoid or not paranoid enough; but
> > > without it it's just too close to a retpoline and it doesn't feel right.
> > 
> > Um, it *is* a retpoline :-)
> > 
> > Can you just use the RETPOLINE macro?  Along with a comment stating why
> > it can't just do a JMP_NOSPEC?
> 
> There is no RETPOLINE macro; or rather it is completely contained in
> lib/retpoline.S and I'd sorta like to keep it that way.
> 
> That said, I can stick a comment on.

The only reason it's in retpoline.S is because nobody else needed it.

It just seems weird to reinvent the wheel, especially with a slightly
different method of stopping speculation.

And I could envision other cases where we might want an unconditional
retpoline.

Your call though...

-- 
Josh