linux-kernel - Re: [PATCH V3 5/9] objtool: Add return address unwind hints

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Date:   Tue, 14 Apr 2020 19:56:04 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexandre Chartre <alexandre.chartre@...cle.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, jpoimboe@...hat.com,
        jthierry@...hat.com, tglx@...utronix.de
Subject: Re: [PATCH V3 5/9] objtool: Add return address unwind hints

On Tue, Apr 14, 2020 at 06:40:12PM +0200, Alexandre Chartre wrote:
> 
> On 4/14/20 6:16 PM, Peter Zijlstra wrote:
> > On Tue, Apr 14, 2020 at 12:36:14PM +0200, Alexandre Chartre wrote:
> > > Add the UNWIND_HINT_RADDR_DELETE and UNWIND_HINT_RADDR_ALTER unwind
> > > hint macros to flag instructions which remove or modify return
> > > addresses.
> > 
> > I'm confused by this thing; why? AFAICT the rest of the patches are
> > actually simpler without this one.
> > 
> 
> This is required to indicate to objtool that assembly instructions are
> changing return addresses. For example, in patch 8:
> 
> For retpoline:
> 
> @@ -88,6 +96,7 @@
>  	lfence
>  	jmp	.Lspec_trap_\@
>  .Ldo_rop_\@:
> +	UNWIND_HINT_RADDR_ALTER
>  	mov	\reg, (%_ASM_SP)
>  	ret
>  .endm
> 
> The unwind hint indicates that the return address has been altered, so the
> code won't return to the return address which was on the stack.

But if you hadn't added that return stack stuff in the first place,
you'd not have needed that HINT.

So what actual problem is it solving?