linux-kernel - Re: [PATCH 2/2] x86/retpoline: Fix return buffer filling

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Message-ID: <20180213143202.7logf7thlozro6op@two.firstfloor.org>
Date:   Tue, 13 Feb 2018 06:32:02 -0800
From:   Andi Kleen <andi@...stfloor.org>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     Andi Kleen <andi@...stfloor.org>, tglx@...utronix.de,
        x86@...nel.org, linux-kernel@...r.kernel.org, bp@...en8.de,
        jpoimboe@...hat.com, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 2/2] x86/retpoline: Fix return buffer filling

> However... that was supposed to be a 'clear RSB' operation, with 32
> CALLs in sequence. And Boris changed it to 16 by calling __fill_rsb()
> instead of __clear_rsb():

True. That's even worse.
> 
> -       asm volatile (ANNOTATE_NOSPEC_ALTERNATIVE
> -                     ALTERNATIVE("jmp 910f",
> -                                 __stringify(__FILL_RETURN_BUFFER(%0, RSB_CLEAR_LOOPS, %1)),
> -                                 X86_FEATURE_RETPOLINE)
> -                     "910:"
> -                     : "=r" (loops), ASM_CALL_CONSTRAINT
> -                     : : "memory" );
> +       alternative_input("",
> +                         "call __fill_rsb",
> +                         X86_FEATURE_RETPOLINE,
> +                         ASM_NO_INPUT_CLOBBER(_ASM_BX, "memory"));
> 
> I think we do need to revert that patch. And perhaps stop accepting any
> more similar bikeshedding.

Yes revertion would be the right way.

I already regret the time I wasted trying to fix it.

-Andi