linux-kernel - Re: [PATCH v3 01/13] x86/retpoline: Add initial retpoline support

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Date:   Mon, 08 Jan 2018 21:50:50 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Borislav Petkov <bp@...e.de>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "tim.c.chen@...ux.intel.com" <tim.c.chen@...ux.intel.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "riel@...hat.com" <riel@...hat.com>,
        "keescook@...gle.com" <keescook@...gle.com>,
        "gnomes@...rguk.ukuu.org.uk" <gnomes@...rguk.ukuu.org.uk>,
        "pjt@...gle.com" <pjt@...gle.com>,
        "dave.hansen@...el.com" <dave.hansen@...el.com>,
        "luto@...capital.net" <luto@...capital.net>,
        "jikos@...nel.org" <jikos@...nel.org>,
        "gregkh@...ux-foundation.org" <gregkh@...ux-foundation.org>
Subject: Re: [PATCH v3 01/13] x86/retpoline: Add initial retpoline support

On Sun, 2018-01-07 at 15:03 +0100, Borislav Petkov wrote:
> 
> My fear is if some funky compiler changes the sizes of the insns in
> RETPOLINE_CALL/JMP and then the padding becomes wrong. But looking at the
> labels, they're all close so you have a 2-byte jmp already and the
> 
> call    1112f
> 
> should be ok. The MOV is reg,(reg) which should not change size of 4...
> 
> But I'm remaining cautious here.

Right. I forget the specifics, but I've *watched* LLVM break carefully
hand-crafted asm code by emitting 4-byte variants when we expected 2-
byte, etc.

On the whole, I'm sufficiently unhappy with making such assumptions,
that I think the cure is worse than the disease. We can live with that
*one* out-of-line call to the thunk in the syscall case, and that was
the *only* one that really needed the call to be at the end.

Note that in the alternative case there, we don't even need to load it
into a register at all. We could do our own alternatives specially for
that case, and hand-tune the lengths only for them. But *with* a sanity
check to break the build on mismatch.

I don't think it's worth it at this point though.
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