linux-kernel - Re: [PATCH 24/29] x86/text-patching: Make text_gen

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Date:   Thu, 24 Feb 2022 10:10:12 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Joao Moreira <joao@...rdrivepizza.com>
Cc:     x86@...nel.org, hjl.tools@...il.com, jpoimboe@...hat.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
Subject: Re: [PATCH 24/29] x86/text-patching: Make text_gen_insn() IBT aware

On Wed, Feb 23, 2022 at 05:18:04PM -0800, Joao Moreira wrote:
> > +#ifdef CONFIG_X86_IBT
> > +	if (is_endbr(dest))
> > +		dest += 4;
> > +#endif
> 
> Hi, FWIIW I saw this snippet trigger a bug in the jump_label infra where the
> target displacement would not fit in a JMP8 operand.

Bah, I was afraid of seening that :/

> For these cases perhaps we can verify if the displacement fits the operand
> and, if not, simply ignore and lose the decode cycle which may not be a huge
> problem and remains semantically correct. Seems more sensible than padding
> jump tables with nops. In the meantime I'll investigate clang's behavior and
> if it is really a bug, I'll work on a patch.

Urgh, trouble is, we're going to be re-writing a bunch of ENDBR to be
UD1 0x0(%eax),%eax, and you really don't want to try and execute those.