linux-kernel - Re: [PATCH] x86/alternatives: Use C int3 selftest but disable KASAN

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Date:   Tue, 12 Nov 2019 08:57:46 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/alternatives: Use C int3 selftest but disable KASAN

On Mon, Nov 11, 2019 at 01:51:16PM -0800, Kees Cook wrote:
> Instead of using inline asm for the int3 selftest (which confuses the
> Clang's ThinLTO pass), 

What is that and why do we care?

> this restores the C function but disables KASAN
> (and tracing for good measure) to keep the things simple and avoid
> unexpected side-effects. This attempts to keep the fix from commit
> ecc606103837 ("x86/alternatives: Fix int3_emulate_call() selftest stack
> corruption") without using inline asm.

See, I don't much like that. The selftest basically does a naked CALL
and hard relies on the callee saving everything if required, which is
very much against the C calling convention.

Sure, by disabling KASAN and all the other crap the compiler probably
does the right thing by accident, but it is still a C ABI violation.

We use ASM all over the kernel, why is this one a problem?