linux-kernel - Re: [PATCH v2 2/2] x86: Rewrite ret_from

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Date:   Wed, 19 Jul 2023 22:41:16 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Joe Lawrence <joe.lawrence@...hat.com>
Cc:     Petr Mladek <pmladek@...e.com>, Brian Gerst <brgerst@...il.com>,
        jpoimboe@...nel.org, linux-kernel@...r.kernel.org, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        alyssa.milburn@...ux.intel.com, keescook@...omium.org,
        joao@...rdrivepizza.com, tim.c.chen@...ux.intel.com,
        live-patching@...r.kernel.org
Subject: Re: [PATCH v2 2/2] x86: Rewrite ret_from_fork() in C

On Wed, Jul 19, 2023 at 04:33:26PM -0400, Joe Lawrence wrote:
> On 7/19/23 11:21, Petr Mladek wrote:
> > On Fri 2023-06-23 18:55:29, Brian Gerst wrote:
> >> When kCFI is enabled, special handling is needed for the indirect call
> >> to the kernel thread function.  Rewrite the ret_from_fork() function in
> >> C so that the compiler can properly handle the indirect call.
> > 
> > This patch broke livepatching. Kthreads never have a reliable stack.
> > It works when I revert it.
> > 
> 
> Just curious -- did the selftests catch this anywhere?  I'm not 100%
> clear on what trees / frequency they all run, so maybe Petr you found
> this by code inspection or other means?

I suspect Petr ran the selftests himself, they're fairly easy to run
(once you figure out the magic incantation) and insta fail.

I'm not sure the robots consistently run this stuff -- I've had these
patches exposed to 0day for weeks...