linux-kernel - Re: [GIT PULL] livepatching for 5.9-rc5

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Date:   Tue, 8 Sep 2020 13:32:39 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Petr Mladek <pmladek@...e.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        live-patching@...r.kernel.org
Subject: Re: [GIT PULL] livepatching for 5.9-rc5

On Tue, Sep 08, 2020 at 11:13:58AM -0700, Linus Torvalds wrote:
> Josh,
> 
> On Mon, Sep 7, 2020 at 1:20 AM Petr Mladek <pmladek@...e.com> wrote:
> >
> > - Workaround "unreachable instruction" objtool warnings that happen
> >   with some compiler versions.
> 
> I know I said this fixes things for me, but I just realized it doesn't entirely.
> 
> I wonder how I missed the remaining one:
> 
>    arch/x86/kvm/vmx/vmx.o: warning: objtool:
> vmx_handle_exit_irqoff()+0x142: unreachable instruction
> 
> so apparently gcc and objtool can still disagree even without that
> '-flive-patching'.
> 
> The unreachable code in question is after the call to
> handle_external_interrupt_irqoff(), and while that function is a bit
> odd, in this case I think it's objtool that is wrong.
> 
> I think that what happens is that the function doesn't have a 'ret'
> instruction, and instead returns by doing a tail-call to
> __sanitizer_cov_trace_pc with my config. And maybe that is what
> confuses objtool.
> 
> This is current tip-of-git of my tree, with a allmodconfig build (but
> the actual config will then depend on things like the gcc plugins
> being there too, so you may not get exactly the same thing as I do)
> 
> Josh? Am I missing something, and the objtool warning is valid? But
> yes, that code is doing some very very special stuff with that thunk
> call asm, so it's hard to read the asm.

Hm, I don't think I've seen that one.  We saw a similar warning in that
function before, but it was caused by the combination of
CONFIG_UBSAN_ALIGNMENT and CONFIG_UBSAN_TRAP, which I think Kees fixed.

Can you share the .o file?  At least I can't recreate with GCC 9.3.1,
which is all I have at the moment.

-- 
Josh