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

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Date:   Tue, 8 Sep 2020 11:13:58 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Petr Mladek <pmladek@...e.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        live-patching@...r.kernel.org
Subject: Re: [GIT PULL] livepatching for 5.9-rc5

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.

                  Linus