linux-kernel - Re: [RFC][PATCH 04/16] objtool: Annotate identity

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Date:   Fri, 13 Mar 2020 18:22:57 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Brian Gerst <brgerst@...il.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>
Subject: Re: [RFC][PATCH 04/16] objtool: Annotate identity_mapped()

On Fri, Mar 13, 2020 at 12:46:05PM -0400, Brian Gerst wrote:
> On Thu, Mar 12, 2020 at 9:53 AM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > Normally identity_mapped is not visible to objtool, due to:
> >
> >   arch/x86/kernel/Makefile:OBJECT_FILES_NON_STANDARD_relocate_kernel_$(BITS).o := y
> >
> > However, when we want to run objtool on vmlinux.o there is no hiding
> > it. Without the annotation we'll get complaints about the:
> >
>          call 1f
> 1:      popq %r8
>         subq $(1b - relocate_kernel), %r8
> 
> It looks to me that this code is simply trying to get the virtual
> address of relocate_kernel using the old 32-bit method of PIC address
> calculation.  On 64-bit can be done with leaq relocate_kernel(%rip),
> %r8.

Indeed. Objtool would be happy with that. And it seems I can still kexec
a kernel too.

Thanks!