linux-kernel - Re: [PATCH v6 22/25] x86/asm: annotate indirect jumps

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Date:   Thu, 22 Oct 2020 09:25:53 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Sami Tolvanen <samitolvanen@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Will Deacon <will@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-kbuild <linux-kbuild@...r.kernel.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        linux-pci@...r.kernel.org
Subject: Re: [PATCH v6 22/25] x86/asm: annotate indirect jumps

On Wed, Oct 21, 2020 at 04:27:47PM -0500, Josh Poimboeuf wrote:
> On Wed, Oct 21, 2020 at 11:32:13AM +0200, Peter Zijlstra wrote:
> > On Wed, Oct 21, 2020 at 10:56:06AM +0200, Peter Zijlstra wrote:
> > 
> > > I do not see these in particular, although I do see a lot of:
> > > 
> > >   "sibling call from callable instruction with modified stack frame"
> > 
> > defconfig-build/vmlinux.o: warning: objtool: msr_write()+0x10a: sibling call from callable instruction with modified stack frame
> > defconfig-build/vmlinux.o: warning: objtool:   msr_write()+0x99: (branch)
> > defconfig-build/vmlinux.o: warning: objtool:   msr_write()+0x3e: (branch)
> > defconfig-build/vmlinux.o: warning: objtool:   msr_write()+0x0: <=== (sym)
> > 
> > $ nm defconfig-build/vmlinux.o | grep msr_write
> > 0000000000043250 t msr_write
> > 00000000004289c0 T msr_write
> > 0000000000003056 t msr_write.cold
> > 
> > Below 'fixes' it. So this is also caused by duplicate symbols.
> 
> There's a new linker flag for renaming duplicates:
> 
>   https://sourceware.org/bugzilla/show_bug.cgi?id=26391
> 
> But I guess that doesn't help us now.

Well, depends a bit if clang can do it; we only need this for LTO builds
for now.

> I don't have access to GCC 10 at the moment so I can't recreate it.
> Does this fix it?

Doesn't seem to do the trick :/ I'll try and have a poke later.