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

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Message-ID: <CABCJKuewXazmBpXz5irWgy+W537x1Lws5YAsFqMgo+Yio8iyRg@mail.gmail.com>
Date:   Wed, 21 Oct 2020 08:01:38 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.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 1:56 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Tue, Oct 20, 2020 at 12:24:37PM -0700, Sami Tolvanen wrote:
> > > > Building allyesconfig with this series and LTO enabled, I still see
> > > > the following objtool warnings for vmlinux.o, grouped by source file:
> > > >
> > > > arch/x86/entry/entry_64.S:
> > > > __switch_to_asm()+0x0: undefined stack state
> > > > .entry.text+0xffd: sibling call from callable instruction with
> > > > modified stack frame
> > > > .entry.text+0x48: stack state mismatch: cfa1=7-8 cfa2=-1+0
> > >
> > > Not sure what this one's about, there's no OBJECT_FILES_NON_STANDARD?
> >
> > Correct, because with LTO, we won't have an ELF binary to process
> > until we compile everything into vmlinux.o, and at that point we can
> > no longer skip individual object files.
>
> I think what Josh was trying to say is; this file is subject to objtool
> on a normal build and does not generate warnings. So why would it
> generate warnings when subject to objtool as result of a vmlinux run
> (due to LTO or otherwise).

Ah, right. It also doesn't generate warnings when I build defconfig
with LTO, so clearly something confuses objtool here.

Sami