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

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Date:   Fri, 13 Nov 2020 13:54:08 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        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 Tue, Nov 10, 2020 at 10:59:55AM -0800, Sami Tolvanen wrote:
> On Tue, Nov 10, 2020 at 9:46 AM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> >
> > On Mon, Nov 09, 2020 at 08:29:24PM -0600, Josh Poimboeuf wrote:
> > > On Mon, Nov 09, 2020 at 03:11:41PM -0800, Sami Tolvanen wrote:
> > > > CONFIG_XEN
> > > >
> > > > __switch_to_asm()+0x0: undefined stack state
> > > >   xen_hypercall_set_trap_table()+0x0: <=== (sym)
> >
> > With your branch + GCC 9 I can recreate all the warnings except this
> > one.
> 
> In a gcc build this warning is replaced with a different one:
> 
> vmlinux.o: warning: objtool: __startup_secondary_64()+0x7: return with
> modified stack frame
> 
> This just seems to depend on which function is placed right after the
> code in xen-head.S. With gcc, the disassembly looks like this:
> 
> 0000000000000000 <asm_cpu_bringup_and_idle>:
>        0:       e8 00 00 00 00          callq  5 <asm_cpu_bringup_and_idle+0x5>
>                         1: R_X86_64_PLT32       cpu_bringup_and_idle-0x4
>        5:       e9 f6 0f 00 00          jmpq   1000
> <xen_hypercall_set_trap_table>
> ...
> 0000000000001000 <xen_hypercall_set_trap_table>:
>         ...
> ...
> 0000000000002000 <__startup_secondary_64>:
> 
> With Clang+LTO, we end up with __switch_to_asm here instead of
> __startup_secondary_64.

I still don't see this warning for some reason.

Is it fixed by adding cpu_bringup_and_idle() to global_noreturns[] in
tools/objtool/check.c?

-- 
Josh