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

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Date:   Tue, 10 Nov 2020 10:59:55 -0800
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Josh Poimboeuf <jpoimboe@...hat.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 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.

> Will do some digging on the others...

Thanks!

Sami