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

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Message-ID: <CABCJKufA-aOcsOqb1NiMQeBGm9Q-JxjoPjsuNpHh0kL4LzfO0w@mail.gmail.com>
Date:   Fri, 13 Nov 2020 12:24:32 -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 Fri, Nov 13, 2020 at 11:54 AM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
>
> 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.

Do you have CONFIG_XEN enabled? I can reproduce this on ToT master as follows:

$ git rev-parse HEAD
585e5b17b92dead8a3aca4e3c9876fbca5f7e0ba
$ make defconfig && \
./scripts/config -e HYPERVISOR_GUEST -e PARAVIRT -e XEN && \
make olddefconfig && \
make -j110
...
$ ./tools/objtool/objtool check -arfld vmlinux.o 2>&1 | grep secondary
vmlinux.o: warning: objtool: __startup_secondary_64()+0x2: return with
modified stack frame

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

No, that didn't fix the warning. Here's what I tested:

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index c6ab44543c92..f1f65f5688cf 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -156,6 +156,7 @@ static bool __dead_end_function(struct
objtool_file *file, struct symbol *func,
                "machine_real_restart",
                "rewind_stack_do_exit",
                "kunit_try_catch_throw",
+               "cpu_bringup_and_idle",
        };

        if (!func)

Sami