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

Josh Poimboeuf
2020-11-13T20:52:40Z

On Fri, Nov 13, 2020 at 12:24:32PM -0800, Sami Tolvanen wrote:
> On Fri, Nov 13, 2020 at 11:54 AM Josh Poimboeuf <jpoimboe@redhat.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@redhat.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

Ok, I see it now on Linus' tree. I just didn't see it on your clang-lto
branch.

--
Josh