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

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Date:   Wed, 21 Oct 2020 11:51:33 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Sami Tolvanen <samitolvanen@...gle.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 Tue, Oct 20, 2020 at 01:52:17PM -0500, Josh Poimboeuf wrote:
> > arch/x86/lib/retpoline.S:
> > __x86_retpoline_rdi()+0x10: return with modified stack frame
> > __x86_retpoline_rdi()+0x0: stack state mismatch: cfa1=7+32 cfa2=7+8
> > __x86_retpoline_rdi()+0x0: stack state mismatch: cfa1=7+32 cfa2=-1+0
> 
> Is this with upstream?  I thought we fixed that with
> UNWIND_HINT_RET_OFFSET.

I can't reproduce this one either; but I do get different warnings:

gcc (Debian 10.2.0-13) 10.2.0, x86_64-defconfig:

defconfig-build/vmlinux.o: warning: objtool: __x86_indirect_thunk_rax() falls through to next function __x86_retpoline_rax()
defconfig-build/vmlinux.o: warning: objtool:   .altinstr_replacement+0x1063: (branch)
defconfig-build/vmlinux.o: warning: objtool:   __x86_indirect_thunk_rax()+0x0: (alt)
defconfig-build/vmlinux.o: warning: objtool:   __x86_indirect_thunk_rax()+0x0: <=== (sym)

(for every single register, not just rax)

Which is daft as well, because the retpoline.o run is clean. It also
doesn't make sense because __x86_retpoline_rax isn't in fact STT_FUNC,
so WTH ?!