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

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Message-ID: <20201021183009.qbvhz7hsrm46vksn@treble>
Date:   Wed, 21 Oct 2020 13:30:09 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 Wed, Oct 21, 2020 at 11:51:33AM +0200, Peter Zijlstra wrote:
> 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 ?!

It is STT_FUNC:

  SYM_FUNC_START_NOALIGN(__x86_retpoline_\reg)

  $ readelf -s vmlinux.o |grep __x86_retpoline_rax
  129749: 0000000000000005    17 FUNC    GLOBAL DEFAULT   39 __x86_retpoline_rax

-- 
Josh