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Date:   Tue, 20 Apr 2021 15:58:35 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     X86 ML <x86@...nel.org>, Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Sedat Dilek <sedat.dilek@...il.com>,
        linux-hardening@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH 01/15] objtool: Find a destination for jumps beyond the
 section end

On Tue, Apr 20, 2021 at 3:57 PM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
>
> On Tue, Apr 20, 2021 at 01:25:43PM -0700, Sami Tolvanen wrote:
> > On Tue, Apr 20, 2021 at 11:14 AM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> > >
> > > On Fri, Apr 16, 2021 at 01:38:30PM -0700, Sami Tolvanen wrote:
> > > > With -ffunction-sections, Clang can generate a jump beyond the end of
> > > > a section when the section ends in an unreachable instruction.
> > >
> > > Why?  Can you show an example?
> >
> > Here's the warning I'm seeing when building allyesconfig + CFI:
> >
> > vmlinux.o: warning: objtool:
> > rockchip_spi_transfer_one.f088382d97b74759d70e27e891fe8f1c()+0x149:
> > can't find jump dest instruction at
> > .text.rockchip_spi_transfer_one.f088382d97b74759d70e27e891fe8f1c+0x7dc
> >
> > $ objdump -d -r -j
> > .text.rockchip_spi_transfer_one.f088382d97b74759d70e27e891fe8f1c
> > vmlinux.o
> > 0000000000000000 <rockchip_spi_transfer_one.f088382d97b74759d70e27e891fe8f1c>:
> >   ...
> >  149:   0f 85 8d 06 00 00       jne    7dc <.compoundliteral.4>
> >   ...
> >  7d7:   e8 00 00 00 00          callq  7dc <.compoundliteral.4>
> >                         7d8: R_X86_64_PLT32     __stack_chk_fail-0x4
>
> Instead of silencing the warning by faking the jump destination, I'd
> rather improve the warning to something like
>
>   "warning: rockchip_spi_transfer_one() falls through to the next function"
>
> which is what we normally do in this type of situation.
>
> It may be caused by UB, or a compiler bug, but either way we should
> figure out the root cause.

We probably want to creduce or cvise this.  IIRC we still have
outstanding issues with switch statements with user-annotated
unreachable branches not getting eliminated.
-- 
Thanks,
~Nick Desaulniers

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