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Date:   Tue, 20 Apr 2021 17:55:50 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.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>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        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>
Subject: Re: [PATCH 01/15] objtool: Find a destination for jumps beyond the
 section end

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.

-- 
Josh

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