linux-kernel - Re: [PATCH 01/15] objtool: Find a destination for jumps beyond the section end

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Date:   Tue, 20 Apr 2021 13:25:43 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Josh Poimboeuf <jpoimboe@...hat.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 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

Sami