linux-kernel - Re: [PATCH] x86/vmlinux: Fix linker fill bytes for ld.lld

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Message-ID: <CAPhsuW4eGj7ByybEF3c_fURys367U3_gyF_Y5zpsnKUtiqOr+Q@mail.gmail.com>
Date:   Wed, 6 Sep 2023 13:02:45 -0700
From:   Song Liu <song@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, kernel-team@...a.com,
        Kees Cook <keescook@...omium.org>, x86@...nel.org
Subject: Re: [PATCH] x86/vmlinux: Fix linker fill bytes for ld.lld

On Wed, Sep 6, 2023 at 12:58 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Wed, Sep 06, 2023 at 10:52:15AM -0700, Song Liu wrote:
> > With ":text =0xcccc", ld.lld fills unused text area with 0xcccc0000.
> > Example objdump -D output:
> >
> > ffffffff82b04203:       00 00                   add    %al,(%rax)
> > ffffffff82b04205:       cc                      int3
> > ffffffff82b04206:       cc                      int3
> > ffffffff82b04207:       00 00                   add    %al,(%rax)
> > ffffffff82b04209:       cc                      int3
> > ffffffff82b0420a:       cc                      int3
> >
> > Replace it with ":text =0xcccccccc", so we get the following instead:
> >
> > ffffffff82b04203:       cc                      int3
> > ffffffff82b04204:       cc                      int3
> > ffffffff82b04205:       cc                      int3
> > ffffffff82b04206:       cc                      int3
> > ffffffff82b04207:       cc                      int3
> > ffffffff82b04208:       cc                      int3
> >
> > gcc/ld doesn't seem to have the same issue. The generated code stays the
> > same for gcc/ld.
> >
> > Cc: Kees Cook <keescook@...omium.org>
> > Cc: x86@...nel.org
> > Signed-off-by: Song Liu <song@...nel.org>
>
> Please provide a Fixes tag, I'm thinking this (otherwise trivial commit)
> wants to be backported for sanity.

I guess we need:

Fixes: 7705dc855797 ("x86/vmlinux: Use INT3 instead of NOP for linker
fill bytes")

Thanks,
Song

>
> Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>