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

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Date:   Wed, 6 Sep 2023 11:06:02 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Song Liu <song@...nel.org>, Fangrui Song <maskray@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kernel-team@...a.com, x86@...nel.org,
        clang-built-linux <llvm@...ts.linux.dev>,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH] x86/vmlinux: Fix linker fill bytes for ld.lld

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>

Ah! Thanks for the catch... I wonder if ld.lld should be fixed too? My
understanding was that ":text =...." was defined as being explicitly
u16?

Reviewed-by: Kees Cook <keescook@...omium.org>

-- 
Kees Cook