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Message-ID: <34CC4A68-CE36-4DE5-B1A1-AB7FD075C0EB@fb.com>
Date: Wed, 6 Sep 2023 18:16:48 +0000
From: Song Liu <songliubraving@...a.com>
To: Kees Cook <keescook@...omium.org>
CC: Song Liu <song@...nel.org>, Fangrui Song <maskray@...gle.com>,
LKML <linux-kernel@...r.kernel.org>,
Kernel Team <kernel-team@...a.com>,
"x86@...nel.org" <x86@...nel.org>,
clang-built-linux <llvm@...ts.linux.dev>,
"linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>
Subject: Re: [PATCH] x86/vmlinux: Fix linker fill bytes for ld.lld
> On Sep 6, 2023, at 11:06 AM, Kees Cook <keescook@...omium.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>
>
> 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?
Per my experiment, gcc/ld gives same output for :text =0xcc, :text =0xcccc,
and :text =0xcccccccc; while ld.lld handles :text = as u32, so :text =0xcc
with ld.lld gives:
ffffffff82b042a1: 00 cc add %cl,%ah
ffffffff82b042a3: 00 00 add %al,(%rax)
ffffffff82b042a5: 00 cc add %cl,%ah
ffffffff82b042a7: 00 00 add %al,(%rax)
ffffffff82b042a9: 00 cc add %cl,%ah
ffffffff82b042ab: 00 00 add %al,(%rax)
I am not sure what the right behavior is per specification.
Thanks,
Song
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