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

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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