* Re: [PATCH] x86/vmlinux: Fix linker fill bytes for ld.lld
[not found] <20230906175215.2236033-1-song@kernel.org>
@ 2023-09-06 18:06 ` Kees Cook
2023-09-06 18:16 ` Song Liu
0 siblings, 1 reply; 3+ messages in thread
From: Kees Cook @ 2023-09-06 18:06 UTC (permalink / raw)
To: Song Liu, Fangrui Song
Cc: linux-kernel, kernel-team, x86, clang-built-linux,
linux-hardening
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@chromium.org>
> Cc: x86@kernel.org
> Signed-off-by: Song Liu <song@kernel.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@chromium.org>
--
Kees Cook
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] x86/vmlinux: Fix linker fill bytes for ld.lld
2023-09-06 18:06 ` [PATCH] x86/vmlinux: Fix linker fill bytes for ld.lld Kees Cook
@ 2023-09-06 18:16 ` Song Liu
2023-09-06 18:27 ` Fangrui Song
0 siblings, 1 reply; 3+ messages in thread
From: Song Liu @ 2023-09-06 18:16 UTC (permalink / raw)
To: Kees Cook
Cc: Song Liu, Fangrui Song, LKML, Kernel Team, x86@kernel.org,
clang-built-linux, linux-hardening@vger.kernel.org
> On Sep 6, 2023, at 11:06 AM, Kees Cook <keescook@chromium.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@chromium.org>
>> Cc: x86@kernel.org
>> Signed-off-by: Song Liu <song@kernel.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
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] x86/vmlinux: Fix linker fill bytes for ld.lld
2023-09-06 18:16 ` Song Liu
@ 2023-09-06 18:27 ` Fangrui Song
0 siblings, 0 replies; 3+ messages in thread
From: Fangrui Song @ 2023-09-06 18:27 UTC (permalink / raw)
To: Song Liu
Cc: Kees Cook, Song Liu, LKML, Kernel Team, x86@kernel.org,
clang-built-linux, linux-hardening@vger.kernel.org
On Wed, Sep 6, 2023 at 11:16 AM Song Liu <songliubraving@meta.com> wrote:
>
>
>
> > On Sep 6, 2023, at 11:06 AM, Kees Cook <keescook@chromium.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@chromium.org>
> >> Cc: x86@kernel.org
> >> Signed-off-by: Song Liu <song@kernel.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
AFAIK GNU ld's behavior is not documented here
https://sourceware.org/binutils/docs/ld/Output-Section-Fill.html
The Output Section Fill syntax allows an expression. It seems that if
you use =0xcc+0, GNU ld will use a 4-byte filler as well, similar to
gold and ld.lld.
Frankly, I feel that GNU ld behavior should not be relied upon. lld's
comment states that it is a deliberate choice to follow gold instead
of GNU ld here.
I'll elaborate a bit and add this discrepancy to my
https://maskray.me/blog/2020-12-19-lld-and-gnu-linker-incompatibilities
:)
--
宋方睿
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2023-09-06 18:06 ` [PATCH] x86/vmlinux: Fix linker fill bytes for ld.lld Kees Cook
2023-09-06 18:16 ` Song Liu
2023-09-06 18:27 ` Fangrui Song
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