* 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 ^ permalink raw reply [flat|nested] 3+ messages in thread
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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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