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

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Message-ID: <20230906195814.GE28278@noisy.programming.kicks-ass.net>
Date:   Wed, 6 Sep 2023 21:58:14 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Song Liu <song@...nel.org>
Cc:     linux-kernel@...r.kernel.org, kernel-team@...a.com,
        Kees Cook <keescook@...omium.org>, x86@...nel.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>

Please provide a Fixes tag, I'm thinking this (otherwise trivial commit)
wants to be backported for sanity.

Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>

> ---
>  arch/x86/kernel/vmlinux.lds.S | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
> index 83d41c2601d7..41d56fb9bf92 100644
> --- a/arch/x86/kernel/vmlinux.lds.S
> +++ b/arch/x86/kernel/vmlinux.lds.S
> @@ -156,7 +156,7 @@ SECTIONS
>  		ALIGN_ENTRY_TEXT_END
>  		*(.gnu.warning)
>  
> -	} :text =0xcccc
> +	} :text =0xcccccccc
>  
>  	/* End of text section, which should occupy whole number of pages */
>  	_etext = .;
> -- 
> 2.34.1
>