linux-kernel - Re: [PATCH] x86/build: Simplify patterns for unwanted section

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Date: Wed, 14 Feb 2024 14:07:23 -0800
From: Kees Cook <keescook@...omium.org>
To: Fangrui Song <maskray@...gle.com>
Cc: Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	x86@...nel.org, linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
	Nathan Chancellor <nathan@...nel.org>,
	Heiko Carstens <hca@...ux.ibm.com>
Subject: Re: [PATCH] x86/build: Simplify patterns for unwanted section

On Wed, Feb 14, 2024 at 01:29:29PM -0800, Fangrui Song wrote:
> A s390 patch modeling its --orphan-handling= after x86 [1] sparked my
> motivation to simplify patterns. Commit 5354e84598f2 ("x86/build: Add
> asserts for unwanted sections") added asserts that certain input
> sections must be absent or empty. The patterns can be simplified.
> 
> For dynamic relocations,
> 
> *(.rela.*) is sufficient to match all dynamic relocations synthesized by
> GNU ld and LLD. .rela_* is unnecessary. --emit-relocs may create .rela_*
> sections for section names prefixed with _, but they are not matched by
> linker scripts.
> 
> .plt instead of .plt.* is sufficient to match synthesized PLT entries.

Do you mean ".plt.foo" matches ".plt" ?

> .igot and .igot.plt are for non-preemptible STT_GNU_IFUNC in GNU ld (LLD
> just uses .got), which the kernel does not use. In addition, if .igot or
> .igot.plt is ever non-empty, there will be .rela.* dynamic relocations
> leading to an assert failure anyway.

I think at the time I was dealing with avoid multiple warnings out of
the linker, as I was getting orphan warnings in addition to the
non-empty warnings.

> 
> [1]: https://lore.kernel.org/all/20240207-s390-lld-and-orphan-warn-v1-6-8a665b3346ab@kernel.org/
> 
> Signed-off-by: Fangrui Song <maskray@...gle.com>

Is anything harmed by leaving all of this as-is?

-Kees

> ---
>  arch/x86/boot/compressed/vmlinux.lds.S | 6 +++---
>  arch/x86/kernel/vmlinux.lds.S          | 8 ++++----
>  2 files changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/boot/compressed/vmlinux.lds.S b/arch/x86/boot/compressed/vmlinux.lds.S
> index 083ec6d7722a..9f288f67972a 100644
> --- a/arch/x86/boot/compressed/vmlinux.lds.S
> +++ b/arch/x86/boot/compressed/vmlinux.lds.S
> @@ -104,17 +104,17 @@ SECTIONS
>  	ASSERT(SIZEOF(.got) == 0, "Unexpected GOT entries detected!")
>  
>  	.plt : {
> -		*(.plt) *(.plt.*)
> +		*(.plt)
>  	}
>  	ASSERT(SIZEOF(.plt) == 0, "Unexpected run-time procedure linkages detected!")
>  
>  	.rel.dyn : {
> -		*(.rel.*) *(.rel_*)
> +		*(.rel.*)
>  	}
>  	ASSERT(SIZEOF(.rel.dyn) == 0, "Unexpected run-time relocations (.rel) detected!")
>  
>  	.rela.dyn : {
> -		*(.rela.*) *(.rela_*)
> +		*(.rela.*)
>  	}
>  	ASSERT(SIZEOF(.rela.dyn) == 0, "Unexpected run-time relocations (.rela) detected!")
>  }
> diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
> index a349dbfc6d5a..b3da7b81d2b3 100644
> --- a/arch/x86/kernel/vmlinux.lds.S
> +++ b/arch/x86/kernel/vmlinux.lds.S
> @@ -463,22 +463,22 @@ SECTIONS
>  	 * explicitly check instead of blindly discarding.
>  	 */
>  	.got : {
> -		*(.got) *(.igot.*)
> +		*(.got)
>  	}
>  	ASSERT(SIZEOF(.got) == 0, "Unexpected GOT entries detected!")
>  
>  	.plt : {
> -		*(.plt) *(.plt.*) *(.iplt)
> +		*(.plt)
>  	}
>  	ASSERT(SIZEOF(.plt) == 0, "Unexpected run-time procedure linkages detected!")
>  
>  	.rel.dyn : {
> -		*(.rel.*) *(.rel_*)
> +		*(.rel.*)
>  	}
>  	ASSERT(SIZEOF(.rel.dyn) == 0, "Unexpected run-time relocations (.rel) detected!")
>  
>  	.rela.dyn : {
> -		*(.rela.*) *(.rela_*)
> +		*(.rela.*)
>  	}
>  	ASSERT(SIZEOF(.rela.dyn) == 0, "Unexpected run-time relocations (.rela) detected!")
>  }
> -- 
> 2.43.0.687.g38aa6559b0-goog
> 

-- 
Kees Cook