linux-kernel - Re: [PATCH] arm64: vmlinux.lds.S: keep .entry.tramp.text section

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Date:   Fri, 26 Feb 2021 12:59:17 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Ard Biesheuvel <ardb@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Marc Zyngier <maz@...nel.org>,
        David Brazdil <dbrazdil@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Fangrui Song <maskray@...gle.com>
Subject: Re: [PATCH] arm64: vmlinux.lds.S: keep .entry.tramp.text section

On Fri, Feb 26, 2021 at 03:03:39PM +0100, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@...db.de>
> 
> When building with CONFIG_LD_DEAD_CODE_DATA_ELIMINATION,
> I sometimes see an assertion
> 
>  ld.lld: error: Entry trampoline text too big

Heh, "too big" seems a weird report for having it discarded. :)

Any idea on this Fangrui?

( I see this is https://github.com/ClangBuiltLinux/linux/issues/1311 )

> 
> This happens when any reference to the trampoline is discarded at link
> time. Marking the section as KEEP() avoids the assertion, but I have
> not figured out whether this is the correct solution for the underlying
> problem.
> 
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

As a work-around, it seems fine to me.

Reviewed-by: Kees Cook <keescook@...omium.org>

-Kees

> ---
>  arch/arm64/kernel/vmlinux.lds.S | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
> index 926cdb597a45..c5ee9d5842db 100644
> --- a/arch/arm64/kernel/vmlinux.lds.S
> +++ b/arch/arm64/kernel/vmlinux.lds.S
> @@ -96,7 +96,7 @@ jiffies = jiffies_64;
>  #define TRAMP_TEXT					\
>  	. = ALIGN(PAGE_SIZE);				\
>  	__entry_tramp_text_start = .;			\
> -	*(.entry.tramp.text)				\
> +	KEEP(*(.entry.tramp.text))			\
>  	. = ALIGN(PAGE_SIZE);				\
>  	__entry_tramp_text_end = .;
>  #else
> -- 
> 2.29.2
> 

-- 
Kees Cook