linux-kernel - Re: [PATCH 3/5] x86/alternative: record .altinstructions section entity size

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Date:   Sat, 12 Mar 2022 22:17:40 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexey Dobriyan <adobriyan@...il.com>
Cc:     x86@...nel.org, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/5] x86/alternative: record .altinstructions section
 entity size

On Fri, Mar 11, 2022 at 05:43:10PM +0300, Alexey Dobriyan wrote:
> .altinstructions entry was 12 bytes in size, then it was 13 bytes,
> now it is 12 again. It was 24 bytes on some distros as well.
> Record this information as section sh_entsize value so that tools
> which parse .altinstructions have easier time.

Which tools would that be? Because afaict you've not actually updated
objtool.

> Signed-off-by: Alexey Dobriyan (CloudLinux) <adobriyan@...il.com>
> ---
>  arch/x86/include/asm/alternative.h | 13 ++++++++-----
>  1 file changed, 8 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h
> index 58eee6402832..cf7722a106b3 100644
> --- a/arch/x86/include/asm/alternative.h
> +++ b/arch/x86/include/asm/alternative.h
> @@ -9,6 +9,8 @@
>  #define ALTINSTR_FLAG_INV	(1 << 15)
>  #define ALT_NOT(feat)		((feat) | ALTINSTR_FLAG_INV)
>  
> +#define sizeof_struct_alt_instr 12
> +
>  #ifndef __ASSEMBLY__
>  
>  #include <linux/stddef.h>
> @@ -66,6 +68,7 @@ struct alt_instr {
>  	u8  instrlen;		/* length of original instruction */
>  	u8  replacementlen;	/* length of new instruction */
>  } __packed;
> +_Static_assert(sizeof(struct alt_instr) == sizeof_struct_alt_instr, "");

Would it not be much simpler to have this in asm-offsets.h ?

> +	".pushsection .altinstructions,\"aM\",@progbits," __stringify(sizeof_struct_alt_instr) "\n"\
> +	".pushsection .altinstructions,\"aM\",@progbits," __stringify(sizeof_struct_alt_instr) "\n"\
> +	".pushsection .altinstructions,\"aM\",@progbits," __stringify(sizeof_struct_alt_instr) "\n"\

> +	.pushsection .altinstructions,"aM",@progbits,sizeof_struct_alt_instr
> +	.pushsection .altinstructions,"aM",@progbits,sizeof_struct_alt_instr

Aside of adding entsize, you're also adding the M(ergable) bit. Also,
those lines are on the unwieldy side of things.