linux-kernel - Re: [PATCH 1/5] x86/alternative: simplify DUMP

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Date:   Mon, 4 Apr 2022 00:25:51 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Alexey Dobriyan <adobriyan@...il.com>
Cc:     x86@...nel.org, tglx@...utronix.de, mingo@...hat.com,
        dave.hansen@...ux.intel.com, hpa@...or.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/5] x86/alternative: simplify DUMP_BYTES macro

On Fri, Mar 11, 2022 at 05:43:08PM +0300, Alexey Dobriyan wrote:
> Avoid zero length check with clever whitespace placement in the format
> string.
> 
> Signed-off-by: Alexey Dobriyan (CloudLinux) <adobriyan@...il.com>
> ---
>  arch/x86/kernel/alternative.c | 21 +++++++++------------
>  1 file changed, 9 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index 5007c3ffe96f..6c9758ee6810 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -66,13 +66,10 @@ do {									\
>  	if (unlikely(debug_alternative)) {				\
>  		int j;							\
>  									\
> -		if (!(len))						\
> -			break;						\
> -									\
>  		printk(KERN_DEBUG pr_fmt(fmt), ##args);			\
> -		for (j = 0; j < (len) - 1; j++)				\
> -			printk(KERN_CONT "%02hhx ", buf[j]);		\
> -		printk(KERN_CONT "%02hhx\n", buf[j]);			\
> +		for (j = 0; j < (len); j++)				\
> +			printk(KERN_CONT " %02hhx", buf[j]);		\
> +		printk(KERN_CONT "\n");					\
>  	}								\
>  } while (0)

That doesn't work always.

Before:

SMP alternatives: feat: 9*32+0, old: (current_save_fsgs+0x32/0xa0 (ffffffff81017762) len: 5), repl: (ffffffff89997c78, len: 0)
SMP alternatives: ffffffff81017762:   old_insn: eb 32 0f 1f 00
SMP alternatives: ffffffff81017762: final_insn: 90 90 90 90 90
SMP alternatives: ffffffff81017762: [0:5) optimized NOPs: 0f 1f 44 00 00


After:

SMP alternatives: feat: 9*32+0, old: (current_save_fsgs+0x32/0xa0 (ffffffff81017762) len: 5), repl: (ffffffff89997c78, len: 0)
SMP alternatives: ffffffff81017762:   old_insn: eb 32 0f 1f 00
SMP alternatives: ffffffff89997c78:   rpl_insn:			<----- *
SMP alternatives: ffffffff81017762: final_insn: 90 90 90 90 90
SMP alternatives: ffffffff81017762: [0:5) optimized NOPs: 0f 1f 44 00 00

there is no replacement insn in this case:

static __always_inline bool _static_cpu_has(u16 bit)
{
	asm_volatile_goto(
		ALTERNATIVE_TERNARY("jmp 6f", %P[feature], "", "jmp %l[t_no]")
						   	   ^^

-- 
Regards/Gruss,
    Boris.

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