Received-SPF: softfail (google.com: domain of transitioning linux-kernel-owner@vger.kernel.org does not designate 23.128.96.19 as permitted sender) client-ip=23.128.96.19;
Date:   Mon, 4 Apr 2022 00:25:51 +0200
From:   Borislav Petkov <bp@alien8.de>
To:     Alexey Dobriyan <adobriyan@gmail.com>
Cc:     x86@kernel.org, tglx@linutronix.de, mingo@redhat.com,
        dave.hansen@linux.intel.com, hpa@zytor.com,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/5] x86/alternative: simplify DUMP_BYTES macro
Message-ID: <Ykoe7zdOBAwDjSh+@zn.tnic>
References: <20220311144312.88466-1-adobriyan@gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <20220311144312.88466-1-adobriyan@gmail.com>
Precedence: bulk

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@gmail.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.

https://people.kernel.org/tglx/notes-about-netiquette