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

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Date:   Sun, 13 Mar 2022 21:09:58 +0300
From:   Alexey Dobriyan <adobriyan@...il.com>
To:     Joe Perches <joe@...ches.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 1/5] x86/alternative: simplify DUMP_BYTES macro

On Sat, Mar 12, 2022 at 08:36:11AM -0800, Joe Perches wrote:
> On Fri, 2022-03-11 at 17:43 +0300, Alexey Dobriyan wrote:
> > Avoid zero length check with clever whitespace placement in the format
> > string.
> []
> > diff --git 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");					\
> >  	}								\
> 
> This could also use %02x and not %02hhx

I doubt as there is funky stuff possible with 255 and such values.
Format specifiers aren't the purpose of the patch anyway.

> And MAX_PATCH_LEN is 255 but is that really possible?

Yes if you try hard enough.

> Maybe if the actual patch length is always <= 64 this could use
> 	printk(KERN_CONT "%*ph\n", (int)len, buf);
> instead and avoid all possible interleaving?

It is for debugging feature nobody uses (because it works).