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

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Message-ID: <e9c77d12092a4f048992f67d3fa0cf363b8614d4.camel@perches.com>
Date:   Sat, 12 Mar 2022 08:36:11 -0800
From:   Joe Perches <joe@...ches.com>
To:     Alexey Dobriyan <adobriyan@...il.com>, x86@...nel.org
Cc:     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 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

And MAX_PATCH_LEN is 255 but is that really possible?

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?

If so, maybe just remove DUMP_BYTES and use DPRINTK directly.

Perhaps:
---
 arch/x86/kernel/alternative.c | 31 ++++++++++---------------------
 1 file changed, 10 insertions(+), 21 deletions(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 018b61febf0e7..74fa946093467 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -61,21 +61,6 @@ do {									\
 		printk(KERN_DEBUG pr_fmt(fmt) "\n", ##args);		\
 } while (0)
 
-#define DUMP_BYTES(buf, len, fmt, args...)				\
-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]);			\
-	}								\
-} while (0)
-
 static const unsigned char x86nops[] =
 {
 	BYTES_NOP1,
@@ -214,7 +199,8 @@ static __always_inline int optimize_nops_range(u8 *instr, u8 instrlen, int off)
 	add_nops(instr + off, nnops);
 	local_irq_restore(flags);
 
-	DUMP_BYTES(instr, instrlen, "%px: [%d:%d) optimized NOPs: ", instr, off, i);
+	DPRINTK("%px: [%d:%d) optimized NOPs: %*ph",
+		instr, off, i, (int)instrlen, instr);
 
 	return nnops;
 }
@@ -303,8 +289,10 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
 			instr, instr, a->instrlen,
 			replacement, a->replacementlen);
 
-		DUMP_BYTES(instr, a->instrlen, "%px:   old_insn: ", instr);
-		DUMP_BYTES(replacement, a->replacementlen, "%px:   rpl_insn: ", replacement);
+		DPRINTK("%px:   old_insn: %*ph",
+			instr, (int)a->instrlen, instr);
+		DPRINTK("%px:   rpl_insn: %*ph",
+			replacement, (int)a->replacementlen, replacement);
 
 		memcpy(insn_buff, replacement, a->replacementlen);
 		insn_buff_sz = a->replacementlen;
@@ -328,7 +316,8 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
 		for (; insn_buff_sz < a->instrlen; insn_buff_sz++)
 			insn_buff[insn_buff_sz] = 0x90;
 
-		DUMP_BYTES(insn_buff, insn_buff_sz, "%px: final_insn: ", instr);
+		DPRINTK("%px: final_insn: %*ph",
+			instr, (int)insn_buff_sz, insn_buff);
 
 		text_poke_early(instr, insn_buff, insn_buff_sz);
 
@@ -499,8 +488,8 @@ void __init_or_module noinline apply_retpolines(s32 *start, s32 *end)
 		len = patch_retpoline(addr, &insn, bytes);
 		if (len == insn.length) {
 			optimize_nops(bytes, len);
-			DUMP_BYTES(((u8*)addr),  len, "%px: orig: ", addr);
-			DUMP_BYTES(((u8*)bytes), len, "%px: repl: ", addr);
+			DPRINTK("%px: orig: %*ph", addr, (int)len, addr);
+			DPRINTK("%px: repl: %*ph", addr, (int)len, bytes);
 			text_poke_early(addr, bytes, len);
 		}
 	}