linux-kernel - Re: [PATCH] x86: Avoid pr_cont() in show

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Message-ID: <f486debf-e824-0d0c-c0c9-cf1e278ee9eb@i-love.sakura.ne.jp>
Date:   Tue, 17 Jul 2018 22:53:55 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Rasmus Villemoes <mail@...musvillemoes.dk>,
        Ingo Molnar <mingo@...nel.org>
Cc:     mingo@...hat.com, linux-kernel@...r.kernel.org,
        Tetsuo Handa <penguin-kernel@...ove.SKAURA.ne.jp>,
        Andy Lutomirski <luto@...capital.net>,
        Borislav Petkov <bp@...e.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] x86: Avoid pr_cont() in show_opcodes()

On 2018/07/17 18:01, Rasmus Villemoes wrote:
> Why not this instead? Less stack use, less code, no intermediary
> snprintfs, no pr_cont...

Excellent! I didn't notice %ph extension.

> Not compile-tested, probably whitespace-damaged, but you get the idea.

Yes, it works well.



>From 96d9d4d135994a081e54d33d23f5007c53d9b5dd Mon Sep 17 00:00:00 2001
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Date: Tue, 17 Jul 2018 22:47:11 +0900
Subject: [PATCH v3] x86: Avoid pr_cont() in show_opcodes()

Since syzbot is confused by concurrent printk() messages [1],
this patch changes show_opcodes() to use %*ph format string.

When we start adding prefix to each line of printk() output,
we will be able to handle concurrent printk() messages.

[1] https://syzkaller.appspot.com/text?tag=CrashReport&x=139d342c400000

Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SKAURA.ne.jp>
Signed-off-by: Rasmus Villemoes <mail@...musvillemoes.dk>
Cc: Borislav Petkov <bp@...e.de>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Josh Poimboeuf <jpoimboe@...hat.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Andy Lutomirski <luto@...capital.net>
---
 arch/x86/kernel/dumpstack.c | 25 +++++++------------------
 1 file changed, 7 insertions(+), 18 deletions(-)

diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
index 666a284..ffdd484 100644
--- a/arch/x86/kernel/dumpstack.c
+++ b/arch/x86/kernel/dumpstack.c
@@ -93,26 +93,15 @@ static void printk_stack_address(unsigned long address, int reliable,
  */
 void show_opcodes(u8 *rip, const char *loglvl)
 {
-	unsigned int code_prologue = OPCODE_BUFSIZE * 2 / 3;
+	const unsigned int prologue = OPCODE_BUFSIZE * 2 / 3;
 	u8 opcodes[OPCODE_BUFSIZE];
-	u8 *ip;
-	int i;
 
-	printk("%sCode: ", loglvl);
-
-	ip = (u8 *)rip - code_prologue;
-	if (probe_kernel_read(opcodes, ip, OPCODE_BUFSIZE)) {
-		pr_cont("Bad RIP value.\n");
-		return;
-	}
-
-	for (i = 0; i < OPCODE_BUFSIZE; i++, ip++) {
-		if (ip == rip)
-			pr_cont("<%02x> ", opcodes[i]);
-		else
-			pr_cont("%02x ", opcodes[i]);
-	}
-	pr_cont("\n");
+	if (probe_kernel_read(opcodes, rip - prologue, OPCODE_BUFSIZE))
+		printk("%sCode: Bad RIP value.\n", loglvl);
+	else
+		printk("%sCode: %*ph <%02x> %*ph\n", loglvl, prologue, opcodes,
+		       opcodes[prologue], OPCODE_BUFSIZE - prologue - 1,
+		       &opcodes[prologue + 1]);
 }
 
 void show_ip(struct pt_regs *regs, const char *loglvl)
-- 
1.8.3.1