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From:   Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
To:     mingo@redhat.com
Cc:     linux-kernel@vger.kernel.org,
        Tetsuo Handa <penguin-kernel@I-love.SKAURA.ne.jp>,
        Andy Lutomirski <luto@amacapital.net>,
        Borislav Petkov <bp@suse.de>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: [PATCH] x86: Avoid pr_cont() in show_opcodes()
Date:   Sat,  7 Jul 2018 15:07:05 +0900
Message-Id: <1530943625-3288-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

From: Tetsuo Handa <penguin-kernel@I-love.SKAURA.ne.jp>

Since syzbot is confused by concurrent printk() messages [1],
this patch changes show_opcodes() to use snprintf().

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@I-love.SKAURA.ne.jp>
Cc: Borislav Petkov <bp@suse.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
---
 arch/x86/kernel/dumpstack.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
index 666a284..bb47426 100644
--- a/arch/x86/kernel/dumpstack.c
+++ b/arch/x86/kernel/dumpstack.c
@@ -97,22 +97,24 @@ void show_opcodes(u8 *rip, const char *loglvl)
 	u8 opcodes[OPCODE_BUFSIZE];
 	u8 *ip;
 	int i;
-
-	printk("%sCode: ", loglvl);
+	int pos = 0;
+	char buf[(3 * OPCODE_BUFSIZE + 2) + 1];
 
 	ip = (u8 *)rip - code_prologue;
 	if (probe_kernel_read(opcodes, ip, OPCODE_BUFSIZE)) {
-		pr_cont("Bad RIP value.\n");
+		printk("%sCode: Bad RIP value.\n", loglvl);
 		return;
 	}
 
 	for (i = 0; i < OPCODE_BUFSIZE; i++, ip++) {
 		if (ip == rip)
-			pr_cont("<%02x> ", opcodes[i]);
+			pos += snprintf(buf + pos, sizeof(buf) - pos,
+					"<%02x> ", opcodes[i]);
 		else
-			pr_cont("%02x ", opcodes[i]);
+			pos += snprintf(buf + pos, sizeof(buf) - pos,
+					"%02x ", opcodes[i]);
 	}
-	pr_cont("\n");
+	printk("%sCode: %s\n", loglvl, buf);
 }
 
 void show_ip(struct pt_regs *regs, const char *loglvl)
-- 
2.7.4