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

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Date:   Sat, 7 Jul 2018 22:54:28 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     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 2018/07/07 20:12, Ingo Molnar wrote:
> 
> * Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp> wrote:
> 
>> From: Tetsuo Handa <penguin-kernel@...ove.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
> 
> Does this change the output?
> 
> - If yes, could you show the before/after output in the changelog,
> 
> - If not (i.e. if only the number of printk calls is changed, the output is the 
>   same), could you say so in the changelog?

This patch will not change the output unless multiple threads concurrently
call printk(). The purpose of this patch is to help parsing kernel messages
when multiple threads are concurrently calling printk() for multiple times
(e.g. backtrace) by reducing pr_cont()/KERN_CONT usage.

> 
> Also, 3*OPCODE_BUFSIZE+2+1 is 195 bytes - isn't that a bit too much on-stack 
> footprint?

Then, we can reduce it by OPCODE_BUFSIZE bytes by unionizing opcodes[] and buf[].



>From 61752cef56fad2a910f6bfd277e1b9b028aeab43 Mon Sep 17 00:00:00 2001
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Date: Sat, 7 Jul 2018 22:45:30 +0900
Subject: [PATCH v2] x86: Avoid pr_cont() in show_opcodes()

Since syzbot is confused by concurrent printk() messages [1], this patch
changes show_opcodes() to use snprintf(). By this change, the Code: line
will always be printed as one line even if multiple threads concurrently
called printk().

To save on-stack footprint, this patch shares opcodes[] and buf[] because
we sequentially reads from opcodes[] and sequentially writes to buf[].

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>
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 | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
index 666a284..6408123 100644
--- a/arch/x86/kernel/dumpstack.c
+++ b/arch/x86/kernel/dumpstack.c
@@ -94,25 +94,27 @@ 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;
-	u8 opcodes[OPCODE_BUFSIZE];
 	u8 *ip;
 	int i;
-
-	printk("%sCode: ", loglvl);
+	int pos = 0;
+	char buf[(3 * OPCODE_BUFSIZE + 2) + 1];
+	u8 *opcodes = (u8 *) buf + sizeof(buf) - OPCODE_BUFSIZE;
 
 	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)
-- 
1.8.3.1