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

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Date:   Mon, 16 Jul 2018 21:47:22 +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,
        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()

Ingo, is this patch acceptable?

On 2018/07/07 22:54, Tetsuo Handa wrote:
>>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)
>