linux-kernel - Re: [PATCH 2/2] panic: Enable to print out all printk msg in buffer

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Date:   Wed, 10 Apr 2019 18:12:25 +0800
From:   Feng Tang <feng.tang@...el.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>,
        Borislav Petkov <bp@...e.de>
Subject: Re: [PATCH 2/2] panic: Enable to print out all printk msg in buffer

Hi Petr,

On Tue, Apr 09, 2019 at 04:14:30PM +0200, Petr Mladek wrote:
> On Mon 2019-04-01 18:48:04, Feng Tang wrote:
> > Currently on panic, kernel will lower the loglevel and print out
> > new printk msg only. With this patch, user can configure the
> > "panic_print" to see all dmesg in buffer, some of which they may
> > have never seen due to the loglevel setting.
> > 
> > Signed-off-by: Feng Tang <feng.tang@...el.com>
> > ---
> > diff --git a/kernel/panic.c b/kernel/panic.c
> > index fb77e01..afe023e 100644
> > --- a/kernel/panic.c
> > +++ b/kernel/panic.c
> > @@ -51,6 +51,7 @@ EXPORT_SYMBOL_GPL(panic_timeout);
> >  #define PANIC_PRINT_TIMER_INFO		0x00000004
> >  #define PANIC_PRINT_LOCK_INFO		0x00000008
> >  #define PANIC_PRINT_FTRACE_INFO		0x00000010
> > +#define PANIC_PRINT_ALL_PRINTK_MSG	0x00000020
> >  unsigned long panic_print;
> >  
> >  ATOMIC_NOTIFIER_HEAD(panic_notifier_list);
> > @@ -134,6 +135,13 @@ EXPORT_SYMBOL(nmi_panic);
> >  
> >  static void panic_print_sys_info(void)
> >  {
> > +	bool flush_all_dmesg = false;
> > +
> > +	if (panic_print & PANIC_PRINT_ALL_PRINTK_MSG)
> > +		flush_all_dmesg = true;
> > +
> > +	console_flush_on_panic(flush_all_dmesg);
> 
> We should:
> 
>    + Flush the latest messages before we replay the log.
> 
>    + Show some header line before replaing the log.
> 
> 
> Therefore, I would keep console_flush_on_panic() as is.
> Then I would add:
> 
> int console_replay;
> 
> void console_replay_on_panic(void)
> {
> 	/*
> 	 * If someone else is holding the console lock, trylock will fail
> 	 * and may_schedule may be set.  Ignore and proceed to unlock so
> 	 * that messages are flushed out.  As this can be called from any
> 	 * context and we don't want to get preempted while flushing,
> 	 * ensure may_schedule is cleared.
> 	 */
> 	console_trylock();
> 	console_may_schedule = 0;
> 	console_replay = 1;
> 	console_unlock();
> }

Many thanks for the review and detailed sample codes.

Your code of moving the console_seq and console_index changing
code into concole_unlock() is much safer, which is protected by
the locl.

Initially I thought of adding a new function similar to this, but
was afraid that it is too similar to the console_flush_on_panic() :)

> 
> Then I would update cosole_unlock() with something like:
> 
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 02ca827b8fac..14ef4e2431e7 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -2386,21 +2386,32 @@ void console_unlock(void)
>  
>  	for (;;) {
>  		struct printk_log *msg;
> +		int reset_idx = 0;
>  		size_t ext_len = 0;
> -		size_t len;
> +		size_t len = 0;
>  
>  		printk_safe_enter_irqsave(flags);
>  		raw_spin_lock(&logbuf_lock);
> +
>  		if (console_seq < log_first_seq) {
>  			len = sprintf(text,
>  				      "** %llu printk messages dropped **\n",
>  				      log_first_seq - console_seq);
>  
>  			/* messages are gone, move to first one */
> +			reset_idx = 1;
> +		}
> +
> +		if (console_replay) {
> +			len += sprintf(text + len,
> +				       "Replaying the entire log:\n");
> +			reset_idx = 1;
> +			console_replay = 0;
> +		}
> +
> +		if (reset_idx) {
>  			console_seq = log_first_seq;
>  			console_idx = log_first_idx;
> -		} else {
> -			len = 0;
>  		}
>  skip:
>  		if (console_seq == log_next_seq)
> 
> 
> Finally, it can get called from panic_print_sys_info(void)
> the following way:
> 
> 	/* Replay existing messages before adding other sys info. */
> 	if (panic_print & PANIC_PRINT_ALL_PRINTK_MSG)
> 		console_replay_on_panic();

Yes, it should be the first inside panic_print_sys_info().

Thanks,
Feng