linux-kernel - Re: [PATCH v2 3.18-rc3] kdb: Avoid printing KERN

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Date:	Fri, 07 Nov 2014 08:04:32 -0800
From:	Joe Perches <joe@...ches.com>
To:	Daniel Thompson <daniel.thompson@...aro.org>
Cc:	Jason Wessel <jason.wessel@...driver.com>,
	linux-kernel@...r.kernel.org, kgdb-bugreport@...ts.sourceforge.net,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>, patches@...aro.org,
	linaro-kernel@...ts.linaro.org,
	John Stultz <john.stultz@...aro.org>,
	Sumit Semwal <sumit.semwal@...aro.org>, stable@...r.kernel.org
Subject: Re: [PATCH v2 3.18-rc3] kdb: Avoid printing KERN_ levels to consoles

On Fri, 2014-11-07 at 12:01 +0000, Daniel Thompson wrote:
> Currently when kdb traps printk messages then the raw log level prefix
> (consisting of '\001' followed by a numeral) does not get stripped off
> before the message is issued to the various I/O handlers supported by
> kdb. This causes annoying visual noise as well as causing problems
> grepping for ^. It is also a change of behaviour compared to normal usage
> of printk() usage. For example <SysRq>-h ends up with different output to
> that of kdb's "sr h".
> 
> This patch addresses the problem by stripping log levels from messages
> before they are issued to the I/O handlers. printk() which can also
> act as an i/o handler in some cases is special cased; if the caller
> provided a log level then this will be preserved when sent to printk().
> 
> The addition of non-printable characters to the output of kdb commands is a
> regression, albeit and extremely elderly one, introduced by commit
> 04d2c8c83d0e ("printk: convert the format for KERN_<LEVEL> to a 2 byte
> pattern"). Note also that this patch does *not* restore the original
> behaviour from v3.5. Instead it makes printk() from within a kdb command
> display the message without any prefix (i.e. like printk() normally does).
[]
> diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c
[]
> @@ -711,7 +712,10 @@ kdb_printit:
>  	if (logging) {
>  		saved_loglevel = console_loglevel;
>  		console_loglevel = CONSOLE_LOGLEVEL_SILENT;
> -		printk(KERN_INFO "%s", kdb_buffer);
> +		if (cp == kdb_buffer)
> +			printk(KERN_INFO "%s", kdb_buffer);
> +		else
> +			printk("%s", kdb_buffer);

The first part of the patch seem fine, but I'm
confused about this bit above.

Here, isn't the "if (cp == kdb_buffer)" unnecessary?

if "(cp != kdb_buffer)", the buffer does have
a prefix and it's emitted.

If (cp == kdb_buffer), the buffer does _not_ have
a prefix (meaning it's either a naked printk or
a continuation line. (KERN_CONT is "")

So why insert KERN_INFO?

I believe the code should be:

-		printk(KERN_INFO "%s", kdb_buffer);
+		printk("%s", kdb_buffer);


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