linux-kernel - Re: [x86] only print out DR registers if they are not power-on defaults.

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Message-ID: <20130618084356.GA13123@pd.tnic>
Date:	Tue, 18 Jun 2013 10:43:56 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Dave Jones <davej@...hat.com>
Cc:	Linux Kernel <linux-kernel@...r.kernel.org>, x86@...nel.org
Subject: Re: [x86] only print out DR registers if they are not power-on
 defaults.

On Tue, Jun 18, 2013 at 12:11:32AM -0400, Dave Jones wrote:
> The DR registers are rarely useful when decoding oopses.
> With screen real estate during oopses at a premium, we can save two lines
> by only printing out these registers when they are set to something other
> than they power-on state.

Makes sense, except...

> 
> Signed-off-by: Dave Jones <davej@...hat.com>
> 
> diff -durpN '--exclude-from=/home/davej/.exclude' /home/davej/src/kernel/git-trees/linux/arch/x86/kernel/process_64.c linux-dj/arch/x86/kernel/process_64.c
> --- /home/davej/src/kernel/git-trees/linux/arch/x86/kernel/process_64.c	2013-05-01 10:02:52.064151923 -0400
> +++ linux-dj/arch/x86/kernel/process_64.c	2013-05-06 20:35:09.219868881 -0400
> @@ -105,11 +105,18 @@ void __show_regs(struct pt_regs *regs, i
>  	get_debugreg(d0, 0);
>  	get_debugreg(d1, 1);
>  	get_debugreg(d2, 2);
> -	printk(KERN_DEFAULT "DR0: %016lx DR1: %016lx DR2: %016lx\n", d0, d1, d2);
>  	get_debugreg(d3, 3);
>  	get_debugreg(d6, 6);
>  	get_debugreg(d7, 7);
> +
> +	/* Only print out debug registers if they are in their non-default state. */
> +	if ((d0 == 0) && (d1 == 0) && (d2 == 0) && (d3 == 0) &&
> +	    (d6 & ~DR6_RESERVED) && (d7 == 0x400))

... I'm not sure about %dr6. So we're not dumping %dr6 when, a.o.
any of the bits in the bit slices [3:0], [15:13] are set (bit 12
is Read-As-Zero). Now those mean stuff like Breakpoint-Condition
Detected or Debug-Register-Access Detected and so on and I think this is
meaningful information.

So actually, we wouldn't want to dump %dr6 when its contents are its
default contents, i.e, 0xffff0ff0, i.e. the test should be:

	(d6 == DR6_RESERVED)

no?

> +		return;
> +
> +	printk(KERN_DEFAULT "DR0: %016lx DR1: %016lx DR2: %016lx\n", d0, d1, d2);
>  	printk(KERN_DEFAULT "DR3: %016lx DR6: %016lx DR7: %016lx\n", d3, d6, d7);
> +
>  }
>  
>  void release_thread(struct task_struct *dead_task)
> 
> diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
> index 7305f7d..5905dc5 100644
> --- a/arch/x86/kernel/process_32.c
> +++ b/arch/x86/kernel/process_32.c
> @@ -110,11 +110,16 @@ void __show_regs(struct pt_regs *regs, int all)
>  	get_debugreg(d1, 1);
>  	get_debugreg(d2, 2);
>  	get_debugreg(d3, 3);
> -	printk(KERN_DEFAULT "DR0: %08lx DR1: %08lx DR2: %08lx DR3: %08lx\n",
> -			d0, d1, d2, d3);
> -
>  	get_debugreg(d6, 6);
>  	get_debugreg(d7, 7);
> +
> +	/* Only print out debug registers if they are in their non-default state. */
> +	if ((d0 == 0) && (d1 == 0) && (d2 == 0) && (d3 == 0) &&
> +	    (d6 & ~DR6_RESERVED) && (d7 == 0x400))

ditto.

> +		return;
> +
> +	printk(KERN_DEFAULT "DR0: %08lx DR1: %08lx DR2: %08lx DR3: %08lx\n",
> +			d0, d1, d2, d3);
>  	printk(KERN_DEFAULT "DR6: %08lx DR7: %08lx\n",
>  			d6, d7);

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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