linux-kernel - Re: [Patch 01/11] Introducing generic hardware breakpoint handler interfaces

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Message-ID: <20090323204220.GA19602@in.ibm.com>
Date:	Tue, 24 Mar 2009 02:12:20 +0530
From:	"K.Prasad" <prasad@...ux.vnet.ibm.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Benjamin Herrenschmidt <benh@....ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Maneesh Soni <maneesh@...ibm.com>,
	Roland McGrath <roland@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [Patch 01/11] Introducing generic hardware breakpoint handler
	interfaces

On Mon, Mar 23, 2009 at 03:21:49PM -0400, Alan Stern wrote:
> On Tue, 24 Mar 2009, K.Prasad wrote:
> 
> > > > Ok. Will do something like:
> > > > return (va <= (TASK_SIZE - (hw_breakpoint_length * word_size)));
> > > 
> > > What is the purpose of word_size here?  The breakpoint length should be 
> > > specified in bytes, not words.
> > > 
> > > Don't forget that that in arch_check_va_in_kernelspace() you need to 
> > > check both for values that are too low and values that are too high 
> > > (they overflow and wrap around back to a user address).
> > > 
> > 
> > While I understand the user-space checking using the length of the HW
> > Breakpoint, I don't really see how I can check for an upper-bound for
> > kernel-space virtual addresses. Most usage in the kernel only checks for
> > the address >= TASK_SIZE (while they check for add + len if the length
> > of the memory is known). I will be glad to have any suggestions in this
> > regard.
> 
> Isn't that exactly the check you need to implement?
> 
> 	addr >= TASK_SIZE && (addr + len) >= TASK_SIZE,
> 
> or perhaps better,
> 
> 	addr >= TASK_SIZE && (addr + len) >= addr.
> 
> In this case you _do_ know the length of the breakpoint.
> 
> Alan Stern
>

Aren't we just checking if len is a positive number through the above
checks? The validation checks in the patchset should take care of
negative lengths. Or am I missing something?

I thought you wanted the code to check for an upper sane limit for addr
in kernel-space, say something like this:

TASK_SIZE <= addr <= (Upper limit for Kernel Virtual Address)

When I referred to 'len' in my previous mail, it meant the length
of the kernel virtual memory area (which can be used to find the upper
bound).

Thanks,
K.Prasad

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