netdev - Re: [RFC PATCH] qos: Limit a filter's priority to a 16 bit value

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Message-Id: <20090501.155155.58484825.davem@davemloft.net>
Date:	Fri, 01 May 2009 15:51:55 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	robert.w.love@...el.com
Cc:	hadi@...erus.ca, netdev@...r.kernel.org,
	jeffrey.t.kirsher@...el.com
Subject: Re: [RFC PATCH] qos: Limit a filter's priority to a 16 bit value

From: Robert Love <robert.w.love@...el.com>
Date: Fri, 01 May 2009 11:42:42 -0700

> Currently the kernel is storing the priority value for
> a filter as a 32 bit value. However, the netlink messages
> between the kernel and userspace are passing only 16 bits
> for the priority. This means that the kernel will always
> try to compare the passed down filter's priority using the
> correct upper 16 bits, but all 0s for the lower 16 bits.
> 
> Most comparisons will fail when checking against the filter
> chain unless the actual filter has a priority with all 0s
> for the last 16 bits.
> 
> The fallout is that users cannot modify or delete most
> filters that are added.

Really?

I'm looking at the code and I don't see this failure case.
Can you describe the problem with specific examples?

All of the code I see either 1) uses u32's to store the
priorities and just compares them (cls_u32.c) or 2)
always uses TC_H_MAJ() of the user's provided value
and uses that to compare with tp->prio values.

Both of which should always just work.

We're just wasting the low 16-bits, but that shouldn't
cause problems like the one you're describing as long
as everyone extracts and compares the top bits properly
which as far as I can tell is the case.
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