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Date:	Thu, 24 May 2007 12:59:26 -0700
From:	Andrew Morton <>
Cc:	"" 
Subject: Re: [Bugme-new] [Bug 8536] New: Kernel drops UDP packets silently
 when reading from certain proc file entries

On Thu, 24 May 2007 12:48:29 -0700 wrote:

>            Summary: Kernel drops UDP packets silently when reading from
>                     certain proc file entries
>     Kernel Version: 2.6.x
>             Status: NEW
>           Severity: high
>              Owner:
>          Submitter:
> Most recent kernel where this bug did *NOT* occur:
> I do not know, but I now that it exists in RHEL4 2.6.9.x kernels
> Distribution:
> All
> Hardware Environment:
> Multi core SMP
> Software Environment:
> All
> Problem Description:
> It is possible to introduce UDP packet losses by reading
> the proc file entry /proc/net/tcp. The really strange thing is that
> the error counters for packet drops are not increased. 
> This means that the kernel introduce "silent" packet drops by just reading a
> proc statistics entry which is Not a good thing! I can most probably be used for
> denial of service attacks from no root users.
> When looking at the network code it does not seem possible that silent packet
> drops can ocurr so it is probably a quite nasty kernel bug.
> Steps to reproduce:
> * Send high speed RTP/UDP multicast traffic towards the system, 50Mbit/s. 
> * Receive the RTP packets and check/validate the RTP counters and print out when
> the counter is not continous.
> * Do a while loop cat:ing from the /proc/net/tcp and see the packets beeing
> dropped but not accounted for in the counter statistics.
> I have reproduced this behavior on all our systems ranging from dual to quad
> core Xeon and Opteron and also on different OS releases, RHEL4, RHEL5, Fedora
> Core 5 and 6
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