Re: [PATCH 1/1] ipv4: off-by-one in continuation handling in /proc/net/route

From: Alexander Duyck
Date: Thu Aug 13 2015 - 12:07:45 EST

On 08/13/2015 08:10 AM, Eric Dumazet wrote:
On Thu, 2015-08-13 at 11:21 +0100, Andy Whitcroft wrote:
When generating /proc/net/route we emit a header followed by a line for
each route. When a short read is performed we will restart this process
based on the open file descriptor. When calculating the start point we
fail to take into account that the 0th entry is the header. This leads
us to skip the first entry when doing a continuation read.

This can be easily seen with the comparison below:

while read l; do echo "$l"; done </proc/net/route >A
cat /proc/net/route >B
diff -bu A B | grep '^[+-]'

On my example machine I have approximatly 10KB of route output. There we
see the very first non-title element is lost in the while read case,
and an entry around the 8K mark in the cat case:

+wlan0 00000000 02021EAC 0003 0 0 400 00000000 0 0 0
-tun1 00C0AC0A 00000000 0001 0 0 950 00C0FFFF 0 0 0

Fix up the off-by-one when reaquiring position on continuation.

Signed-off-by: Andy Whitcroft <apw@xxxxxxxxxxxxx>
net/ipv4/fib_trie.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

From code inspection I belive this was introduced by the Fixes
below, but I have not tested this to confirm.

Fixes: 8be33e955cb9 ("ipv4: off-by-one in continuation handling in /proc/net/route")
You probably meant

Fixes: 8be33e955cb9 ("fib_trie: Fib walk rcu should take a tnode and key instead of a trie and a leaf")

CC Alexander for review/comment

I agree the Fixes line needs to be updated. Other than that the fix looks good. Once you get the comment fixed feel free to add my acked-by. Also you might call out that this is needed for net, and 4.1 stable as well.

Acked-by: Alexander Duyck <alexander.h.duyck@xxxxxxxxxx>
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