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

From: Andy Whitcroft
Date: Thu Aug 13 2015 - 06:21:15 EST

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.

BugLink: http://bugs.launchpad.net/bugs/1483440
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")

diff --git a/net/ipv4/fib_trie.c b/net/ipv4/fib_trie.c
index 37c4bb8..b0c6258 100644
--- a/net/ipv4/fib_trie.c
+++ b/net/ipv4/fib_trie.c
@@ -2465,7 +2465,7 @@ static struct key_vector *fib_route_get_idx(struct fib_route_iter *iter,
key = l->key + 1;

- if (pos-- <= 0)
+ if (--pos <= 0)

l = NULL;

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