Comment 7 for bug 1483440

commit 25b97c016b26039982daaa2c11d83979f93b71ab
Author: Andy Whitcroft <email address hidden>
Date: Thu Aug 13 20:49:01 2015 +0100

    ipv4: off-by-one in continuation handling in /proc/net/route

    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.

    Fixes: 8be33e955cb9 ("fib_trie: Fib walk rcu should take a tnode and key instead of a trie and a leaf")
    BugLink: http://bugs.launchpad.net/bugs/1483440
    Acked-by: Alexander Duyck <email address hidden>
    Signed-off-by: Andy Whitcroft <email address hidden>
    Signed-off-by: David S. Miller <email address hidden>