From a0917e0bc6efc05834c0c1eafebd579a9c75e6e9 Mon Sep 17 00:00:00 2001 From: Matthew Dawson Date: Fri, 18 Aug 2017 15:04:54 -0400 Subject: datagram: When peeking datagrams with offset < 0 don't skip empty skbs Due to commit e6afc8ace6dd5cef5e812f26c72579da8806f5ac ("udp: remove headers from UDP packets before queueing"), when udp packets are being peeked the requested extra offset is always 0 as there is no need to skip the udp header. However, when the offset is 0 and the next skb is of length 0, it is only returned once. The behaviour can be seen with the following python script: from socket import *; f=socket(AF_INET6, SOCK_DGRAM | SOCK_NONBLOCK, 0); g=socket(AF_INET6, SOCK_DGRAM | SOCK_NONBLOCK, 0); f.bind(('::', 0)); addr=('::1', f.getsockname()[1]); g.sendto(b'', addr) g.sendto(b'b', addr) print(f.recvfrom(10, MSG_PEEK)); print(f.recvfrom(10, MSG_PEEK)); Where the expected output should be the empty string twice. Instead, make sk_peek_offset return negative values, and pass those values to __skb_try_recv_datagram/__skb_try_recv_from_queue. If the passed offset to __skb_try_recv_from_queue is negative, the checked skb is never skipped. __skb_try_recv_from_queue will then ensure the offset is reset back to 0 if a peek is requested without an offset, unless no packets are found. Also simplify the if condition in __skb_try_recv_from_queue. If _off is greater then 0, and off is greater then or equal to skb->len, then (_off || skb->len) must always be true assuming skb->len >= 0 is always true. Also remove a redundant check around a call to sk_peek_offset in af_unix.c, as it double checked if MSG_PEEK was set in the flags. V2: - Moved the negative fixup into __skb_try_recv_from_queue, and remove now redundant checks - Fix peeking in udp{,v6}_recvmsg to report the right value when the offset is 0 V3: - Marked new branch in __skb_try_recv_from_queue as unlikely. Signed-off-by: Matthew Dawson Acked-by: Willem de Bruijn Signed-off-by: David S. Miller --- net/core/datagram.c | 12 +++++++++--- 1 file changed, 9 insertions(+), 3 deletions(-) (limited to 'net/core') diff --git a/net/core/datagram.c b/net/core/datagram.c index ee5647bd91b3..a21ca8dee5ea 100644 --- a/net/core/datagram.c +++ b/net/core/datagram.c @@ -169,14 +169,20 @@ struct sk_buff *__skb_try_recv_from_queue(struct sock *sk, int *peeked, int *off, int *err, struct sk_buff **last) { + bool peek_at_off = false; struct sk_buff *skb; - int _off = *off; + int _off = 0; + + if (unlikely(flags & MSG_PEEK && *off >= 0)) { + peek_at_off = true; + _off = *off; + } *last = queue->prev; skb_queue_walk(queue, skb) { if (flags & MSG_PEEK) { - if (_off >= skb->len && (skb->len || _off || - skb->peeked)) { + if (peek_at_off && _off >= skb->len && + (_off || skb->peeked)) { _off -= skb->len; continue; } -- cgit v1.2.3