Re: [PATCH net v3] datagram: When peeking datagrams with offset < 0 don't skip empty skbs

[David Miller <davem@davemloft.net>]

From: Matthew Dawson <matthew@mjdsystems.ca>
Date: Fri, 18 Aug 2017 15:04:54 -0400

> 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 <matthew@mjdsystems.ca>
> Acked-by: Willem de Bruijn <willemb@google.com>

Applied and queued up for -stable, thanks.