Commit a0917e0b authored by Matthew Dawson's avatar Matthew Dawson Committed by David S. Miller
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datagram: When peeking datagrams with offset < 0 don't skip empty skbs

Due to commit e6afc8ac

 ("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: default avatarMatthew Dawson <matthew@mjdsystems.ca>
Acked-by: default avatarWillem de Bruijn <willemb@google.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 2110ba58
......@@ -507,9 +507,7 @@ int sk_set_peek_off(struct sock *sk, int val);
static inline int sk_peek_offset(struct sock *sk, int flags)
{
if (unlikely(flags & MSG_PEEK)) {
s32 off = READ_ONCE(sk->sk_peek_off);
if (off >= 0)
return off;
return READ_ONCE(sk->sk_peek_off);
}
return 0;
......
......@@ -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;
}
......
......@@ -1574,7 +1574,8 @@ int udp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len, int noblock,
return ip_recv_error(sk, msg, len, addr_len);
try_again:
peeking = off = sk_peek_offset(sk, flags);
peeking = flags & MSG_PEEK;
off = sk_peek_offset(sk, flags);
skb = __skb_recv_udp(sk, flags, noblock, &peeked, &off, &err);
if (!skb)
return err;
......
......@@ -362,7 +362,8 @@ int udpv6_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
return ipv6_recv_rxpmtu(sk, msg, len, addr_len);
try_again:
peeking = off = sk_peek_offset(sk, flags);
peeking = flags & MSG_PEEK;
off = sk_peek_offset(sk, flags);
skb = __skb_recv_udp(sk, flags, noblock, &peeked, &off, &err);
if (!skb)
return err;
......
......@@ -2304,10 +2304,7 @@ static int unix_stream_read_generic(struct unix_stream_read_state *state,
*/
mutex_lock(&u->iolock);
if (flags & MSG_PEEK)
skip = sk_peek_offset(sk, flags);
else
skip = 0;
skip = max(sk_peek_offset(sk, flags), 0);
do {
int chunk;
......
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