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 <matthew@mjdsystems.ca>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

5 files changed, 15 insertions, 12 deletions

diff --git a/include/net/sock.h b/include/net/sock.h
index 7c0632c7e870..aeeec62992ca 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -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;
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;
                         }
diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index a7c804f73990..cd1d044a7fa5 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -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;
diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
index 578142b7ca3e..20039c8501eb 100644
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -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;
diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 7b52a380d710..be8982b4f8c0 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -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;