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Message-ID: <1789862.HUSStbeWG9@ring00>
Date: Fri, 18 Aug 2017 12:39:20 -0400
From: Matthew Dawson <matthew@...systems.ca>
To: Paolo Abeni <pabeni@...hat.com>
Cc: netdev@...r.kernel.org,
"Macieira, Thiago" <thiago.macieira@...el.com>,
willemdebruijn.kernel@...il.com
Subject: Re: [PATCH net v2] datagram: When peeking datagrams with offset < 0 don't skip empty skbs
On Friday, August 18, 2017 10:05:18 AM EDT Paolo Abeni wrote:
> On Thu, 2017-08-17 at 22:11 -0400, Matthew Dawson wrote:
> > 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
> >
> > Signed-off-by: Matthew Dawson <matthew@...systems.ca>
> > ---
> >
> > include/net/sock.h | 4 +---
> > net/core/datagram.c | 12 +++++++++---
> > net/ipv4/udp.c | 3 ++-
> > net/ipv6/udp.c | 3 ++-
> > net/unix/af_unix.c | 5 +----
> > 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..4b558503bef5 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 (flags & MSG_PEEK && *off >= 0) {
> > + peek_at_off = true;
> > + _off = *off;
> > + }
>
> I think that unlikely() will fit the above condition
Sounds good.
>
> > *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;
>
> later we have:
>
> chunk = min_t(unsigned int, unix_skb_len(skb) - skip, size);
>
> without any call to __skb_try_recv_from_queue(), so we will get
> bad/unexpected values from the above assignment when 'skip' is
> negative.
The assignment to skip should ensure it is never less then zero, thanks to the
max(sk...(), 0). Thus that shouldn't be an issue?
>
> Overall I still think that adding/using an explicit MSG_PEEK_OFF bit
> would produce a simpler code, but is just a personal preference.
I don't mind either way, that just seemed to be the preference I saw from the
discussion around the patch. I think either way will work, so whatever the
list prefers I'm happy with.
--
Matthew
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