* [PATCH] udp: change MSG_TRUNC return behaviour for MSG_PEEK in recvmsg
@ 2022-03-22 19:03 Torin Carey
2022-03-23 1:57 ` David Laight
0 siblings, 1 reply; 2+ messages in thread
From: Torin Carey @ 2022-03-22 19:03 UTC (permalink / raw)
To: netdev
Cc: David S. Miller, Hideaki YOSHIFUJI, David Ahern, Jakub Kicinski,
linux-kernel
Make UDP recvmsg only return the MSG_TRUNC flag if the read does not
copy the tail end of the datagram. Specifically, this targets MSG_PEEK
when we're using a positive peek offset.
The current behaviour means that if we have a positive peek offset `off`
and we're reading `r` bytes from a datagram of `ulen` length, we respond
with MSG_TRUNC if and only if `r <= ulen - off`. This is odd behaviour
as we return MSG_TRUNC if the user requests exactly `ulen - off` which
has no truncation.
The behaviour could be corrected in two ways:
This patch returns MSG_TRUNC only for tail-end truncation and not head
truncation. This is more consistent with recv(2):
> MSG_TRUNC
> indicates that the trailing portion of a datagram was discarded
> because the datagram was larger than the buffer supplied.
although this isn't written with SO_PEEK_OFF in mind.
The second option is to always return MSG_TRUNC if `off > 0` like the
man-pages socket(7) page states:
> For datagram sockets, if the "peek offset" points to the middle of a
> packet, the data returned will be marked with the MSG_TRUNC flag.
Signed-off-by: Torin Carey <torin@tcarey.uk>
---
net/ipv4/udp.c | 2 +-
net/ipv6/udp.c | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)
diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 319dd7bbfe33..e57740a2c308 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -1855,7 +1855,7 @@ int udp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len, int noblock,
copied = len;
if (copied > ulen - off)
copied = ulen - off;
- else if (copied < ulen)
+ else if (copied < ulen - off)
msg->msg_flags |= MSG_TRUNC;
/*
diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
index 14a94cddcf0b..d6c0eed94564 100644
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -348,7 +348,7 @@ int udpv6_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
copied = len;
if (copied > ulen - off)
copied = ulen - off;
- else if (copied < ulen)
+ else if (copied < ulen - off)
msg->msg_flags |= MSG_TRUNC;
is_udp4 = (skb->protocol == htons(ETH_P_IP));
--
2.34.1
^ permalink raw reply related [flat|nested] 2+ messages in thread
* RE: [PATCH] udp: change MSG_TRUNC return behaviour for MSG_PEEK in recvmsg
2022-03-22 19:03 [PATCH] udp: change MSG_TRUNC return behaviour for MSG_PEEK in recvmsg Torin Carey
@ 2022-03-23 1:57 ` David Laight
0 siblings, 0 replies; 2+ messages in thread
From: David Laight @ 2022-03-23 1:57 UTC (permalink / raw)
To: 'Torin Carey', netdev
Cc: David S. Miller, Hideaki YOSHIFUJI, David Ahern, Jakub Kicinski,
linux-kernel
From: Torin Carey
> Sent: 22 March 2022 19:04
>
> Make UDP recvmsg only return the MSG_TRUNC flag if the read does not
> copy the tail end of the datagram. Specifically, this targets MSG_PEEK
> when we're using a positive peek offset.
>
> The current behaviour means that if we have a positive peek offset `off`
> and we're reading `r` bytes from a datagram of `ulen` length, we respond
> with MSG_TRUNC if and only if `r <= ulen - off`. This is odd behaviour
> as we return MSG_TRUNC if the user requests exactly `ulen - off` which
> has no truncation.
>
> The behaviour could be corrected in two ways:
>
> This patch returns MSG_TRUNC only for tail-end truncation and not head
> truncation. This is more consistent with recv(2):
> > MSG_TRUNC
> > indicates that the trailing portion of a datagram was discarded
> > because the datagram was larger than the buffer supplied.
> although this isn't written with SO_PEEK_OFF in mind.
>
> The second option is to always return MSG_TRUNC if `off > 0` like the
> man-pages socket(7) page states:
> > For datagram sockets, if the "peek offset" points to the middle of a
> > packet, the data returned will be marked with the MSG_TRUNC flag.
>
> Signed-off-by: Torin Carey <torin@tcarey.uk>
> ---
> net/ipv4/udp.c | 2 +-
> net/ipv6/udp.c | 2 +-
> 2 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> index 319dd7bbfe33..e57740a2c308 100644
> --- a/net/ipv4/udp.c
> +++ b/net/ipv4/udp.c
> @@ -1855,7 +1855,7 @@ int udp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len, int noblock,
> copied = len;
> if (copied > ulen - off)
> copied = ulen - off;
> - else if (copied < ulen)
> + else if (copied < ulen - off)
> msg->msg_flags |= MSG_TRUNC;
You can remove a test:
if (copied >= ulen - off)
copied = ulen - off;
else
msg->msg_flags |= MSG_TRUNC;
David
>
> /*
> diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
> index 14a94cddcf0b..d6c0eed94564 100644
> --- a/net/ipv6/udp.c
> +++ b/net/ipv6/udp.c
> @@ -348,7 +348,7 @@ int udpv6_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
> copied = len;
> if (copied > ulen - off)
> copied = ulen - off;
> - else if (copied < ulen)
> + else if (copied < ulen - off)
> msg->msg_flags |= MSG_TRUNC;
>
> is_udp4 = (skb->protocol == htons(ETH_P_IP));
> --
> 2.34.1
>
-
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