RE: [PATCH] udp: change MSG_TRUNC return behaviour for MSG_PEEK in recvmsg

[David Laight <David.Laight@ACULAB.COM>]

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
> 

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)