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udp: make sendto(2) on unconnected UDP socket use public inpcb KPIs

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UDP allows to sendto(2) on unconnected socket.  The original BSD devise
was that such action would create a temporary (for the duration of the
syscall) connection between our inpcb and remote addr:port specified in
sockaddr 'to' of the syscall.  This devise was broken in 2002 in
90162a4e87.  For more motivation on the removal of the temporary
connection see email [1].

Since the removal of the true temporary connection the sendto(2) on
unconnected socket has the following side effects:

1) After first sendto(2) the "unconnected" socket will receive datagrams
   destined to the selected port.
2) All subsequent sendto(2) calls will use the same source port.

Effectively, such sendto(2) acts like a bind(2) to INADDR_ANY:0.  Indeed,
if you do this:

	s1 = socket(PF_INET, SOCK_DGRAM, 0);
	s2 = socket(PF_INET, SOCK_DGRAM, 0);
	sendto(s1, ..., &somedestination, ...);
	bind(s2, &{ .sin_addr = INADDR_ANY, sin_port = 0 });

And then look into kgdb at resulting inpcbs, you would find them equal in
all means modulo bound to different anonymous ports.

What is even more interesting is that Linux kernel had picked up same
behavior, including that "unconnected" socket will receive datagrams.  So
it seems that such behavior is now an undocumented standard, thus I
covered it in recently added tests/sys/netinet/udp_bindings.

Now, with the above knowledge at hand, why are we using
in_pcbconnect_setup() and in_pcbinshash(), which are supposed to be
private to in_pcb.c, to achieve the binding?  Let's use public KPI
in_pcbbind() on the first sendto(2) and use in_pcbladdr() on all
sendto(2)s.  Apart from finally hiding these two should be private
functions, we no longer acquire global INP_HASH_WLOCK() for every
sendto(2) on unconnected socket as well as remove a couple workarounds.

[1] https://mail-archive.FreeBSD.org/cgi/mid.cgi?200210141935.aa83883

Reviewed by:		markj
Differential Revision:	https://reviews.freebsd.org/D49043
This commit is contained in:
Gleb Smirnoff 2025-02-21 18:11:17 -08:00
parent ad77d3177f
commit 69c05f4287
1 changed files with 30 additions and 60 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

90
sys/netinet/udp_usrreq.c
View file

@ -1135,10 +1135,9 @@ udp_send(struct socket *so, int flags, struct mbuf *m, struct sockaddr *addr,
sin = (struct sockaddr_in *)addr;
/*
* udp_send() may need to temporarily bind or connect the current
* inpcb. As such, we don't know up front whether we will need the
* pcbinfo lock or not. Do any work to decide what is needed up
* front before acquiring any locks.
* udp_send() may need to bind the current inpcb. As such, we don't
* know up front whether we will need the pcbinfo lock or not. Do any
* work to decide what is needed up front before acquiring any locks.
*
* We will need network epoch in either case, to safely lookup into
* pcb hash.
@ -1292,66 +1291,37 @@ udp_send(struct socket *so, int flags, struct mbuf *m, struct sockaddr *addr,
error = prison_remote_ip4(td->td_ucred, &sin->sin_addr);
if (error)
goto release;
/*
* If a local address or port hasn't yet been selected, or if
* the destination address needs to be rewritten due to using
* a special INADDR_ constant, invoke in_pcbconnect_setup()
* to do the heavy lifting. Once a port is selected, we
* commit the binding back to the socket; we also commit the
* binding of the address if in jail.
*
* If we already have a valid binding and we're not
* requesting a destination address rewrite, use a fast path.
* sendto(2) on unconnected UDP socket results in implicit
* binding to INADDR_ANY and anonymous port. This has two
* side effects:
* 1) after first sendto(2) the socket will receive datagrams
* destined to the selected port.
* 2) subsequent sendto(2) calls will use the same source port.
*/
if (inp->inp_laddr.s_addr == INADDR_ANY ||
inp->inp_lport == 0 ||
sin->sin_addr.s_addr == INADDR_ANY ||
sin->sin_addr.s_addr == INADDR_BROADCAST) {
if ((flags & PRUS_IPV6) != 0) {
vflagsav = inp->inp_vflag;
inp->inp_vflag |= INP_IPV4;
inp->inp_vflag &= ~INP_IPV6;
}
INP_HASH_WLOCK(pcbinfo);
error = in_pcbconnect_setup(inp, sin, &laddr.s_addr,
&lport, &faddr.s_addr, &fport, td->td_ucred);
if ((flags & PRUS_IPV6) != 0)
inp->inp_vflag = vflagsav;
if (error) {
INP_HASH_WUNLOCK(pcbinfo);
goto release;
}
if (inp->inp_lport == 0) {
struct sockaddr_in wild = {
.sin_family = AF_INET,
.sin_len = sizeof(struct sockaddr_in),
};
/*
* XXXRW: Why not commit the port if the address is
* !INADDR_ANY?
*/
/* Commit the local port if newly assigned. */
if (inp->inp_laddr.s_addr == INADDR_ANY &&
inp->inp_lport == 0) {
INP_WLOCK_ASSERT(inp);
/*
* Remember addr if jailed, to prevent
* rebinding.
*/
if (prison_flag(td->td_ucred, PR_IP4))
inp->inp_laddr = laddr;
inp->inp_lport = lport;
error = in_pcbinshash(inp);
INP_HASH_WUNLOCK(pcbinfo);
if (error != 0) {
inp->inp_lport = 0;
error = EAGAIN;
goto release;
}
inp->inp_flags |= INP_ANONPORT;
} else
INP_HASH_WUNLOCK(pcbinfo);
} else {
faddr = sin->sin_addr;
fport = sin->sin_port;
INP_HASH_WLOCK(pcbinfo);
error = in_pcbbind(inp, &wild, V_udp_bind_all_fibs ?
0 : INPBIND_FIB, td->td_ucred);
INP_HASH_WUNLOCK(pcbinfo);
if (error)
goto release;
lport = inp->inp_lport;
laddr = inp->inp_laddr;
}
if (laddr.s_addr == INADDR_ANY) {
error = in_pcbladdr(inp, &sin->sin_addr, &laddr,
td->td_ucred);
if (error)
goto release;
}
faddr = sin->sin_addr;
fport = sin->sin_port;
} else {
INP_LOCK_ASSERT(inp);
faddr = inp->inp_faddr;