Re: [PATCH 7/7] Introduce passt-repair

[Stefano Brivio <sbrivio@redhat.com>]

On Tue, 28 Jan 2025 12:51:59 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Tue, Jan 28, 2025 at 12:15:32AM +0100, Stefano Brivio wrote:
> > A privileged helper to set/clear TCP_REPAIR on sockets on behalf of
> > passt. Not used yet.
> > 
> > Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
> > ---
> >  Makefile       |  10 +++--
> >  passt-repair.c | 111 +++++++++++++++++++++++++++++++++++++++++++++++++
> >  2 files changed, 118 insertions(+), 3 deletions(-)
> >  create mode 100644 passt-repair.c
> > 
> > diff --git a/Makefile b/Makefile
> > index 1383875..1b71cb0 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -42,7 +42,8 @@ PASST_SRCS = arch.c arp.c checksum.c conf.c dhcp.c dhcpv6.c flow.c fwd.c \
> >  	tcp.c tcp_buf.c tcp_splice.c tcp_vu.c udp.c udp_flow.c udp_vu.c util.c \
> >  	vhost_user.c virtio.c vu_common.c
> >  QRAP_SRCS = qrap.c
> > -SRCS = $(PASST_SRCS) $(QRAP_SRCS)
> > +PASST_REPAIR_SRCS = passt-repair.c
> > +SRCS = $(PASST_SRCS) $(QRAP_SRCS) $(PASST_REPAIR_SRCS)
> >  
> >  MANPAGES = passt.1 pasta.1 qrap.1
> >  
> > @@ -72,9 +73,9 @@ mandir		?= $(datarootdir)/man
> >  man1dir		?= $(mandir)/man1
> >  
> >  ifeq ($(TARGET_ARCH),x86_64)
> > -BIN := passt passt.avx2 pasta pasta.avx2 qrap
> > +BIN := passt passt.avx2 pasta pasta.avx2 qrap passt-repair
> >  else
> > -BIN := passt pasta qrap
> > +BIN := passt pasta qrap passt-repair
> >  endif
> >  
> >  all: $(BIN) $(MANPAGES) docs
> > @@ -101,6 +102,9 @@ pasta.avx2 pasta.1 pasta: pasta%: passt%
> >  qrap: $(QRAP_SRCS) passt.h
> >  	$(CC) $(FLAGS) $(CFLAGS) $(CPPFLAGS) -DARCH=\"$(TARGET_ARCH)\" $(QRAP_SRCS) -o qrap $(LDFLAGS)
> >  
> > +passt-repair: $(PASST_REPAIR_SRCS)
> > +	$(CC) $(FLAGS) $(CFLAGS) $(CPPFLAGS) $(PASST_REPAIR_SRCS) -o passt-repair $(LDFLAGS)
> > +
> >  valgrind: EXTRA_SYSCALLS += rt_sigprocmask rt_sigtimedwait rt_sigaction	\
> >  			    rt_sigreturn getpid gettid kill clock_gettime mmap \
> >  			    mmap2 munmap open unlink gettimeofday futex statx \
> > diff --git a/passt-repair.c b/passt-repair.c
> > new file mode 100644
> > index 0000000..e9b9609
> > --- /dev/null
> > +++ b/passt-repair.c
> > @@ -0,0 +1,111 @@
> > +// SPDX-License-Identifier: GPL-2.0-or-later
> > +
> > +/* PASST - Plug A Simple Socket Transport
> > + *  for qemu/UNIX domain socket mode
> > + *
> > + * passt-repair.c - Privileged helper to set/clear TCP_REPAIR on sockets
> > + *
> > + * Copyright (c) 2025 Red Hat GmbH
> > + * Author: Stefano Brivio <sbrivio@redhat.com>
> > + *
> > + * Connect to passt via UNIX domain socket, receive sockets via SCM_RIGHTS along
> > + * with commands mapping to TCP_REPAIR values, and switch repair mode on or
> > + * off. Reply by echoing the command. Exit if the command is INT_MAX.
> > + */
> > +
> > +#include <sys/types.h>
> > +#include <sys/socket.h>
> > +#include <sys/un.h>
> > +#include <errno.h>
> > +#include <stdio.h>
> > +#include <stdlib.h>
> > +#include <string.h>
> > +#include <limits.h>
> > +#include <unistd.h>
> > +#include <netdb.h>
> > +
> > +#include <netinet/tcp.h>
> > +
> > +#define SCM_MAX_FD 253 /* From Linux kernel (include/net/scm.h), not in UAPI */
> > +
> > +int main(int argc, char **argv)
> > +{
> > +	char buf[CMSG_SPACE(sizeof(int) * SCM_MAX_FD)]
> > +	     __attribute__ ((aligned(__alignof__(struct cmsghdr))));
> > +	struct sockaddr_un a = { AF_UNIX, "" };
> > +	int cmd, fds[SCM_MAX_FD], s, ret, i;
> > +	struct cmsghdr *cmsg;
> > +	struct msghdr msg;
> > +	struct iovec iov;
> > +
> > +	iov = (struct iovec){ &cmd, sizeof(cmd) };  
> 
> I mean, local to local, it's *probably* fine, but still a network
> protocol not defined in terms of explicit width fields makes me
> nervous.  I'd prefer to see the cmd being a packed structure with
> fixed width elements.

It actually is, because:

	struct {
		int cmd;
	};

is a packet structure with fixed width elements. Any architecture we
build for (at least the ones I'm aware of) has a 32-bit int. We can
make it uint32_t if it makes you feel better.

> I also think we should do some sort of basic magic / version exchange.
> I don't see any reason we'd need to extend the protocol, but I'd
> rather have the option if we have to.

passt-repair will be packaged and distributed together with passt,
though. Versions must match. And latency here might matter more than in
the rest of the migration process.

> Plus checking a magic number
> should make things less damaging and more debuggable if you were to
> point the repair helper at an entirely unrelated unix socket instead
> of passt's repair socket.

Maybe, yes, even though I don't really see good chances for that
mistake to happen. Feel free to post a proposal, of course.

> > +	msg = (struct msghdr){ NULL, 0, &iov, 1, buf, sizeof(buf), 0 };
> > +	cmsg = CMSG_FIRSTHDR(&msg);
> > +
> > +	if (argc != 2) {
> > +		fprintf(stderr, "Usage: %s PATH\n", argv[0]);
> > +		return -1;
> > +	}
> > +
> > +	ret = snprintf(a.sun_path, sizeof(a.sun_path), "%s", argv[1]);
> > +	if (ret <= 0 || ret >= (int)sizeof(a.sun_path)) {
> > +		fprintf(stderr, "Invalid socket path: %s\n", argv[1]);
> > +		return -1;
> > +	}
> > +
> > +	if ((s = socket(AF_UNIX, SOCK_STREAM, 0)) < 0) {  
> 
> Hmm.. would a datagram socket better suit our needs here?

We need a connection though, so that passt knows when the helper is
ready to get messages. It could be done with a synchronisation datagram
but it looks more complicated to handle.

By the way, with a connection, we could probably just close() the
socket here instead of having a "quit" command.

If you're referring to the fact we don't keep message boundaries, so we
would in theory need to add short read handling to the recvmsg() below:
I'd rather switch cmd to a single byte instead. You can't transfer less
than that.

> > +		perror("Failed to create AF_UNIX socket");
> > +		return -1;
> > +	}
> > +
> > +	if (connect(s, (struct sockaddr *)&a, sizeof(a))) {
> > +		fprintf(stderr, "Failed to connect to %s: %s\n", argv[1],
> > +			strerror(errno));
> > +		return -1;
> > +	}
> > +
> > +	while (1) {
> > +		int n;
> > +
> > +		if (recvmsg(s, &msg, 0) < 0) {
> > +			perror("Failed to receive message");
> > +			return -1;
> > +		}
> > +
> > +		if (!cmsg ||
> > +		    cmsg->cmsg_len < CMSG_LEN(sizeof(int)) ||
> > +		    cmsg->cmsg_len > CMSG_LEN(sizeof(int) * SCM_MAX_FD) ||
> > +		    cmsg->cmsg_type != SCM_RIGHTS)
> > +			return -1;
> > +
> > +		n = cmsg->cmsg_len / CMSG_LEN(sizeof(int));
> > +		memcpy(fds, CMSG_DATA(cmsg), sizeof(int) * n);
> > +
> > +		switch (cmd) {
> > +		case INT_MAX:
> > +			return 0;
> > +		case TCP_REPAIR_ON:
> > +		case TCP_REPAIR_OFF:
> > +		case TCP_REPAIR_OFF_NO_WP:
> > +			for (i = 0; i < n; i++) {
> > +				if (setsockopt(fds[i], SOL_TCP, TCP_REPAIR,
> > +					       &cmd, sizeof(int))) {
> > +					perror("Setting TCP_REPAIR");
> > +					return -1;  
> 
> We probably want this to report errors back to passt, rather than just
> dying in this case.  That way if for some weird reason one socket
> can't be placed in repair mode, we can still migrate all the other
> connections.

We implicitly report the error in the sense that we close the
connection and passt will abort the migration. If you look at the
handling of TCP_REPAIR in do_tcp_setsockopt(), you'll see that it
either always fails (EPERM), or always succeeds.

I mean, it's straightforward to implement, and we can just reply with a
different command. But it's probably more meaningful and fitting to
abort altogether.

Besides, if we have to report exactly on which socket we failed, we
won't be able to switch to a single-byte command protocol.

> > +				}
> > +			}
> > +
> > +			/* Confirm setting by echoing the command back */
> > +			if (send(s, &cmd, sizeof(int), 0) < 0) {
> > +				fprintf(stderr, "Reply to command %i: %s\n",
> > +					cmd, strerror(errno));
> > +				return -1;
> > +			}
> > +
> > +			break;
> > +		default:
> > +			fprintf(stderr, "Unsupported command 0x%04x\n", cmd);
> > +			return -1;
> > +		}
> > +	}
> > +}  

-- 
Stefano