Re: [PATCH 2/3] conf, tap, repair: Uniformly use non-blocking accept() on Unix sockets

On Wed, May 13, 2026 at 02:14:22PM +1000, David Gibson wrote:

sock_unix(), which creates a listening Unix socket, doesn't set the SOCK_NONBLOCK flag, meaning that accept() will block if called with no pending connections. Generally, this doesn't matter because we only accept() once we've received an epoll event indicating there's a pending connection request.

Control connections (pesto) are an exception, because the way we queue connections requires that we call accept() when we close one connection to see if there's another one waiting. We rely on an EAGAIN here to know that there's nothing waiting. To handle these we have an explicit fcntl() to enable NONBLOCK on the control listening socket.

However, always using non-blocking accept() for Unix sockets would make things a bit more uniform, and should be a bit less fragile in the case that we ever somehow got a spurious connection event. So, alter sock_unix() to always use the SOCK_NONBLOCK flag. Remove the control socket's special case fcntl(), and adjust the error handling on each Unix socket accept() for the new behaviour. As a bonus the last adds reporting for accept() errors on tap socket connections.

I didn't realise it, but adding that reporting also removes a valid, if fairly minor coverity warning (at least with coverity 2026.3.0).

we will need non-blocking accept() for the upcoming control/configuration socket. Always add SOCK_NONBLOCK, which is more robust and in keeping with the normal non-blocking style of passt.

Oops. This paragraph is left over from a previous version. Can you remove on merge, if there's no other reason to respin?

Signed-off-by: David Gibson --- conf.c | 4 +--- repair.c | 4 ++-- tap.c | 5 +++++ util.c | 2 +- 4 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/conf.c b/conf.c index 029b9c7c..dec43fca 100644 --- a/conf.c +++ b/conf.c @@ -1091,8 +1091,6 @@ static void conf_open_files(struct ctx *c) die_perror("Couldn't open control socket %s", c->control_path); } - if (fcntl(c->fd_control_listen, F_SETFL, O_NONBLOCK)) - die_perror("Couldn't set O_NONBLOCK on control socket"); } else { c->fd_control_listen = -1; } @@ -2087,7 +2085,7 @@ retry: fd = accept4(c->fd_control_listen, NULL, NULL, SOCK_CLOEXEC); if (fd < 0) { if (errno != EAGAIN) - warn_perror("accept4() on configuration listening socket"); + warn_perror("Error accept()ing configuration socket"); return; }

diff --git a/repair.c b/repair.c index 3e0e3e0a..42c4ae97 100644 --- a/repair.c +++ b/repair.c @@ -101,8 +101,8 @@ int repair_listen_handler(struct ctx *c, uint32_t events)

if ((c->fd_repair = accept4(c->fd_repair_listen, NULL, NULL, SOCK_CLOEXEC)) < 0) { - rc = errno; - debug_perror("accept4() on TCP_REPAIR helper listening socket"); + if ((rc = errno) != EAGAIN) + warn_perror("Error accept()ing repair helper"); return rc; }

diff --git a/tap.c b/tap.c index e7cac9df..fda2da9b 100644 --- a/tap.c +++ b/tap.c @@ -1491,6 +1491,11 @@ void tap_listen_handler(struct ctx *c, uint32_t events) }

c->fd_tap = accept4(c->fd_tap_listen, NULL, NULL, SOCK_CLOEXEC); + if (c->fd_tap < 0) { + if (errno != EAGAIN) + warn_perror("Error accepting tap client"); + return; + }

if (!getsockopt(c->fd_tap, SOL_SOCKET, SO_PEERCRED, &ucred, &len)) info("accepted connection from PID %i", ucred.pid); diff --git a/util.c b/util.c index 73c9d51d..204391c7 100644 --- a/util.c +++ b/util.c @@ -238,7 +238,7 @@ int sock_l4_dualstack_any(const struct ctx *c, enum epoll_type type, */ int sock_unix(char *sock_path) { - int fd = socket(AF_UNIX, SOCK_STREAM | SOCK_CLOEXEC, 0); + int fd = socket(AF_UNIX, SOCK_STREAM | SOCK_CLOEXEC | SOCK_NONBLOCK, 0); struct sockaddr_un addr = { .sun_family = AF_UNIX, }; -- 2.54.0

-- David Gibson (he or they) | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you, not the other way | around. http://www.ozlabs.org/~dgibson