Re: [PATCH v2 2/3] tcp: Register fds with epoll at flow creation

[Stefano Brivio <sbrivio@redhat.com>]

On Mon, 19 Jan 2026 17:19:14 +0100
Laurent Vivier <lvivier@redhat.com> wrote:

> Register connection sockets with epoll using empty events
> (events=0) in tcp_conn_from_tap(), tcp_tap_conn_from_sock()
> and tcp_flow_repair_socket().
> 
> This allows tcp_epoll_ctl() to always use EPOLL_CTL_MOD, removing
> the need to check whether fds are already registered. As a result, the
> conditional ADD/MOD logic is no longer needed, simplifying the function.
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> ---
>  flow.c |  1 +
>  tcp.c  | 46 ++++++++++++++++++++++++----------------------
>  2 files changed, 25 insertions(+), 22 deletions(-)
> 
> diff --git a/flow.c b/flow.c
> index cefe6c8b5b24..532339ce7fe1 100644
> --- a/flow.c
> +++ b/flow.c
> @@ -357,6 +357,7 @@ static void flow_set_state(struct flow_common *f, enum flow_state state)
>   *
>   * Return: true if flow is registered with epoll, false otherwise
>   */
> +/* cppcheck-suppress unusedFunction */
>  bool flow_in_epoll(const struct flow_common *f)
>  {
>  	return f->epollid != EPOLLFD_ID_INVALID;
> diff --git a/tcp.c b/tcp.c
> index 1db861705ddb..29d69354bd94 100644
> --- a/tcp.c
> +++ b/tcp.c
> @@ -528,37 +528,22 @@ static uint32_t tcp_conn_epoll_events(uint8_t events, uint8_t conn_flags)
>  static int tcp_epoll_ctl(struct tcp_tap_conn *conn)
>  {
>  	uint32_t events;
> -	int m;
>  
>  	if (conn->events == CLOSED) {
> -		if (flow_in_epoll(&conn->f)) {
> -			int epollfd = flow_epollfd(&conn->f);
> +		int epollfd = flow_epollfd(&conn->f);
>  
> -			epoll_del(epollfd, conn->sock);
> -			if (conn->timer != -1)
> -				epoll_del(epollfd, conn->timer);
> -		}
> +		epoll_del(epollfd, conn->sock);
> +		if (conn->timer != -1)
> +			epoll_del(epollfd, conn->timer);
>  
>  		return 0;
>  	}
>  
>  	events = tcp_conn_epoll_events(conn->events, conn->flags);
>  
> -	if (flow_in_epoll(&conn->f)) {
> -		m = EPOLL_CTL_MOD;
> -	} else {
> -		flow_epollid_set(&conn->f, EPOLLFD_ID_DEFAULT);
> -		m = EPOLL_CTL_ADD;
> -	}
> -
> -	if (flow_epoll_set(&conn->f, m, events, conn->sock,
> -			   !TAPSIDE(conn)) < 0) {
> -		int ret = -errno;
> -
> -		if (m == EPOLL_CTL_ADD)
> -			flow_epollid_clear(&conn->f);
> -		return ret;
> -	}
> +	if (flow_epoll_set(&conn->f, EPOLL_CTL_MOD, events, conn->sock,
> +			   !TAPSIDE(conn)) < 0)
> +		return -errno;
>  
>  	return 0;
>  }
> @@ -1710,6 +1695,11 @@ static void tcp_conn_from_tap(const struct ctx *c, sa_family_t af,
>  	conn->sock = s;
>  	conn->timer = -1;
>  	conn->listening_sock = -1;
> +	flow_epollid_set(&conn->f, EPOLLFD_ID_DEFAULT);
> +	if (flow_epoll_set(&conn->f, EPOLL_CTL_ADD, 0, s, TGTSIDE) < 0) {
> +		flow_perror(flow, "Can't register with epoll");
> +		goto cancel;
> +	}
>  	conn_event(c, conn, TAP_SYN_RCVD);
>  
>  	conn->wnd_to_tap = WINDOW_DEFAULT;
> @@ -2433,6 +2423,15 @@ static void tcp_tap_conn_from_sock(const struct ctx *c, union flow *flow,
>  	conn->sock = s;
>  	conn->timer = -1;
>  	conn->ws_to_tap = conn->ws_from_tap = 0;
> +
> +	flow_epollid_set(&conn->f, EPOLLFD_ID_DEFAULT);
> +	if (flow_epoll_set(&conn->f, EPOLL_CTL_ADD, 0, s, INISIDE) < 0) {
> +		flow_perror(flow, "Can't register with epoll");
> +		conn_flag(c, conn, CLOSING);
> +		FLOW_ACTIVATE(conn);
> +		return;
> +	}
> +
>  	conn_event(c, conn, SOCK_ACCEPTED);
>  
>  	hash = flow_hash_insert(c, TAP_SIDX(conn));
> @@ -3825,6 +3824,9 @@ int tcp_flow_migrate_target(struct ctx *c, int fd)
>  		return 0;
>  	}
>  
> +	flow_epollid_set(&conn->f, EPOLLFD_ID_DEFAULT);
> +	flow_epoll_set(&conn->f, EPOLL_CTL_ADD, 0, conn->sock, !TAPSIDE(conn));

Oops, I overlooked this, because I missed re-checking on v2 after David
pointed it out: this causes Coverity Scan to (reasonably, I think)
complain that:

/home/sbrivio/passt/tcp.c:3693:2:
  Type: Unchecked return value (CHECKED_RETURN)

/home/sbrivio/passt/tcp.c:3648:2: Unchecked call to function
  1. path: Condition "!(flow = flow_alloc())", taking false branch.
/home/sbrivio/passt/tcp.c:3653:2:
  2. path: Condition "read_all_buf(fd, &t, 112UL /* sizeof (t) */)", taking false branch.
/home/sbrivio/passt/tcp.c:3685:2:
  3. path: Condition "rc = tcp_flow_repair_socket(c, conn)", taking false branch.
/home/sbrivio/passt/tcp.c:3693:2:
  4. path: Condition "!(conn->f.pif[1] == PIF_TAP)", taking true branch.
/home/sbrivio/passt/tcp.c:3693:2:
  5. check_return: Calling "flow_epoll_set" without checking return value (as is done elsewhere 6 out of 7 times).
/home/sbrivio/passt/icmp.c:213:2: Examples where return value from this function is checked
  6. example_checked: Example 1: "flow_epoll_set(&pingf->f, 1, EPOLLIN, pingf->sock, 1U)" has its value checked in "flow_epoll_set(&pingf->f, 1, EPOLLIN, pingf->sock, 1U) < 0".
/home/sbrivio/passt/tcp.c:1695:2: Examples where return value from this function is checked
  7. example_checked: Example 2: "flow_epoll_set(&conn->f, 1, 0U, s, 1U)" has its value checked in "flow_epoll_set(&conn->f, 1, 0U, s, 1U) < 0".
/home/sbrivio/passt/tcp_splice.c:364:2: Examples where return value from this function is checked
  8. example_checked: Example 3: "flow_epoll_set(&conn->f, 1, 0U, conn->s[1], 1U)" has its value checked in "flow_epoll_set(&conn->f, 1, 0U, conn->s[1], 1U)".
/home/sbrivio/passt/tcp_splice.c:364:2: Examples where return value from this function is checked
  9. example_checked: Example 4: "flow_epoll_set(&conn->f, 1, 0U, conn->s[0], 0U)" has its value checked in "flow_epoll_set(&conn->f, 1, 0U, conn->s[0], 0U)".
/home/sbrivio/passt/udp_flow.c:87:2: Examples where return value from this function is checked
  10. example_checked: Example 5: "flow_epoll_set(&uflow->f, 1, EPOLLIN, s, sidei)" has its value checked in "flow_epoll_set(&uflow->f, 1, EPOLLIN, s, sidei) < 0".

I don't think it's overly problematic but epoll_ctl() can, in theory,
fail regardless of the arguments.

> +
>  	flow_hash_insert(c, TAP_SIDX(conn));
>  	FLOW_ACTIVATE(conn);
>  

-- 
Stefano