From cf3eeba6c0d7c7b33824b6e1c53f14dcb90437ae Mon Sep 17 00:00:00 2001 From: David Gibson Date: Thu, 9 Nov 2023 20:54:00 +1100 Subject: tcp: Don't use TCP_WINDOW_CLAMP On the L2 tap side, we see TCP headers and know the TCP window that the ultimate receiver is advertising. In order to avoid unnecessary buffering within passt/pasta (or by the kernel on passt/pasta's behalf) we attempt to advertise that window back to the original sock-side sender using TCP_WINDOW_CLAMP. However, TCP_WINDOW_CLAMP just doesn't work like this. Prior to kernel commit 3aa7857fe1d7 ("tcp: enable mid stream window clamp"), it simply had no effect on established sockets. After that commit, it does affect established sockets but doesn't behave the way we need: * It appears to be designed only to shrink the window, not to allow it to re-expand. * More importantly, that commit has a serious bug where if the setsockopt() is made when the existing kernel advertised window for the socket happens to be zero, it will now become locked at zero, stopping any further data from being received on the socket. Since this has never worked as intended, simply remove it. It might be possible to re-implement the intended behaviour by manipulating SO_RCVBUF, so we leave a comment to that effect. This kernel bug is the underlying cause of both the linked passt bug and the linked podman bug. We attempted to fix this before with passt commit d3192f67 ("tcp: Force TCP_WINDOW_CLAMP before resetting STALLED flag"). However while that commit masked the bug for some cases, it didn't really address the problem. Fixes: d3192f67c492 ("tcp: Force TCP_WINDOW_CLAMP before resetting STALLED flag") Link: https://github.com/containers/podman/issues/20170 Link: https://bugs.passt.top/show_bug.cgi?id=74 Signed-off-by: David Gibson Signed-off-by: Stefano Brivio --- tcp.c | 65 +++++++++----------------------------------------------------- tcp_conn.h | 7 +++---- 2 files changed, 12 insertions(+), 60 deletions(-) diff --git a/tcp.c b/tcp.c index 13e82ca..cfcd40a 100644 --- a/tcp.c +++ b/tcp.c @@ -46,8 +46,8 @@ * - avoiding port and address translations whenever possible * - mirroring TCP dynamics by observation of socket parameters (TCP_INFO * socket option) and TCP headers of packets coming from the tap interface, - * reapplying those parameters in both flow directions (including TCP_MSS, - * TCP_WINDOW_CLAMP socket options) + * reapplying those parameters in both flow directions (including TCP_MSS + * socket option) * - simplicity: only a small subset of TCP logic is implemented here and * delegated as much as possible to the TCP implementations of guest and host * kernel. This is achieved by: @@ -236,12 +236,10 @@ * - update @seq_ack_from_tap from ack_seq in header * - on two duplicated ACKs, reset @seq_to_tap to @seq_ack_from_tap, and * resend with steps listed above - * - set TCP_WINDOW_CLAMP from TCP header from tap * * - from tap/guest to socket: * - on packet from tap/guest: * - set @ts_tap_act - * - set TCP_WINDOW_CLAMP from TCP header from tap * - check seq from header against @seq_from_tap, if data is missing, send * two ACKs with number @seq_ack_to_tap, discard packet * - otherwise queue data to socket, set @seq_from_tap to seq from header @@ -420,7 +418,7 @@ static const char *tcp_state_str[] __attribute((__unused__)) = { }; static const char *tcp_flag_str[] __attribute((__unused__)) = { - "STALLED", "LOCAL", "WND_CLAMPED", "ACTIVE_CLOSE", "ACK_TO_TAP_DUE", + "STALLED", "LOCAL", "ACTIVE_CLOSE", "ACK_TO_TAP_DUE", "ACK_FROM_TAP_DUE", }; @@ -1790,58 +1788,16 @@ static void tcp_get_tap_ws(struct tcp_tap_conn *conn, /** * tcp_tap_window_update() - Process an updated window from tap side - * @c: Execution context * @conn: Connection pointer * @window: Window value, host order, unscaled */ -static void tcp_tap_window_update(const struct ctx *c, - struct tcp_tap_conn *conn, unsigned wnd) +static void tcp_tap_window_update(struct tcp_tap_conn *conn, unsigned wnd) { - uint32_t prev_scaled = conn->wnd_from_tap << conn->ws_from_tap; - int s = conn->sock; - wnd = MIN(MAX_WINDOW, wnd << conn->ws_from_tap); conn->wnd_from_tap = MIN(wnd >> conn->ws_from_tap, USHRT_MAX); - /* TODO: With (at least) Linux kernel versions 6.1 to 6.5, if we end up - * with a zero-sized window on a TCP socket, dropping data (once - * acknowledged by the guest) with recv() and MSG_TRUNC doesn't appear - * to be enough to make the kernel advertise a non-zero window to the - * sender. Forcing a TCP_WINDOW_CLAMP setting, even with the existing - * value, fixes this. - * - * The STALLED flag on a connection is a sufficient indication that we - * might have a zero-sized window on the socket, because it's set if we - * exhausted the tap-side window, or if everything we receive from a - * socket is already in flight to the guest. - * - * So, if STALLED is set, and we received a window value from the tap, - * force a TCP_WINDOW_CLAMP setsockopt(). This should be investigated - * further and fixed in the kernel instead, if confirmed. - */ - if (!(conn->flags & STALLED) && conn->flags & WND_CLAMPED) { - if (prev_scaled == wnd) - return; - - /* Discard +/- 1% updates to spare some syscalls. */ - /* TODO: cppcheck, starting from commit b4d455df487c ("Fix - * 11349: FP negativeIndex for clamped array index (#4627)"), - * reports wnd > prev_scaled as always being true, see also: - * - * https://github.com/danmar/cppcheck/pull/4627 - * - * drop this suppression once that's resolved. - */ - /* cppcheck-suppress [knownConditionTrueFalse, unmatchedSuppression] */ - if ((wnd > prev_scaled && wnd * 99 / 100 < prev_scaled) || - (wnd < prev_scaled && wnd * 101 / 100 > prev_scaled)) - return; - } - - if (setsockopt(s, SOL_TCP, TCP_WINDOW_CLAMP, &wnd, sizeof(wnd))) - trace("TCP: failed to set TCP_WINDOW_CLAMP on socket %i", s); - - conn_flag(c, conn, WND_CLAMPED); + /* FIXME: reflect the tap-side receiver's window back to the sock-side + * sender by adjusting SO_RCVBUF? */ } /** @@ -2452,7 +2408,7 @@ static int tcp_data_from_tap(struct ctx *c, struct tcp_tap_conn *conn, if (ack && !tcp_sock_consume(conn, max_ack_seq)) tcp_update_seqack_from_tap(c, conn, max_ack_seq); - tcp_tap_window_update(c, conn, max_ack_seq_wnd); + tcp_tap_window_update(conn, max_ack_seq_wnd); if (retr) { trace("TCP: fast re-transmit, ACK: %u, previous sequence: %u", @@ -2537,7 +2493,7 @@ static void tcp_conn_from_sock_finish(struct ctx *c, struct tcp_tap_conn *conn, const struct tcphdr *th, const char *opts, size_t optlen) { - tcp_tap_window_update(c, conn, ntohs(th->window)); + tcp_tap_window_update(conn, ntohs(th->window)); tcp_get_tap_ws(conn, opts, optlen); /* First value is not scaled */ @@ -2645,7 +2601,7 @@ int tcp_tap_handler(struct ctx *c, uint8_t pif, int af, if (!th->ack) goto reset; - tcp_tap_window_update(c, conn, ntohs(th->window)); + tcp_tap_window_update(conn, ntohs(th->window)); tcp_data_from_sock(c, conn); @@ -2669,9 +2625,6 @@ int tcp_tap_handler(struct ctx *c, uint8_t pif, int af, if (count == -1) goto reset; - /* Note: STALLED matters for tcp_tap_window_update(): unset it only - * after processing data (and window) from the tap side - */ conn_flag(c, conn, ~STALLED); if (conn->seq_ack_to_tap != conn->seq_from_tap) diff --git a/tcp_conn.h b/tcp_conn.h index 01d31d4..0c6a35b 100644 --- a/tcp_conn.h +++ b/tcp_conn.h @@ -85,10 +85,9 @@ struct tcp_tap_conn { uint8_t flags; #define STALLED BIT(0) #define LOCAL BIT(1) -#define WND_CLAMPED BIT(2) -#define ACTIVE_CLOSE BIT(3) -#define ACK_TO_TAP_DUE BIT(4) -#define ACK_FROM_TAP_DUE BIT(5) +#define ACTIVE_CLOSE BIT(2) +#define ACK_TO_TAP_DUE BIT(3) +#define ACK_FROM_TAP_DUE BIT(4) #define TCP_MSS_BITS 14 -- cgit v1.2.3