From 969e50b61a285b0cc8dea6d4d2ade3f758d5ecc7 Mon Sep 17 00:00:00 2001 From: Bin Meng Date: Wed, 17 Mar 2021 14:26:29 +0800 Subject: [PATCH] net: Pad short frames to minimum size before sending from SLiRP/TAP MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit The minimum Ethernet frame length is 60 bytes. For short frames with smaller length like ARP packets (only 42 bytes), on a real world NIC it can choose either padding its length to the minimum required 60 bytes, or sending it out directly to the wire. Such behavior can be hardcoded or controled by a register bit. Similarly on the receive path, NICs can choose either dropping such short frames directly or handing them over to software to handle. On the other hand, for the network backends like SLiRP/TAP, they don't expose a way to control the short frame behavior. As of today they just send/receive data from/to the other end connected to them, which means any sized packet is acceptable. So they can send and receive short frames without any problem. It is observed that ARP packets sent from SLiRP/TAP are 42 bytes, and SLiRP/TAP just send these ARP packets to the other end which might be a NIC model that does not allow short frames to pass through. To provide better compatibility, for packets sent from QEMU network backends like SLiRP/TAP, we change to pad short frames before sending it out to the other end, if the other end does not forbid it via the nc->do_not_pad flag. This ensures a backend as an Ethernet sender does not violate the spec. But with this change, the behavior of dropping short frames from SLiRP/TAP interfaces in the NIC model cannot be emulated because it always receives a packet that is spec complaint. The capability of sending short frames from NIC models is still supported and short frames can still pass through SLiRP/TAP. This commit should be able to fix the issue as reported with some NIC models before, that ARP requests get dropped, preventing the guest from becoming visible on the network. It was workarounded in these NIC models on the receive path, that when a short frame is received, it is padded up to 60 bytes. The following 2 commits seem to be the one to workaround this issue in e1000 and vmxenet3 before, and should probably be reverted. commit 78aeb23eded2 ("e1000: Pad short frames to minimum size (60 bytes)") commit 40a87c6c9b11 ("vmxnet3: Pad short frames to minimum size (60 bytes)") Signed-off-by: Bin Meng Reviewed-by: Philippe Mathieu-Daudé Signed-off-by: Jason Wang --- net/slirp.c | 10 ++++++++++ net/tap-win32.c | 10 ++++++++++ net/tap.c | 10 ++++++++++ 3 files changed, 30 insertions(+) diff --git a/net/slirp.c b/net/slirp.c index 9454a673d6..a9fdc7a08f 100644 --- a/net/slirp.c +++ b/net/slirp.c @@ -31,6 +31,7 @@ #include #include #endif +#include "net/eth.h" #include "net/net.h" #include "clients.h" #include "hub.h" @@ -115,6 +116,15 @@ static ssize_t net_slirp_send_packet(const void *pkt, size_t pkt_len, void *opaque) { SlirpState *s = opaque; + uint8_t min_pkt[ETH_ZLEN]; + size_t min_pktsz = sizeof(min_pkt); + + if (!s->nc.peer->do_not_pad) { + if (eth_pad_short_frame(min_pkt, &min_pktsz, pkt, pkt_len)) { + pkt = min_pkt; + pkt_len = min_pktsz; + } + } return qemu_send_packet(&s->nc, pkt, pkt_len); } diff --git a/net/tap-win32.c b/net/tap-win32.c index 21e451107b..d7c2a8759c 100644 --- a/net/tap-win32.c +++ b/net/tap-win32.c @@ -31,6 +31,7 @@ #include "qemu-common.h" #include "clients.h" /* net_init_tap */ +#include "net/eth.h" #include "net/net.h" #include "net/tap.h" /* tap_has_ufo, ... */ #include "qemu/error-report.h" @@ -688,9 +689,18 @@ static void tap_win32_send(void *opaque) uint8_t *buf; int max_size = 4096; int size; + uint8_t min_pkt[ETH_ZLEN]; + size_t min_pktsz = sizeof(min_pkt); size = tap_win32_read(s->handle, &buf, max_size); if (size > 0) { + if (!s->nc.peer->do_not_pad) { + if (eth_pad_short_frame(min_pkt, &min_pktsz, buf, size)) { + buf = min_pkt; + size = min_pktsz; + } + } + qemu_send_packet(&s->nc, buf, size); tap_win32_free_buffer(s->handle, buf); } diff --git a/net/tap.c b/net/tap.c index 12a08d54fe..d6d8456188 100644 --- a/net/tap.c +++ b/net/tap.c @@ -32,6 +32,7 @@ #include #include +#include "net/eth.h" #include "net/net.h" #include "clients.h" #include "monitor/monitor.h" @@ -189,6 +190,8 @@ static void tap_send(void *opaque) while (true) { uint8_t *buf = s->buf; + uint8_t min_pkt[ETH_ZLEN]; + size_t min_pktsz = sizeof(min_pkt); size = tap_read_packet(s->fd, s->buf, sizeof(s->buf)); if (size <= 0) { @@ -200,6 +203,13 @@ static void tap_send(void *opaque) size -= s->host_vnet_hdr_len; } + if (!s->nc.peer->do_not_pad) { + if (eth_pad_short_frame(min_pkt, &min_pktsz, buf, size)) { + buf = min_pkt; + size = min_pktsz; + } + } + size = qemu_send_packet_async(&s->nc, buf, size, tap_send_completed); if (size == 0) { tap_read_poll(s, false);