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-rw-r--r--pkg/iovec/iovec.go71
-rw-r--r--pkg/iovec/iovec_state_autogen.go5
-rw-r--r--pkg/sentry/fs/host/socket_iovec.go4
-rw-r--r--pkg/sentry/fsimpl/host/socket_iovec.go4
-rw-r--r--pkg/sentry/hostfd/hostfd_linux.go9
-rw-r--r--pkg/sentry/hostfd/hostfd_unsafe.go17
-rw-r--r--pkg/tcpip/link/fdbased/endpoint.go207
-rw-r--r--pkg/tcpip/link/rawfile/rawfile_unsafe.go61
8 files changed, 220 insertions, 158 deletions
diff --git a/pkg/iovec/iovec.go b/pkg/iovec/iovec.go
deleted file mode 100644
index a281c05b6..000000000
--- a/pkg/iovec/iovec.go
+++ /dev/null
@@ -1,71 +0,0 @@
-// Copyright 2020 The gVisor Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-// http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-// +build linux
-
-// Package iovec provides helpers to interact with vectorized I/O on host
-// system.
-package iovec
-
-import (
- "golang.org/x/sys/unix"
-)
-
-// MaxIovs is the maximum number of iovecs host platform can accept.
-var MaxIovs = 1024
-
-// Builder is a builder for slice of unix.Iovec.
-type Builder struct {
- iovec []unix.Iovec
- storage [8]unix.Iovec
-
- // overflow tracks the last buffer when iovec length is at MaxIovs.
- overflow []byte
-}
-
-// Add adds buf to b preparing to be written. Zero-length buf won't be added.
-func (b *Builder) Add(buf []byte) {
- if len(buf) == 0 {
- return
- }
- if b.iovec == nil {
- b.iovec = b.storage[:0]
- }
- if len(b.iovec) >= MaxIovs {
- b.addByAppend(buf)
- return
- }
-
- b.iovec = append(b.iovec, unix.Iovec{Base: &buf[0]})
- b.iovec[len(b.iovec)-1].SetLen(len(buf))
-
- // Keep the last buf if iovec is at max capacity. We will need to append to it
- // for later bufs.
- if len(b.iovec) == MaxIovs {
- n := len(buf)
- b.overflow = buf[:n:n]
- }
-}
-
-func (b *Builder) addByAppend(buf []byte) {
- b.overflow = append(b.overflow, buf...)
- b.iovec[len(b.iovec)-1] = unix.Iovec{Base: &b.overflow[0]}
- b.iovec[len(b.iovec)-1].SetLen(len(b.overflow))
-}
-
-// Build returns the final Iovec slice. The length of returned iovec will not
-// excceed MaxIovs.
-func (b *Builder) Build() []unix.Iovec {
- return b.iovec
-}
diff --git a/pkg/iovec/iovec_state_autogen.go b/pkg/iovec/iovec_state_autogen.go
deleted file mode 100644
index c2e428cbe..000000000
--- a/pkg/iovec/iovec_state_autogen.go
+++ /dev/null
@@ -1,5 +0,0 @@
-// automatically generated by stateify.
-
-// +build linux
-
-package iovec
diff --git a/pkg/sentry/fs/host/socket_iovec.go b/pkg/sentry/fs/host/socket_iovec.go
index 7380d75e7..fd48aff11 100644
--- a/pkg/sentry/fs/host/socket_iovec.go
+++ b/pkg/sentry/fs/host/socket_iovec.go
@@ -16,7 +16,7 @@ package host
import (
"golang.org/x/sys/unix"
- "gvisor.dev/gvisor/pkg/iovec"
+ "gvisor.dev/gvisor/pkg/sentry/hostfd"
"gvisor.dev/gvisor/pkg/syserror"
)
@@ -72,7 +72,7 @@ func buildIovec(bufs [][]byte, maxlen int64, truncate bool) (length int64, iovec
}
}
- if iovsRequired > iovec.MaxIovs {
+ if iovsRequired > hostfd.MaxSendRecvMsgIov {
// The kernel will reject our call if we pass this many iovs.
// Use a single intermediate buffer instead.
b := make([]byte, stopLen)
diff --git a/pkg/sentry/fsimpl/host/socket_iovec.go b/pkg/sentry/fsimpl/host/socket_iovec.go
index b123a63ee..e090bb725 100644
--- a/pkg/sentry/fsimpl/host/socket_iovec.go
+++ b/pkg/sentry/fsimpl/host/socket_iovec.go
@@ -16,7 +16,7 @@ package host
import (
"golang.org/x/sys/unix"
- "gvisor.dev/gvisor/pkg/iovec"
+ "gvisor.dev/gvisor/pkg/sentry/hostfd"
"gvisor.dev/gvisor/pkg/syserror"
)
@@ -70,7 +70,7 @@ func buildIovec(bufs [][]byte, maxlen int64, truncate bool) (length int64, iovec
}
}
- if iovsRequired > iovec.MaxIovs {
+ if iovsRequired > hostfd.MaxSendRecvMsgIov {
// The kernel will reject our call if we pass this many iovs.
// Use a single intermediate buffer instead.
b := make([]byte, stopLen)
diff --git a/pkg/sentry/hostfd/hostfd_linux.go b/pkg/sentry/hostfd/hostfd_linux.go
index 1cabc848f..e103e7296 100644
--- a/pkg/sentry/hostfd/hostfd_linux.go
+++ b/pkg/sentry/hostfd/hostfd_linux.go
@@ -14,5 +14,10 @@
package hostfd
-// maxIov is the maximum permitted size of a struct iovec array.
-const maxIov = 1024 // UIO_MAXIOV
+// MaxReadWriteIov is the maximum permitted size of a struct iovec array in a
+// readv, writev, preadv, or pwritev host syscall.
+const MaxReadWriteIov = 1024 // UIO_MAXIOV
+
+// MaxSendRecvMsgIov is the maximum permitted size of a struct iovec array in a
+// sendmsg or recvmsg host syscall.
+const MaxSendRecvMsgIov = 1024 // UIO_MAXIOV
diff --git a/pkg/sentry/hostfd/hostfd_unsafe.go b/pkg/sentry/hostfd/hostfd_unsafe.go
index 03c6d2a16..a43311eb4 100644
--- a/pkg/sentry/hostfd/hostfd_unsafe.go
+++ b/pkg/sentry/hostfd/hostfd_unsafe.go
@@ -23,6 +23,11 @@ import (
"gvisor.dev/gvisor/pkg/safemem"
)
+const (
+ sizeofIovec = unsafe.Sizeof(unix.Iovec{})
+ sizeofMsghdr = unsafe.Sizeof(unix.Msghdr{})
+)
+
// Preadv2 reads up to dsts.NumBytes() bytes from host file descriptor fd into
// dsts. offset and flags are interpreted as for preadv2(2).
//
@@ -44,9 +49,9 @@ func Preadv2(fd int32, dsts safemem.BlockSeq, offset int64, flags uint32) (uint6
}
} else {
iovs := safemem.IovecsFromBlockSeq(dsts)
- if len(iovs) > maxIov {
- log.Debugf("hostfd.Preadv2: truncating from %d iovecs to %d", len(iovs), maxIov)
- iovs = iovs[:maxIov]
+ if len(iovs) > MaxReadWriteIov {
+ log.Debugf("hostfd.Preadv2: truncating from %d iovecs to %d", len(iovs), MaxReadWriteIov)
+ iovs = iovs[:MaxReadWriteIov]
}
n, _, e = unix.Syscall6(unix.SYS_PREADV2, uintptr(fd), uintptr((unsafe.Pointer)(&iovs[0])), uintptr(len(iovs)), uintptr(offset), 0 /* pos_h */, uintptr(flags))
}
@@ -80,9 +85,9 @@ func Pwritev2(fd int32, srcs safemem.BlockSeq, offset int64, flags uint32) (uint
}
} else {
iovs := safemem.IovecsFromBlockSeq(srcs)
- if len(iovs) > maxIov {
- log.Debugf("hostfd.Preadv2: truncating from %d iovecs to %d", len(iovs), maxIov)
- iovs = iovs[:maxIov]
+ if len(iovs) > MaxReadWriteIov {
+ log.Debugf("hostfd.Preadv2: truncating from %d iovecs to %d", len(iovs), MaxReadWriteIov)
+ iovs = iovs[:MaxReadWriteIov]
}
n, _, e = unix.Syscall6(unix.SYS_PWRITEV2, uintptr(fd), uintptr((unsafe.Pointer)(&iovs[0])), uintptr(len(iovs)), uintptr(offset), 0 /* pos_h */, uintptr(flags))
}
diff --git a/pkg/tcpip/link/fdbased/endpoint.go b/pkg/tcpip/link/fdbased/endpoint.go
index 735c28da1..1b56d2b72 100644
--- a/pkg/tcpip/link/fdbased/endpoint.go
+++ b/pkg/tcpip/link/fdbased/endpoint.go
@@ -44,7 +44,6 @@ import (
"sync/atomic"
"golang.org/x/sys/unix"
- "gvisor.dev/gvisor/pkg/iovec"
"gvisor.dev/gvisor/pkg/sync"
"gvisor.dev/gvisor/pkg/tcpip"
"gvisor.dev/gvisor/pkg/tcpip/buffer"
@@ -138,6 +137,20 @@ type endpoint struct {
// gsoKind is the supported kind of GSO.
gsoKind stack.SupportedGSO
+
+ // maxSyscallHeaderBytes has the same meaning as
+ // Options.MaxSyscallHeaderBytes.
+ maxSyscallHeaderBytes uintptr
+
+ // writevMaxIovs is the maximum number of iovecs that may be passed to
+ // rawfile.NonBlockingWriteIovec, as possibly limited by
+ // maxSyscallHeaderBytes. (No analogous limit is defined for
+ // rawfile.NonBlockingSendMMsg, since in that case the maximum number of
+ // iovecs also depends on the number of mmsghdrs. Instead, if sendBatch
+ // encounters a packet whose iovec count is limited by
+ // maxSyscallHeaderBytes, it falls back to writing the packet using writev
+ // via WritePacket.)
+ writevMaxIovs int
}
// Options specify the details about the fd-based endpoint to be created.
@@ -186,6 +199,11 @@ type Options struct {
// RXChecksumOffload if true, indicates that this endpoints capability
// set should include CapabilityRXChecksumOffload.
RXChecksumOffload bool
+
+ // If MaxSyscallHeaderBytes is non-zero, it is the maximum number of bytes
+ // of struct iovec, msghdr, and mmsghdr that may be passed by each host
+ // system call.
+ MaxSyscallHeaderBytes int
}
// fanoutID is used for AF_PACKET based endpoints to enable PACKET_FANOUT
@@ -235,14 +253,25 @@ func New(opts *Options) (stack.LinkEndpoint, error) {
return nil, fmt.Errorf("opts.FD is empty, at least one FD must be specified")
}
+ if opts.MaxSyscallHeaderBytes < 0 {
+ return nil, fmt.Errorf("opts.MaxSyscallHeaderBytes is negative")
+ }
+
e := &endpoint{
- fds: opts.FDs,
- mtu: opts.MTU,
- caps: caps,
- closed: opts.ClosedFunc,
- addr: opts.Address,
- hdrSize: hdrSize,
- packetDispatchMode: opts.PacketDispatchMode,
+ fds: opts.FDs,
+ mtu: opts.MTU,
+ caps: caps,
+ closed: opts.ClosedFunc,
+ addr: opts.Address,
+ hdrSize: hdrSize,
+ packetDispatchMode: opts.PacketDispatchMode,
+ maxSyscallHeaderBytes: uintptr(opts.MaxSyscallHeaderBytes),
+ writevMaxIovs: rawfile.MaxIovs,
+ }
+ if e.maxSyscallHeaderBytes != 0 {
+ if max := int(e.maxSyscallHeaderBytes / rawfile.SizeofIovec); max < e.writevMaxIovs {
+ e.writevMaxIovs = max
+ }
}
// Increment fanoutID to ensure that we don't re-use the same fanoutID for
@@ -470,9 +499,8 @@ func (e *endpoint) WritePacket(r stack.RouteInfo, protocol tcpip.NetworkProtocol
e.AddHeader(r.LocalLinkAddress, r.RemoteLinkAddress, protocol, pkt)
}
- var builder iovec.Builder
-
fd := e.fds[pkt.Hash%uint32(len(e.fds))]
+ var vnetHdrBuf []byte
if e.gsoKind == stack.HWGSOSupported {
vnetHdr := virtioNetHdr{}
if pkt.GSOOptions.Type != stack.GSONone {
@@ -494,71 +522,123 @@ func (e *endpoint) WritePacket(r stack.RouteInfo, protocol tcpip.NetworkProtocol
vnetHdr.gsoSize = pkt.GSOOptions.MSS
}
}
+ vnetHdrBuf = vnetHdr.marshal()
+ }
- vnetHdrBuf := vnetHdr.marshal()
- builder.Add(vnetHdrBuf)
+ views := pkt.Views()
+ numIovecs := len(views)
+ if len(vnetHdrBuf) != 0 {
+ numIovecs++
+ }
+ if numIovecs > e.writevMaxIovs {
+ numIovecs = e.writevMaxIovs
}
- for _, v := range pkt.Views() {
- builder.Add(v)
+ // Allocate small iovec arrays on the stack.
+ var iovecsArr [8]unix.Iovec
+ iovecs := iovecsArr[:0]
+ if numIovecs > len(iovecsArr) {
+ iovecs = make([]unix.Iovec, 0, numIovecs)
+ }
+ iovecs = rawfile.AppendIovecFromBytes(iovecs, vnetHdrBuf, numIovecs)
+ for _, v := range views {
+ iovecs = rawfile.AppendIovecFromBytes(iovecs, v, numIovecs)
}
- return rawfile.NonBlockingWriteIovec(fd, builder.Build())
+ return rawfile.NonBlockingWriteIovec(fd, iovecs)
}
-func (e *endpoint) sendBatch(batchFD int, batch []*stack.PacketBuffer) (int, tcpip.Error) {
+func (e *endpoint) sendBatch(batchFD int, pkts []*stack.PacketBuffer) (int, tcpip.Error) {
// Send a batch of packets through batchFD.
- mmsgHdrs := make([]rawfile.MMsgHdr, 0, len(batch))
- for _, pkt := range batch {
- if e.hdrSize > 0 {
- e.AddHeader(pkt.EgressRoute.LocalLinkAddress, pkt.EgressRoute.RemoteLinkAddress, pkt.NetworkProtocolNumber, pkt)
- }
+ mmsgHdrsStorage := make([]rawfile.MMsgHdr, 0, len(pkts))
+ packets := 0
+ for packets < len(pkts) {
+ mmsgHdrs := mmsgHdrsStorage
+ batch := pkts[packets:]
+ syscallHeaderBytes := uintptr(0)
+ for _, pkt := range batch {
+ if e.hdrSize > 0 {
+ e.AddHeader(pkt.EgressRoute.LocalLinkAddress, pkt.EgressRoute.RemoteLinkAddress, pkt.NetworkProtocolNumber, pkt)
+ }
- var vnetHdrBuf []byte
- if e.gsoKind == stack.HWGSOSupported {
- vnetHdr := virtioNetHdr{}
- if pkt.GSOOptions.Type != stack.GSONone {
- vnetHdr.hdrLen = uint16(pkt.HeaderSize())
- if pkt.GSOOptions.NeedsCsum {
- vnetHdr.flags = _VIRTIO_NET_HDR_F_NEEDS_CSUM
- vnetHdr.csumStart = header.EthernetMinimumSize + pkt.GSOOptions.L3HdrLen
- vnetHdr.csumOffset = pkt.GSOOptions.CsumOffset
- }
- if pkt.GSOOptions.Type != stack.GSONone && uint16(pkt.Data().Size()) > pkt.GSOOptions.MSS {
- switch pkt.GSOOptions.Type {
- case stack.GSOTCPv4:
- vnetHdr.gsoType = _VIRTIO_NET_HDR_GSO_TCPV4
- case stack.GSOTCPv6:
- vnetHdr.gsoType = _VIRTIO_NET_HDR_GSO_TCPV6
- default:
- panic(fmt.Sprintf("Unknown gso type: %v", pkt.GSOOptions.Type))
+ var vnetHdrBuf []byte
+ if e.gsoKind == stack.HWGSOSupported {
+ vnetHdr := virtioNetHdr{}
+ if pkt.GSOOptions.Type != stack.GSONone {
+ vnetHdr.hdrLen = uint16(pkt.HeaderSize())
+ if pkt.GSOOptions.NeedsCsum {
+ vnetHdr.flags = _VIRTIO_NET_HDR_F_NEEDS_CSUM
+ vnetHdr.csumStart = header.EthernetMinimumSize + pkt.GSOOptions.L3HdrLen
+ vnetHdr.csumOffset = pkt.GSOOptions.CsumOffset
+ }
+ if pkt.GSOOptions.Type != stack.GSONone && uint16(pkt.Data().Size()) > pkt.GSOOptions.MSS {
+ switch pkt.GSOOptions.Type {
+ case stack.GSOTCPv4:
+ vnetHdr.gsoType = _VIRTIO_NET_HDR_GSO_TCPV4
+ case stack.GSOTCPv6:
+ vnetHdr.gsoType = _VIRTIO_NET_HDR_GSO_TCPV6
+ default:
+ panic(fmt.Sprintf("Unknown gso type: %v", pkt.GSOOptions.Type))
+ }
+ vnetHdr.gsoSize = pkt.GSOOptions.MSS
}
- vnetHdr.gsoSize = pkt.GSOOptions.MSS
}
+ vnetHdrBuf = vnetHdr.marshal()
}
- vnetHdrBuf = vnetHdr.marshal()
- }
- var builder iovec.Builder
- builder.Add(vnetHdrBuf)
- for _, v := range pkt.Views() {
- builder.Add(v)
- }
- iovecs := builder.Build()
+ views := pkt.Views()
+ numIovecs := len(views)
+ if len(vnetHdrBuf) != 0 {
+ numIovecs++
+ }
+ if numIovecs > rawfile.MaxIovs {
+ numIovecs = rawfile.MaxIovs
+ }
+ if e.maxSyscallHeaderBytes != 0 {
+ syscallHeaderBytes += rawfile.SizeofMMsgHdr + uintptr(numIovecs)*rawfile.SizeofIovec
+ if syscallHeaderBytes > e.maxSyscallHeaderBytes {
+ // We can't fit this packet into this call to sendmmsg().
+ // We could potentially do so if we reduced numIovecs
+ // further, but this might incur considerable extra
+ // copying. Leave it to the next batch instead.
+ break
+ }
+ }
- var mmsgHdr rawfile.MMsgHdr
- mmsgHdr.Msg.Iov = &iovecs[0]
- mmsgHdr.Msg.SetIovlen((len(iovecs)))
- mmsgHdrs = append(mmsgHdrs, mmsgHdr)
- }
+ // We can't easily allocate iovec arrays on the stack here since
+ // they will escape this loop iteration via mmsgHdrs.
+ iovecs := make([]unix.Iovec, 0, numIovecs)
+ iovecs = rawfile.AppendIovecFromBytes(iovecs, vnetHdrBuf, numIovecs)
+ for _, v := range views {
+ iovecs = rawfile.AppendIovecFromBytes(iovecs, v, numIovecs)
+ }
- packets := 0
- for len(mmsgHdrs) > 0 {
- sent, err := rawfile.NonBlockingSendMMsg(batchFD, mmsgHdrs)
- if err != nil {
- return packets, err
+ var mmsgHdr rawfile.MMsgHdr
+ mmsgHdr.Msg.Iov = &iovecs[0]
+ mmsgHdr.Msg.SetIovlen(len(iovecs))
+ mmsgHdrs = append(mmsgHdrs, mmsgHdr)
+ }
+
+ if len(mmsgHdrs) == 0 {
+ // We can't fit batch[0] into a mmsghdr while staying under
+ // e.maxSyscallHeaderBytes. Use WritePacket, which will avoid the
+ // mmsghdr (by using writev) and re-buffer iovecs more aggressively
+ // if necessary (by using e.writevMaxIovs instead of
+ // rawfile.MaxIovs).
+ pkt := batch[0]
+ if err := e.WritePacket(pkt.EgressRoute, pkt.NetworkProtocolNumber, pkt); err != nil {
+ return packets, err
+ }
+ packets++
+ } else {
+ for len(mmsgHdrs) > 0 {
+ sent, err := rawfile.NonBlockingSendMMsg(batchFD, mmsgHdrs)
+ if err != nil {
+ return packets, err
+ }
+ packets += sent
+ mmsgHdrs = mmsgHdrs[sent:]
+ }
}
- packets += sent
- mmsgHdrs = mmsgHdrs[sent:]
}
return packets, nil
@@ -676,8 +756,9 @@ func NewInjectable(fd int, mtu uint32, capabilities stack.LinkEndpointCapabiliti
unix.SetNonblock(fd, true)
return &InjectableEndpoint{endpoint: endpoint{
- fds: []int{fd},
- mtu: mtu,
- caps: capabilities,
+ fds: []int{fd},
+ mtu: mtu,
+ caps: capabilities,
+ writevMaxIovs: rawfile.MaxIovs,
}}
}
diff --git a/pkg/tcpip/link/rawfile/rawfile_unsafe.go b/pkg/tcpip/link/rawfile/rawfile_unsafe.go
index ba92aedbc..43fe57830 100644
--- a/pkg/tcpip/link/rawfile/rawfile_unsafe.go
+++ b/pkg/tcpip/link/rawfile/rawfile_unsafe.go
@@ -19,12 +19,66 @@
package rawfile
import (
+ "reflect"
"unsafe"
"golang.org/x/sys/unix"
"gvisor.dev/gvisor/pkg/tcpip"
)
+// SizeofIovec is the size of a unix.Iovec in bytes.
+const SizeofIovec = unsafe.Sizeof(unix.Iovec{})
+
+// MaxIovs is UIO_MAXIOV, the maximum number of iovecs that may be passed to a
+// host system call in a single array.
+const MaxIovs = 1024
+
+// IovecFromBytes returns a unix.Iovec representing bs.
+//
+// Preconditions: len(bs) > 0.
+func IovecFromBytes(bs []byte) unix.Iovec {
+ iov := unix.Iovec{
+ Base: &bs[0],
+ }
+ iov.SetLen(len(bs))
+ return iov
+}
+
+func bytesFromIovec(iov unix.Iovec) (bs []byte) {
+ sh := (*reflect.SliceHeader)(unsafe.Pointer(&bs))
+ sh.Data = uintptr(unsafe.Pointer(iov.Base))
+ sh.Len = int(iov.Len)
+ sh.Cap = int(iov.Len)
+ return
+}
+
+// AppendIovecFromBytes returns append(iovs, IovecFromBytes(bs)). If len(bs) ==
+// 0, AppendIovecFromBytes returns iovs without modification. If len(iovs) >=
+// max, AppendIovecFromBytes replaces the final iovec in iovs with one that
+// also includes the contents of bs. Note that this implies that
+// AppendIovecFromBytes is only usable when the returned iovec slice is used as
+// the source of a write.
+func AppendIovecFromBytes(iovs []unix.Iovec, bs []byte, max int) []unix.Iovec {
+ if len(bs) == 0 {
+ return iovs
+ }
+ if len(iovs) < max {
+ return append(iovs, IovecFromBytes(bs))
+ }
+ iovs[len(iovs)-1] = IovecFromBytes(append(bytesFromIovec(iovs[len(iovs)-1]), bs...))
+ return iovs
+}
+
+// MMsgHdr represents the mmsg_hdr structure required by recvmmsg() on linux.
+type MMsgHdr struct {
+ Msg unix.Msghdr
+ Len uint32
+ _ [4]byte
+}
+
+// SizeofMMsgHdr is the size of a MMsgHdr in bytes.
+const SizeofMMsgHdr = unsafe.Sizeof(MMsgHdr{})
+
// GetMTU determines the MTU of a network interface device.
func GetMTU(name string) (uint32, error) {
fd, err := unix.Socket(unix.AF_UNIX, unix.SOCK_DGRAM, 0)
@@ -137,13 +191,6 @@ func BlockingReadv(fd int, iovecs []unix.Iovec) (int, tcpip.Error) {
}
}
-// MMsgHdr represents the mmsg_hdr structure required by recvmmsg() on linux.
-type MMsgHdr struct {
- Msg unix.Msghdr
- Len uint32
- _ [4]byte
-}
-
// BlockingRecvMMsg reads from a file descriptor that is set up as non-blocking
// and stores the received messages in a slice of MMsgHdr structures. If no data
// is available, it will block in a poll() syscall until the file descriptor
generated by cgit v1.2.3 (git 2.39.1) at 2024-11-08 21:10:10 +0000