3 files changed, 214 insertions, 0 deletions
diff --git a/pkg/iovec/BUILD b/pkg/iovec/BUILD
new file mode 100644
index 000000000..eda82cfc1
--- /dev/null
+++ b/pkg/iovec/BUILD
@@ -0,0 +1,18 @@
+load("//tools:defs.bzl", "go_library", "go_test")
+
+package(licenses = ["notice"])
+
+go_library(
+    name = "iovec",
+    srcs = ["iovec.go"],
+    visibility = ["//:sandbox"],
+    deps = ["//pkg/abi/linux"],
+)
+
+go_test(
+    name = "iovec_test",
+    size = "small",
+    srcs = ["iovec_test.go"],
+    library = ":iovec",
+    deps = ["@org_golang_x_sys//unix:go_default_library"],
+)
diff --git a/pkg/iovec/iovec.go b/pkg/iovec/iovec.go
new file mode 100644
index 000000000..dd70fe80f
--- /dev/null
+++ b/pkg/iovec/iovec.go
@@ -0,0 +1,75 @@
+// 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 (
+	"syscall"
+
+	"gvisor.dev/gvisor/pkg/abi/linux"
+)
+
+// MaxIovs is the maximum number of iovecs host platform can accept.
+var MaxIovs = linux.UIO_MAXIOV
+
+// Builder is a builder for slice of syscall.Iovec.
+type Builder struct {
+	iovec   []syscall.Iovec
+	storage [8]syscall.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, syscall.Iovec{
+		Base: &buf[0],
+		Len:  uint64(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] = syscall.Iovec{
+		Base: &b.overflow[0],
+		Len:  uint64(len(b.overflow)),
+	}
+}
+
+// Build returns the final Iovec slice. The length of returned iovec will not
+// excceed MaxIovs.
+func (b *Builder) Build() []syscall.Iovec {
+	return b.iovec
+}
diff --git a/pkg/iovec/iovec_test.go b/pkg/iovec/iovec_test.go
new file mode 100644
index 000000000..a3900c299
--- /dev/null
+++ b/pkg/iovec/iovec_test.go
@@ -0,0 +1,121 @@
+// 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
+
+import (
+	"bytes"
+	"fmt"
+	"syscall"
+	"testing"
+	"unsafe"
+
+	"golang.org/x/sys/unix"
+)
+
+func TestBuilderEmpty(t *testing.T) {
+	var builder Builder
+	iovecs := builder.Build()
+	if got, want := len(iovecs), 0; got != want {
+		t.Errorf("len(iovecs) = %d, want %d", got, want)
+	}
+}
+
+func TestBuilderBuild(t *testing.T) {
+	a := []byte{1, 2}
+	b := []byte{3, 4, 5}
+
+	var builder Builder
+	builder.Add(a)
+	builder.Add(b)
+	builder.Add(nil)      // Nil slice won't be added.
+	builder.Add([]byte{}) // Empty slice won't be added.
+	iovecs := builder.Build()
+
+	if got, want := len(iovecs), 2; got != want {
+		t.Fatalf("len(iovecs) = %d, want %d", got, want)
+	}
+	for i, data := range [][]byte{a, b} {
+		if got, want := *iovecs[i].Base, data[0]; got != want {
+			t.Fatalf("*iovecs[%d].Base = %d, want %d", i, got, want)
+		}
+		if got, want := iovecs[i].Len, uint64(len(data)); got != want {
+			t.Fatalf("iovecs[%d].Len = %d, want %d", i, got, want)
+		}
+	}
+}
+
+func TestBuilderBuildMaxIov(t *testing.T) {
+	for _, test := range []struct {
+		numIov int
+	}{
+		{
+			numIov: MaxIovs - 1,
+		},
+		{
+			numIov: MaxIovs,
+		},
+		{
+			numIov: MaxIovs + 1,
+		},
+		{
+			numIov: MaxIovs + 10,
+		},
+	} {
+		name := fmt.Sprintf("numIov=%v", test.numIov)
+		t.Run(name, func(t *testing.T) {
+			var data []byte
+			var builder Builder
+			for i := 0; i < test.numIov; i++ {
+				buf := []byte{byte(i)}
+				builder.Add(buf)
+				data = append(data, buf...)
+			}
+			iovec := builder.Build()
+
+			// Check the expected length of iovec.
+			wantNum := test.numIov
+			if wantNum > MaxIovs {
+				wantNum = MaxIovs
+			}
+			if got, want := len(iovec), wantNum; got != want {
+				t.Errorf("len(iovec) = %d, want %d", got, want)
+			}
+
+			// Test a real read-write.
+			var fds [2]int
+			if err := unix.Pipe(fds[:]); err != nil {
+				t.Fatalf("Pipe: %v", err)
+			}
+			defer syscall.Close(fds[0])
+			defer syscall.Close(fds[1])
+
+			wrote, _, e := syscall.RawSyscall(syscall.SYS_WRITEV, uintptr(fds[1]), uintptr(unsafe.Pointer(&iovec[0])), uintptr(len(iovec)))
+			if int(wrote) != len(data) || e != 0 {
+				t.Fatalf("writev: %v, %v; want %v, 0", wrote, e, len(data))
+			}
+
+			got := make([]byte, len(data))
+			if n, err := syscall.Read(fds[0], got); n != len(got) || err != nil {
+				t.Fatalf("read: %v, %v; want %v, nil", n, err, len(got))
+			}
+
+			if !bytes.Equal(got, data) {
+				t.Errorf("read: got data %v, want %v", got, data)
+			}
+		})
+	}
+}