Update package locations.

Because the abi will depend on the core types for marshalling (usermem,
context, safemem, safecopy), these need to be flattened from the sentry
directory. These packages contain no sentry-specific details.

PiperOrigin-RevId: 291811289

7 files changed, 1408 insertions, 0 deletions

diff --git a/pkg/safemem/BUILD b/pkg/safemem/BUILD
new file mode 100644
index 000000000..ce30382ab
--- /dev/null
+++ b/pkg/safemem/BUILD
@@ -0,0 +1,27 @@
+load("//tools:defs.bzl", "go_library", "go_test")
+
+package(licenses = ["notice"])
+
+go_library(
+    name = "safemem",
+    srcs = [
+        "block_unsafe.go",
+        "io.go",
+        "safemem.go",
+        "seq_unsafe.go",
+    ],
+    visibility = ["//:sandbox"],
+    deps = [
+        "//pkg/safecopy",
+    ],
+)
+
+go_test(
+    name = "safemem_test",
+    size = "small",
+    srcs = [
+        "io_test.go",
+        "seq_test.go",
+    ],
+    library = ":safemem",
+)
diff --git a/pkg/safemem/block_unsafe.go b/pkg/safemem/block_unsafe.go
new file mode 100644
index 000000000..e7fd30743
--- /dev/null
+++ b/pkg/safemem/block_unsafe.go
@@ -0,0 +1,279 @@
+// Copyright 2018 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.
+
+package safemem
+
+import (
+	"fmt"
+	"reflect"
+	"unsafe"
+
+	"gvisor.dev/gvisor/pkg/safecopy"
+)
+
+// A Block is a range of contiguous bytes, similar to []byte but with the
+// following differences:
+//
+// - The memory represented by a Block may require the use of safecopy to
+// access.
+//
+// - Block does not carry a capacity and cannot be expanded.
+//
+// Blocks are immutable and may be copied by value. The zero value of Block
+// represents an empty range, analogous to a nil []byte.
+type Block struct {
+	// [start, start+length) is the represented memory.
+	//
+	// start is an unsafe.Pointer to ensure that Block prevents the represented
+	// memory from being garbage-collected.
+	start  unsafe.Pointer
+	length int
+
+	// needSafecopy is true if accessing the represented memory requires the
+	// use of safecopy.
+	needSafecopy bool
+}
+
+// BlockFromSafeSlice returns a Block equivalent to slice, which is safe to
+// access without safecopy.
+func BlockFromSafeSlice(slice []byte) Block {
+	return blockFromSlice(slice, false)
+}
+
+// BlockFromUnsafeSlice returns a Block equivalent to bs, which is not safe to
+// access without safecopy.
+func BlockFromUnsafeSlice(slice []byte) Block {
+	return blockFromSlice(slice, true)
+}
+
+func blockFromSlice(slice []byte, needSafecopy bool) Block {
+	if len(slice) == 0 {
+		return Block{}
+	}
+	return Block{
+		start:        unsafe.Pointer(&slice[0]),
+		length:       len(slice),
+		needSafecopy: needSafecopy,
+	}
+}
+
+// BlockFromSafePointer returns a Block equivalent to [ptr, ptr+len), which is
+// safe to access without safecopy.
+//
+// Preconditions: ptr+len does not overflow.
+func BlockFromSafePointer(ptr unsafe.Pointer, len int) Block {
+	return blockFromPointer(ptr, len, false)
+}
+
+// BlockFromUnsafePointer returns a Block equivalent to [ptr, ptr+len), which
+// is not safe to access without safecopy.
+//
+// Preconditions: ptr+len does not overflow.
+func BlockFromUnsafePointer(ptr unsafe.Pointer, len int) Block {
+	return blockFromPointer(ptr, len, true)
+}
+
+func blockFromPointer(ptr unsafe.Pointer, len int, needSafecopy bool) Block {
+	if uptr := uintptr(ptr); uptr+uintptr(len) < uptr {
+		panic(fmt.Sprintf("ptr %#x + len %#x overflows", ptr, len))
+	}
+	return Block{
+		start:        ptr,
+		length:       len,
+		needSafecopy: needSafecopy,
+	}
+}
+
+// DropFirst returns a Block equivalent to b, but with the first n bytes
+// omitted. It is analogous to the [n:] operation on a slice, except that if n
+// > b.Len(), DropFirst returns an empty Block instead of panicking.
+//
+// Preconditions: n >= 0.
+func (b Block) DropFirst(n int) Block {
+	if n < 0 {
+		panic(fmt.Sprintf("invalid n: %d", n))
+	}
+	return b.DropFirst64(uint64(n))
+}
+
+// DropFirst64 is equivalent to DropFirst but takes a uint64.
+func (b Block) DropFirst64(n uint64) Block {
+	if n >= uint64(b.length) {
+		return Block{}
+	}
+	return Block{
+		start:        unsafe.Pointer(uintptr(b.start) + uintptr(n)),
+		length:       b.length - int(n),
+		needSafecopy: b.needSafecopy,
+	}
+}
+
+// TakeFirst returns a Block equivalent to the first n bytes of b. It is
+// analogous to the [:n] operation on a slice, except that if n > b.Len(),
+// TakeFirst returns a copy of b instead of panicking.
+//
+// Preconditions: n >= 0.
+func (b Block) TakeFirst(n int) Block {
+	if n < 0 {
+		panic(fmt.Sprintf("invalid n: %d", n))
+	}
+	return b.TakeFirst64(uint64(n))
+}
+
+// TakeFirst64 is equivalent to TakeFirst but takes a uint64.
+func (b Block) TakeFirst64(n uint64) Block {
+	if n == 0 {
+		return Block{}
+	}
+	if n >= uint64(b.length) {
+		return b
+	}
+	return Block{
+		start:        b.start,
+		length:       int(n),
+		needSafecopy: b.needSafecopy,
+	}
+}
+
+// ToSlice returns a []byte equivalent to b.
+func (b Block) ToSlice() []byte {
+	var bs []byte
+	hdr := (*reflect.SliceHeader)(unsafe.Pointer(&bs))
+	hdr.Data = uintptr(b.start)
+	hdr.Len = b.length
+	hdr.Cap = b.length
+	return bs
+}
+
+// Addr returns b's start address as a uintptr. It returns uintptr instead of
+// unsafe.Pointer so that code using safemem cannot obtain unsafe.Pointers
+// without importing the unsafe package explicitly.
+//
+// Note that a uintptr is not recognized as a pointer by the garbage collector,
+// such that if there are no uses of b after a call to b.Addr() and the address
+// is to Go-managed memory, the returned uintptr does not prevent garbage
+// collection of the pointee.
+func (b Block) Addr() uintptr {
+	return uintptr(b.start)
+}
+
+// Len returns b's length in bytes.
+func (b Block) Len() int {
+	return b.length
+}
+
+// NeedSafecopy returns true if accessing b.ToSlice() requires the use of safecopy.
+func (b Block) NeedSafecopy() bool {
+	return b.needSafecopy
+}
+
+// String implements fmt.Stringer.String.
+func (b Block) String() string {
+	if uintptr(b.start) == 0 && b.length == 0 {
+		return "<nil>"
+	}
+	var suffix string
+	if b.needSafecopy {
+		suffix = "*"
+	}
+	return fmt.Sprintf("[%#x-%#x)%s", uintptr(b.start), uintptr(b.start)+uintptr(b.length), suffix)
+}
+
+// Copy copies src.Len() or dst.Len() bytes, whichever is less, from src
+// to dst and returns the number of bytes copied.
+//
+// If src and dst overlap, the data stored in dst is unspecified.
+func Copy(dst, src Block) (int, error) {
+	if !dst.needSafecopy && !src.needSafecopy {
+		return copy(dst.ToSlice(), src.ToSlice()), nil
+	}
+
+	n := dst.length
+	if n > src.length {
+		n = src.length
+	}
+	if n == 0 {
+		return 0, nil
+	}
+
+	switch {
+	case dst.needSafecopy && !src.needSafecopy:
+		return safecopy.CopyOut(dst.start, src.TakeFirst(n).ToSlice())
+	case !dst.needSafecopy && src.needSafecopy:
+		return safecopy.CopyIn(dst.TakeFirst(n).ToSlice(), src.start)
+	case dst.needSafecopy && src.needSafecopy:
+		n64, err := safecopy.Copy(dst.start, src.start, uintptr(n))
+		return int(n64), err
+	default:
+		panic("unreachable")
+	}
+}
+
+// Zero sets all bytes in dst to 0 and returns the number of bytes zeroed.
+func Zero(dst Block) (int, error) {
+	if !dst.needSafecopy {
+		bs := dst.ToSlice()
+		for i := range bs {
+			bs[i] = 0
+		}
+		return len(bs), nil
+	}
+
+	n64, err := safecopy.ZeroOut(dst.start, uintptr(dst.length))
+	return int(n64), err
+}
+
+// Safecopy atomics are no slower than non-safecopy atomics, so use the former
+// even when !b.needSafecopy to get consistent alignment checking.
+
+// SwapUint32 invokes safecopy.SwapUint32 on the first 4 bytes of b.
+//
+// Preconditions: b.Len() >= 4.
+func SwapUint32(b Block, new uint32) (uint32, error) {
+	if b.length < 4 {
+		panic(fmt.Sprintf("insufficient length: %d", b.length))
+	}
+	return safecopy.SwapUint32(b.start, new)
+}
+
+// SwapUint64 invokes safecopy.SwapUint64 on the first 8 bytes of b.
+//
+// Preconditions: b.Len() >= 8.
+func SwapUint64(b Block, new uint64) (uint64, error) {
+	if b.length < 8 {
+		panic(fmt.Sprintf("insufficient length: %d", b.length))
+	}
+	return safecopy.SwapUint64(b.start, new)
+}
+
+// CompareAndSwapUint32 invokes safecopy.CompareAndSwapUint32 on the first 4
+// bytes of b.
+//
+// Preconditions: b.Len() >= 4.
+func CompareAndSwapUint32(b Block, old, new uint32) (uint32, error) {
+	if b.length < 4 {
+		panic(fmt.Sprintf("insufficient length: %d", b.length))
+	}
+	return safecopy.CompareAndSwapUint32(b.start, old, new)
+}
+
+// LoadUint32 invokes safecopy.LoadUint32 on the first 4 bytes of b.
+//
+// Preconditions: b.Len() >= 4.
+func LoadUint32(b Block) (uint32, error) {
+	if b.length < 4 {
+		panic(fmt.Sprintf("insufficient length: %d", b.length))
+	}
+	return safecopy.LoadUint32(b.start)
+}
diff --git a/pkg/safemem/io.go b/pkg/safemem/io.go
new file mode 100644
index 000000000..f039a5c34
--- /dev/null
+++ b/pkg/safemem/io.go
@@ -0,0 +1,392 @@
+// Copyright 2018 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.
+
+package safemem
+
+import (
+	"errors"
+	"io"
+	"math"
+)
+
+// ErrEndOfBlockSeq is returned by BlockSeqWriter when attempting to write
+// beyond the end of the BlockSeq.
+var ErrEndOfBlockSeq = errors.New("write beyond end of BlockSeq")
+
+// Reader represents a streaming byte source like io.Reader.
+type Reader interface {
+	// ReadToBlocks reads up to dsts.NumBytes() bytes into dsts and returns the
+	// number of bytes read. It may return a partial read without an error
+	// (i.e. (n, nil) where 0 < n < dsts.NumBytes()). It should not return a
+	// full read with an error (i.e. (dsts.NumBytes(), err) where err != nil);
+	// note that this differs from io.Reader.Read (in particular, io.EOF should
+	// not be returned if ReadToBlocks successfully reads dsts.NumBytes()
+	// bytes.)
+	ReadToBlocks(dsts BlockSeq) (uint64, error)
+}
+
+// Writer represents a streaming byte sink like io.Writer.
+type Writer interface {
+	// WriteFromBlocks writes up to srcs.NumBytes() bytes from srcs and returns
+	// the number of bytes written. It may return a partial write without an
+	// error (i.e. (n, nil) where 0 < n < srcs.NumBytes()). It should not
+	// return a full write with an error (i.e. srcs.NumBytes(), err) where err
+	// != nil).
+	WriteFromBlocks(srcs BlockSeq) (uint64, error)
+}
+
+// ReadFullToBlocks repeatedly invokes r.ReadToBlocks until dsts.NumBytes()
+// bytes have been read or ReadToBlocks returns an error.
+func ReadFullToBlocks(r Reader, dsts BlockSeq) (uint64, error) {
+	var done uint64
+	for !dsts.IsEmpty() {
+		n, err := r.ReadToBlocks(dsts)
+		done += n
+		if err != nil {
+			return done, err
+		}
+		dsts = dsts.DropFirst64(n)
+	}
+	return done, nil
+}
+
+// WriteFullFromBlocks repeatedly invokes w.WriteFromBlocks until
+// srcs.NumBytes() bytes have been written or WriteFromBlocks returns an error.
+func WriteFullFromBlocks(w Writer, srcs BlockSeq) (uint64, error) {
+	var done uint64
+	for !srcs.IsEmpty() {
+		n, err := w.WriteFromBlocks(srcs)
+		done += n
+		if err != nil {
+			return done, err
+		}
+		srcs = srcs.DropFirst64(n)
+	}
+	return done, nil
+}
+
+// BlockSeqReader implements Reader by reading from a BlockSeq.
+type BlockSeqReader struct {
+	Blocks BlockSeq
+}
+
+// ReadToBlocks implements Reader.ReadToBlocks.
+func (r *BlockSeqReader) ReadToBlocks(dsts BlockSeq) (uint64, error) {
+	n, err := CopySeq(dsts, r.Blocks)
+	r.Blocks = r.Blocks.DropFirst64(n)
+	if err != nil {
+		return n, err
+	}
+	if n < dsts.NumBytes() {
+		return n, io.EOF
+	}
+	return n, nil
+}
+
+// BlockSeqWriter implements Writer by writing to a BlockSeq.
+type BlockSeqWriter struct {
+	Blocks BlockSeq
+}
+
+// WriteFromBlocks implements Writer.WriteFromBlocks.
+func (w *BlockSeqWriter) WriteFromBlocks(srcs BlockSeq) (uint64, error) {
+	n, err := CopySeq(w.Blocks, srcs)
+	w.Blocks = w.Blocks.DropFirst64(n)
+	if err != nil {
+		return n, err
+	}
+	if n < srcs.NumBytes() {
+		return n, ErrEndOfBlockSeq
+	}
+	return n, nil
+}
+
+// ReaderFunc implements Reader for a function with the semantics of
+// Reader.ReadToBlocks.
+type ReaderFunc func(dsts BlockSeq) (uint64, error)
+
+// ReadToBlocks implements Reader.ReadToBlocks.
+func (f ReaderFunc) ReadToBlocks(dsts BlockSeq) (uint64, error) {
+	return f(dsts)
+}
+
+// WriterFunc implements Writer for a function with the semantics of
+// Writer.WriteFromBlocks.
+type WriterFunc func(srcs BlockSeq) (uint64, error)
+
+// WriteFromBlocks implements Writer.WriteFromBlocks.
+func (f WriterFunc) WriteFromBlocks(srcs BlockSeq) (uint64, error) {
+	return f(srcs)
+}
+
+// ToIOReader implements io.Reader for a (safemem.)Reader.
+//
+// ToIOReader will return a successful partial read iff Reader.ReadToBlocks does
+// so.
+type ToIOReader struct {
+	Reader Reader
+}
+
+// Read implements io.Reader.Read.
+func (r ToIOReader) Read(dst []byte) (int, error) {
+	n, err := r.Reader.ReadToBlocks(BlockSeqOf(BlockFromSafeSlice(dst)))
+	return int(n), err
+}
+
+// ToIOWriter implements io.Writer for a (safemem.)Writer.
+type ToIOWriter struct {
+	Writer Writer
+}
+
+// Write implements io.Writer.Write.
+func (w ToIOWriter) Write(src []byte) (int, error) {
+	// io.Writer does not permit partial writes.
+	n, err := WriteFullFromBlocks(w.Writer, BlockSeqOf(BlockFromSafeSlice(src)))
+	return int(n), err
+}
+
+// FromIOReader implements Reader for an io.Reader by repeatedly invoking
+// io.Reader.Read until it returns an error or partial read. This is not
+// thread-safe.
+//
+// FromIOReader will return a successful partial read iff Reader.Read does so.
+type FromIOReader struct {
+	Reader io.Reader
+}
+
+// ReadToBlocks implements Reader.ReadToBlocks.
+func (r FromIOReader) ReadToBlocks(dsts BlockSeq) (uint64, error) {
+	var buf []byte
+	var done uint64
+	for !dsts.IsEmpty() {
+		dst := dsts.Head()
+		var n int
+		var err error
+		n, buf, err = r.readToBlock(dst, buf)
+		done += uint64(n)
+		if n != dst.Len() {
+			return done, err
+		}
+		dsts = dsts.Tail()
+		if err != nil {
+			if dsts.IsEmpty() && err == io.EOF {
+				return done, nil
+			}
+			return done, err
+		}
+	}
+	return done, nil
+}
+
+func (r FromIOReader) readToBlock(dst Block, buf []byte) (int, []byte, error) {
+	// io.Reader isn't safecopy-aware, so we have to buffer Blocks that require
+	// safecopy.
+	if !dst.NeedSafecopy() {
+		n, err := r.Reader.Read(dst.ToSlice())
+		return n, buf, err
+	}
+	if len(buf) < dst.Len() {
+		buf = make([]byte, dst.Len())
+	}
+	rn, rerr := r.Reader.Read(buf[:dst.Len()])
+	wbn, wberr := Copy(dst, BlockFromSafeSlice(buf[:rn]))
+	if wberr != nil {
+		return wbn, buf, wberr
+	}
+	return wbn, buf, rerr
+}
+
+// FromIOReaderAt implements Reader for an io.ReaderAt. Does not repeatedly
+// invoke io.ReaderAt.ReadAt because ReadAt is more strict than Read. A partial
+// read indicates an error. This is not thread-safe.
+type FromIOReaderAt struct {
+	ReaderAt io.ReaderAt
+	Offset   int64
+}
+
+// ReadToBlocks implements Reader.ReadToBlocks.
+func (r FromIOReaderAt) ReadToBlocks(dsts BlockSeq) (uint64, error) {
+	var buf []byte
+	var done uint64
+	for !dsts.IsEmpty() {
+		dst := dsts.Head()
+		var n int
+		var err error
+		n, buf, err = r.readToBlock(dst, buf)
+		done += uint64(n)
+		if n != dst.Len() {
+			return done, err
+		}
+		dsts = dsts.Tail()
+		if err != nil {
+			if dsts.IsEmpty() && err == io.EOF {
+				return done, nil
+			}
+			return done, err
+		}
+	}
+	return done, nil
+}
+
+func (r FromIOReaderAt) readToBlock(dst Block, buf []byte) (int, []byte, error) {
+	// io.Reader isn't safecopy-aware, so we have to buffer Blocks that require
+	// safecopy.
+	if !dst.NeedSafecopy() {
+		n, err := r.ReaderAt.ReadAt(dst.ToSlice(), r.Offset)
+		r.Offset += int64(n)
+		return n, buf, err
+	}
+	if len(buf) < dst.Len() {
+		buf = make([]byte, dst.Len())
+	}
+	rn, rerr := r.ReaderAt.ReadAt(buf[:dst.Len()], r.Offset)
+	r.Offset += int64(rn)
+	wbn, wberr := Copy(dst, BlockFromSafeSlice(buf[:rn]))
+	if wberr != nil {
+		return wbn, buf, wberr
+	}
+	return wbn, buf, rerr
+}
+
+// FromIOWriter implements Writer for an io.Writer by repeatedly invoking
+// io.Writer.Write until it returns an error or partial write.
+//
+// FromIOWriter will tolerate implementations of io.Writer.Write that return
+// partial writes with a nil error in contravention of io.Writer's
+// requirements, since Writer is permitted to do so. FromIOWriter will return a
+// successful partial write iff Writer.Write does so.
+type FromIOWriter struct {
+	Writer io.Writer
+}
+
+// WriteFromBlocks implements Writer.WriteFromBlocks.
+func (w FromIOWriter) WriteFromBlocks(srcs BlockSeq) (uint64, error) {
+	var buf []byte
+	var done uint64
+	for !srcs.IsEmpty() {
+		src := srcs.Head()
+		var n int
+		var err error
+		n, buf, err = w.writeFromBlock(src, buf)
+		done += uint64(n)
+		if n != src.Len() || err != nil {
+			return done, err
+		}
+		srcs = srcs.Tail()
+	}
+	return done, nil
+}
+
+func (w FromIOWriter) writeFromBlock(src Block, buf []byte) (int, []byte, error) {
+	// io.Writer isn't safecopy-aware, so we have to buffer Blocks that require
+	// safecopy.
+	if !src.NeedSafecopy() {
+		n, err := w.Writer.Write(src.ToSlice())
+		return n, buf, err
+	}
+	if len(buf) < src.Len() {
+		buf = make([]byte, src.Len())
+	}
+	bufn, buferr := Copy(BlockFromSafeSlice(buf[:src.Len()]), src)
+	wn, werr := w.Writer.Write(buf[:bufn])
+	if werr != nil {
+		return wn, buf, werr
+	}
+	return wn, buf, buferr
+}
+
+// FromVecReaderFunc implements Reader for a function that reads data into a
+// [][]byte and returns the number of bytes read as an int64.
+type FromVecReaderFunc struct {
+	ReadVec func(dsts [][]byte) (int64, error)
+}
+
+// ReadToBlocks implements Reader.ReadToBlocks.
+//
+// ReadToBlocks calls r.ReadVec at most once.
+func (r FromVecReaderFunc) ReadToBlocks(dsts BlockSeq) (uint64, error) {
+	if dsts.IsEmpty() {
+		return 0, nil
+	}
+	// Ensure that we don't pass a [][]byte with a total length > MaxInt64.
+	dsts = dsts.TakeFirst64(uint64(math.MaxInt64))
+	dstSlices := make([][]byte, 0, dsts.NumBlocks())
+	// Buffer Blocks that require safecopy.
+	for tmp := dsts; !tmp.IsEmpty(); tmp = tmp.Tail() {
+		dst := tmp.Head()
+		if dst.NeedSafecopy() {
+			dstSlices = append(dstSlices, make([]byte, dst.Len()))
+		} else {
+			dstSlices = append(dstSlices, dst.ToSlice())
+		}
+	}
+	rn, rerr := r.ReadVec(dstSlices)
+	dsts = dsts.TakeFirst64(uint64(rn))
+	var done uint64
+	var i int
+	for !dsts.IsEmpty() {
+		dst := dsts.Head()
+		if dst.NeedSafecopy() {
+			n, err := Copy(dst, BlockFromSafeSlice(dstSlices[i]))
+			done += uint64(n)
+			if err != nil {
+				return done, err
+			}
+		} else {
+			done += uint64(dst.Len())
+		}
+		dsts = dsts.Tail()
+		i++
+	}
+	return done, rerr
+}
+
+// FromVecWriterFunc implements Writer for a function that writes data from a
+// [][]byte and returns the number of bytes written.
+type FromVecWriterFunc struct {
+	WriteVec func(srcs [][]byte) (int64, error)
+}
+
+// WriteFromBlocks implements Writer.WriteFromBlocks.
+//
+// WriteFromBlocks calls w.WriteVec at most once.
+func (w FromVecWriterFunc) WriteFromBlocks(srcs BlockSeq) (uint64, error) {
+	if srcs.IsEmpty() {
+		return 0, nil
+	}
+	// Ensure that we don't pass a [][]byte with a total length > MaxInt64.
+	srcs = srcs.TakeFirst64(uint64(math.MaxInt64))
+	srcSlices := make([][]byte, 0, srcs.NumBlocks())
+	// Buffer Blocks that require safecopy.
+	var buferr error
+	for tmp := srcs; !tmp.IsEmpty(); tmp = tmp.Tail() {
+		src := tmp.Head()
+		if src.NeedSafecopy() {
+			slice := make([]byte, src.Len())
+			n, err := Copy(BlockFromSafeSlice(slice), src)
+			srcSlices = append(srcSlices, slice[:n])
+			if err != nil {
+				buferr = err
+				break
+			}
+		} else {
+			srcSlices = append(srcSlices, src.ToSlice())
+		}
+	}
+	n, err := w.WriteVec(srcSlices)
+	if err != nil {
+		return uint64(n), err
+	}
+	return uint64(n), buferr
+}
diff --git a/pkg/safemem/io_test.go b/pkg/safemem/io_test.go
new file mode 100644
index 000000000..629741bee
--- /dev/null
+++ b/pkg/safemem/io_test.go
@@ -0,0 +1,199 @@
+// Copyright 2018 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.
+
+package safemem
+
+import (
+	"bytes"
+	"io"
+	"testing"
+)
+
+func makeBlocks(slices ...[]byte) []Block {
+	blocks := make([]Block, 0, len(slices))
+	for _, s := range slices {
+		blocks = append(blocks, BlockFromSafeSlice(s))
+	}
+	return blocks
+}
+
+func TestFromIOReaderFullRead(t *testing.T) {
+	r := FromIOReader{bytes.NewBufferString("foobar")}
+	dsts := makeBlocks(make([]byte, 3), make([]byte, 3))
+	n, err := r.ReadToBlocks(BlockSeqFromSlice(dsts))
+	if wantN := uint64(6); n != wantN || err != nil {
+		t.Errorf("ReadToBlocks: got (%v, %v), wanted (%v, nil)", n, err, wantN)
+	}
+	for i, want := range [][]byte{[]byte("foo"), []byte("bar")} {
+		if got := dsts[i].ToSlice(); !bytes.Equal(got, want) {
+			t.Errorf("dsts[%d]: got %q, wanted %q", i, got, want)
+		}
+	}
+}
+
+type eofHidingReader struct {
+	Reader io.Reader
+}
+
+func (r eofHidingReader) Read(dst []byte) (int, error) {
+	n, err := r.Reader.Read(dst)
+	if err == io.EOF {
+		return n, nil
+	}
+	return n, err
+}
+
+func TestFromIOReaderPartialRead(t *testing.T) {
+	r := FromIOReader{eofHidingReader{bytes.NewBufferString("foob")}}
+	dsts := makeBlocks(make([]byte, 3), make([]byte, 3))
+	n, err := r.ReadToBlocks(BlockSeqFromSlice(dsts))
+	// FromIOReader should stop after the eofHidingReader returns (1, nil)
+	// for a 3-byte read.
+	if wantN := uint64(4); n != wantN || err != nil {
+		t.Errorf("ReadToBlocks: got (%v, %v), wanted (%v, nil)", n, err, wantN)
+	}
+	for i, want := range [][]byte{[]byte("foo"), []byte("b\x00\x00")} {
+		if got := dsts[i].ToSlice(); !bytes.Equal(got, want) {
+			t.Errorf("dsts[%d]: got %q, wanted %q", i, got, want)
+		}
+	}
+}
+
+type singleByteReader struct {
+	Reader io.Reader
+}
+
+func (r singleByteReader) Read(dst []byte) (int, error) {
+	if len(dst) == 0 {
+		return r.Reader.Read(dst)
+	}
+	return r.Reader.Read(dst[:1])
+}
+
+func TestSingleByteReader(t *testing.T) {
+	r := FromIOReader{singleByteReader{bytes.NewBufferString("foobar")}}
+	dsts := makeBlocks(make([]byte, 3), make([]byte, 3))
+	n, err := r.ReadToBlocks(BlockSeqFromSlice(dsts))
+	// FromIOReader should stop after the singleByteReader returns (1, nil)
+	// for a 3-byte read.
+	if wantN := uint64(1); n != wantN || err != nil {
+		t.Errorf("ReadToBlocks: got (%v, %v), wanted (%v, nil)", n, err, wantN)
+	}
+	for i, want := range [][]byte{[]byte("f\x00\x00"), []byte("\x00\x00\x00")} {
+		if got := dsts[i].ToSlice(); !bytes.Equal(got, want) {
+			t.Errorf("dsts[%d]: got %q, wanted %q", i, got, want)
+		}
+	}
+}
+
+func TestReadFullToBlocks(t *testing.T) {
+	r := FromIOReader{singleByteReader{bytes.NewBufferString("foobar")}}
+	dsts := makeBlocks(make([]byte, 3), make([]byte, 3))
+	n, err := ReadFullToBlocks(r, BlockSeqFromSlice(dsts))
+	// ReadFullToBlocks should call into FromIOReader => singleByteReader
+	// repeatedly until dsts is exhausted.
+	if wantN := uint64(6); n != wantN || err != nil {
+		t.Errorf("ReadFullToBlocks: got (%v, %v), wanted (%v, nil)", n, err, wantN)
+	}
+	for i, want := range [][]byte{[]byte("foo"), []byte("bar")} {
+		if got := dsts[i].ToSlice(); !bytes.Equal(got, want) {
+			t.Errorf("dsts[%d]: got %q, wanted %q", i, got, want)
+		}
+	}
+}
+
+func TestFromIOWriterFullWrite(t *testing.T) {
+	srcs := makeBlocks([]byte("foo"), []byte("bar"))
+	var dst bytes.Buffer
+	w := FromIOWriter{&dst}
+	n, err := w.WriteFromBlocks(BlockSeqFromSlice(srcs))
+	if wantN := uint64(6); n != wantN || err != nil {
+		t.Errorf("WriteFromBlocks: got (%v, %v), wanted (%v, nil)", n, err, wantN)
+	}
+	if got, want := dst.Bytes(), []byte("foobar"); !bytes.Equal(got, want) {
+		t.Errorf("dst: got %q, wanted %q", got, want)
+	}
+}
+
+type limitedWriter struct {
+	Writer io.Writer
+	Done   int
+	Limit  int
+}
+
+func (w *limitedWriter) Write(src []byte) (int, error) {
+	count := len(src)
+	if count > (w.Limit - w.Done) {
+		count = w.Limit - w.Done
+	}
+	n, err := w.Writer.Write(src[:count])
+	w.Done += n
+	return n, err
+}
+
+func TestFromIOWriterPartialWrite(t *testing.T) {
+	srcs := makeBlocks([]byte("foo"), []byte("bar"))
+	var dst bytes.Buffer
+	w := FromIOWriter{&limitedWriter{&dst, 0, 4}}
+	n, err := w.WriteFromBlocks(BlockSeqFromSlice(srcs))
+	// FromIOWriter should stop after the limitedWriter returns (1, nil) for a
+	// 3-byte write.
+	if wantN := uint64(4); n != wantN || err != nil {
+		t.Errorf("WriteFromBlocks: got (%v, %v), wanted (%v, nil)", n, err, wantN)
+	}
+	if got, want := dst.Bytes(), []byte("foob"); !bytes.Equal(got, want) {
+		t.Errorf("dst: got %q, wanted %q", got, want)
+	}
+}
+
+type singleByteWriter struct {
+	Writer io.Writer
+}
+
+func (w singleByteWriter) Write(src []byte) (int, error) {
+	if len(src) == 0 {
+		return w.Writer.Write(src)
+	}
+	return w.Writer.Write(src[:1])
+}
+
+func TestSingleByteWriter(t *testing.T) {
+	srcs := makeBlocks([]byte("foo"), []byte("bar"))
+	var dst bytes.Buffer
+	w := FromIOWriter{singleByteWriter{&dst}}
+	n, err := w.WriteFromBlocks(BlockSeqFromSlice(srcs))
+	// FromIOWriter should stop after the singleByteWriter returns (1, nil)
+	// for a 3-byte write.
+	if wantN := uint64(1); n != wantN || err != nil {
+		t.Errorf("WriteFromBlocks: got (%v, %v), wanted (%v, nil)", n, err, wantN)
+	}
+	if got, want := dst.Bytes(), []byte("f"); !bytes.Equal(got, want) {
+		t.Errorf("dst: got %q, wanted %q", got, want)
+	}
+}
+
+func TestWriteFullToBlocks(t *testing.T) {
+	srcs := makeBlocks([]byte("foo"), []byte("bar"))
+	var dst bytes.Buffer
+	w := FromIOWriter{singleByteWriter{&dst}}
+	n, err := WriteFullFromBlocks(w, BlockSeqFromSlice(srcs))
+	// WriteFullToBlocks should call into FromIOWriter => singleByteWriter
+	// repeatedly until srcs is exhausted.
+	if wantN := uint64(6); n != wantN || err != nil {
+		t.Errorf("WriteFullFromBlocks: got (%v, %v), wanted (%v, nil)", n, err, wantN)
+	}
+	if got, want := dst.Bytes(), []byte("foobar"); !bytes.Equal(got, want) {
+		t.Errorf("dst: got %q, wanted %q", got, want)
+	}
+}
diff --git a/pkg/safemem/safemem.go b/pkg/safemem/safemem.go
new file mode 100644
index 000000000..3e70d33a2
--- /dev/null
+++ b/pkg/safemem/safemem.go
@@ -0,0 +1,16 @@
+// Copyright 2018 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.
+
+// Package safemem provides the Block and BlockSeq types.
+package safemem
diff --git a/pkg/safemem/seq_test.go b/pkg/safemem/seq_test.go
new file mode 100644
index 000000000..eba4bb535
--- /dev/null
+++ b/pkg/safemem/seq_test.go
@@ -0,0 +1,196 @@
+// Copyright 2018 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.
+
+package safemem
+
+import (
+	"bytes"
+	"reflect"
+	"testing"
+)
+
+type blockSeqTest struct {
+	desc string
+
+	pieces     []string
+	haveOffset bool
+	offset     uint64
+	haveLimit  bool
+	limit      uint64
+
+	want string
+}
+
+func (t blockSeqTest) NonEmptyByteSlices() [][]byte {
+	// t is a value, so we can mutate it freely.
+	slices := make([][]byte, 0, len(t.pieces))
+	for _, str := range t.pieces {
+		if t.haveOffset {
+			strOff := t.offset
+			if strOff > uint64(len(str)) {
+				strOff = uint64(len(str))
+			}
+			str = str[strOff:]
+			t.offset -= strOff
+		}
+		if t.haveLimit {
+			strLim := t.limit
+			if strLim > uint64(len(str)) {
+				strLim = uint64(len(str))
+			}
+			str = str[:strLim]
+			t.limit -= strLim
+		}
+		if len(str) != 0 {
+			slices = append(slices, []byte(str))
+		}
+	}
+	return slices
+}
+
+func (t blockSeqTest) BlockSeq() BlockSeq {
+	blocks := make([]Block, 0, len(t.pieces))
+	for _, str := range t.pieces {
+		blocks = append(blocks, BlockFromSafeSlice([]byte(str)))
+	}
+	bs := BlockSeqFromSlice(blocks)
+	if t.haveOffset {
+		bs = bs.DropFirst64(t.offset)
+	}
+	if t.haveLimit {
+		bs = bs.TakeFirst64(t.limit)
+	}
+	return bs
+}
+
+var blockSeqTests = []blockSeqTest{
+	{
+		desc: "Empty sequence",
+	},
+	{
+		desc:   "Sequence of length 1",
+		pieces: []string{"foobar"},
+		want:   "foobar",
+	},
+	{
+		desc:   "Sequence of length 2",
+		pieces: []string{"foo", "bar"},
+		want:   "foobar",
+	},
+	{
+		desc:   "Empty Blocks",
+		pieces: []string{"", "foo", "", "", "bar", ""},
+		want:   "foobar",
+	},
+	{
+		desc:       "Sequence with non-zero offset",
+		pieces:     []string{"foo", "bar"},
+		haveOffset: true,
+		offset:     2,
+		want:       "obar",
+	},
+	{
+		desc:      "Sequence with non-maximal limit",
+		pieces:    []string{"foo", "bar"},
+		haveLimit: true,
+		limit:     5,
+		want:      "fooba",
+	},
+	{
+		desc:       "Sequence with offset and limit",
+		pieces:     []string{"foo", "bar"},
+		haveOffset: true,
+		offset:     2,
+		haveLimit:  true,
+		limit:      3,
+		want:       "oba",
+	},
+}
+
+func TestBlockSeqNumBytes(t *testing.T) {
+	for _, test := range blockSeqTests {
+		t.Run(test.desc, func(t *testing.T) {
+			if got, want := test.BlockSeq().NumBytes(), uint64(len(test.want)); got != want {
+				t.Errorf("NumBytes: got %d, wanted %d", got, want)
+			}
+		})
+	}
+}
+
+func TestBlockSeqIterBlocks(t *testing.T) {
+	// Tests BlockSeq iteration using Head/Tail.
+	for _, test := range blockSeqTests {
+		t.Run(test.desc, func(t *testing.T) {
+			srcs := test.BlockSeq()
+			// "Note that a non-nil empty slice and a nil slice ... are not
+			// deeply equal." - reflect
+			slices := make([][]byte, 0, 0)
+			for !srcs.IsEmpty() {
+				src := srcs.Head()
+				slices = append(slices, src.ToSlice())
+				nextSrcs := srcs.Tail()
+				if got, want := nextSrcs.NumBytes(), srcs.NumBytes()-uint64(src.Len()); got != want {
+					t.Fatalf("%v.Tail(): got %v (%d bytes), wanted %d bytes", srcs, nextSrcs, got, want)
+				}
+				srcs = nextSrcs
+			}
+			if wantSlices := test.NonEmptyByteSlices(); !reflect.DeepEqual(slices, wantSlices) {
+				t.Errorf("Accumulated slices: got %v, wanted %v", slices, wantSlices)
+			}
+		})
+	}
+}
+
+func TestBlockSeqIterBytes(t *testing.T) {
+	// Tests BlockSeq iteration using Head/DropFirst.
+	for _, test := range blockSeqTests {
+		t.Run(test.desc, func(t *testing.T) {
+			srcs := test.BlockSeq()
+			var dst bytes.Buffer
+			for !srcs.IsEmpty() {
+				src := srcs.Head()
+				var b [1]byte
+				n, err := Copy(BlockFromSafeSlice(b[:]), src)
+				if n != 1 || err != nil {
+					t.Fatalf("Copy: got (%v, %v), wanted (1, nil)", n, err)
+				}
+				dst.WriteByte(b[0])
+				nextSrcs := srcs.DropFirst(1)
+				if got, want := nextSrcs.NumBytes(), srcs.NumBytes()-1; got != want {
+					t.Fatalf("%v.DropFirst(1): got %v (%d bytes), wanted %d bytes", srcs, nextSrcs, got, want)
+				}
+				srcs = nextSrcs
+			}
+			if got := string(dst.Bytes()); got != test.want {
+				t.Errorf("Copied string: got %q, wanted %q", got, test.want)
+			}
+		})
+	}
+}
+
+func TestBlockSeqDropBeyondLimit(t *testing.T) {
+	blocks := []Block{BlockFromSafeSlice([]byte("123")), BlockFromSafeSlice([]byte("4"))}
+	bs := BlockSeqFromSlice(blocks)
+	if got, want := bs.NumBytes(), uint64(4); got != want {
+		t.Errorf("%v.NumBytes(): got %d, wanted %d", bs, got, want)
+	}
+	bs = bs.TakeFirst(1)
+	if got, want := bs.NumBytes(), uint64(1); got != want {
+		t.Errorf("%v.NumBytes(): got %d, wanted %d", bs, got, want)
+	}
+	bs = bs.DropFirst(2)
+	if got, want := bs.NumBytes(), uint64(0); got != want {
+		t.Errorf("%v.NumBytes(): got %d, wanted %d", bs, got, want)
+	}
+}
diff --git a/pkg/safemem/seq_unsafe.go b/pkg/safemem/seq_unsafe.go
new file mode 100644
index 000000000..354a95dde
--- /dev/null
+++ b/pkg/safemem/seq_unsafe.go
@@ -0,0 +1,299 @@
+// Copyright 2018 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.
+
+package safemem
+
+import (
+	"bytes"
+	"fmt"
+	"reflect"
+	"unsafe"
+)
+
+// A BlockSeq represents a sequence of Blocks, each of which has non-zero
+// length.
+//
+// BlockSeqs are immutable and may be copied by value. The zero value of
+// BlockSeq represents an empty sequence.
+type BlockSeq struct {
+	// If length is 0, then the BlockSeq is empty. Invariants: data == 0;
+	// offset == 0; limit == 0.
+	//
+	// If length is -1, then the BlockSeq represents the single Block{data,
+	// limit, false}. Invariants: offset == 0; limit > 0; limit does not
+	// overflow the range of an int.
+	//
+	// If length is -2, then the BlockSeq represents the single Block{data,
+	// limit, true}. Invariants: offset == 0; limit > 0; limit does not
+	// overflow the range of an int.
+	//
+	// Otherwise, length >= 2, and the BlockSeq represents the `length` Blocks
+	// in the array of Blocks starting at address `data`, starting at `offset`
+	// bytes into the first Block and limited to the following `limit` bytes.
+	// Invariants: data != 0; offset < len(data[0]); limit > 0; offset+limit <=
+	// the combined length of all Blocks in the array; the first Block in the
+	// array has non-zero length.
+	//
+	// length is never 1; sequences consisting of a single Block are always
+	// stored inline (with length < 0).
+	data   unsafe.Pointer
+	length int
+	offset int
+	limit  uint64
+}
+
+// BlockSeqOf returns a BlockSeq representing the single Block b.
+func BlockSeqOf(b Block) BlockSeq {
+	bs := BlockSeq{
+		data:   b.start,
+		length: -1,
+		limit:  uint64(b.length),
+	}
+	if b.needSafecopy {
+		bs.length = -2
+	}
+	return bs
+}
+
+// BlockSeqFromSlice returns a BlockSeq representing all Blocks in slice.
+// If slice contains Blocks with zero length, BlockSeq will skip them during
+// iteration.
+//
+// Whether the returned BlockSeq shares memory with slice is unspecified;
+// clients should avoid mutating slices passed to BlockSeqFromSlice.
+//
+// Preconditions: The combined length of all Blocks in slice <= math.MaxUint64.
+func BlockSeqFromSlice(slice []Block) BlockSeq {
+	slice = skipEmpty(slice)
+	var limit uint64
+	for _, b := range slice {
+		sum := limit + uint64(b.Len())
+		if sum < limit {
+			panic("BlockSeq length overflows uint64")
+		}
+		limit = sum
+	}
+	return blockSeqFromSliceLimited(slice, limit)
+}
+
+// Preconditions: The combined length of all Blocks in slice <= limit. If
+// len(slice) != 0, the first Block in slice has non-zero length, and limit >
+// 0.
+func blockSeqFromSliceLimited(slice []Block, limit uint64) BlockSeq {
+	switch len(slice) {
+	case 0:
+		return BlockSeq{}
+	case 1:
+		return BlockSeqOf(slice[0].TakeFirst64(limit))
+	default:
+		return BlockSeq{
+			data:   unsafe.Pointer(&slice[0]),
+			length: len(slice),
+			limit:  limit,
+		}
+	}
+}
+
+func skipEmpty(slice []Block) []Block {
+	for i, b := range slice {
+		if b.Len() != 0 {
+			return slice[i:]
+		}
+	}
+	return nil
+}
+
+// IsEmpty returns true if bs contains no Blocks.
+//
+// Invariants: bs.IsEmpty() == (bs.NumBlocks() == 0) == (bs.NumBytes() == 0).
+// (Of these, prefer to use bs.IsEmpty().)
+func (bs BlockSeq) IsEmpty() bool {
+	return bs.length == 0
+}
+
+// NumBlocks returns the number of Blocks in bs.
+func (bs BlockSeq) NumBlocks() int {
+	// In general, we have to count: if bs represents a windowed slice then the
+	// slice may contain Blocks with zero length, and bs.length may be larger
+	// than the actual number of Blocks due to bs.limit.
+	var n int
+	for !bs.IsEmpty() {
+		n++
+		bs = bs.Tail()
+	}
+	return n
+}
+
+// NumBytes returns the sum of Block.Len() for all Blocks in bs.
+func (bs BlockSeq) NumBytes() uint64 {
+	return bs.limit
+}
+
+// Head returns the first Block in bs.
+//
+// Preconditions: !bs.IsEmpty().
+func (bs BlockSeq) Head() Block {
+	if bs.length == 0 {
+		panic("empty BlockSeq")
+	}
+	if bs.length < 0 {
+		return bs.internalBlock()
+	}
+	return (*Block)(bs.data).DropFirst(bs.offset).TakeFirst64(bs.limit)
+}
+
+// Preconditions: bs.length < 0.
+func (bs BlockSeq) internalBlock() Block {
+	return Block{
+		start:        bs.data,
+		length:       int(bs.limit),
+		needSafecopy: bs.length == -2,
+	}
+}
+
+// Tail returns a BlockSeq consisting of all Blocks in bs after the first.
+//
+// Preconditions: !bs.IsEmpty().
+func (bs BlockSeq) Tail() BlockSeq {
+	if bs.length == 0 {
+		panic("empty BlockSeq")
+	}
+	if bs.length < 0 {
+		return BlockSeq{}
+	}
+	head := (*Block)(bs.data).DropFirst(bs.offset)
+	headLen := uint64(head.Len())
+	if headLen >= bs.limit {
+		// The head Block exhausts the limit, so the tail is empty.
+		return BlockSeq{}
+	}
+	var extSlice []Block
+	extSliceHdr := (*reflect.SliceHeader)(unsafe.Pointer(&extSlice))
+	extSliceHdr.Data = uintptr(bs.data)
+	extSliceHdr.Len = bs.length
+	extSliceHdr.Cap = bs.length
+	tailSlice := skipEmpty(extSlice[1:])
+	tailLimit := bs.limit - headLen
+	return blockSeqFromSliceLimited(tailSlice, tailLimit)
+}
+
+// DropFirst returns a BlockSeq equivalent to bs, but with the first n bytes
+// omitted. If n > bs.NumBytes(), DropFirst returns an empty BlockSeq.
+//
+// Preconditions: n >= 0.
+func (bs BlockSeq) DropFirst(n int) BlockSeq {
+	if n < 0 {
+		panic(fmt.Sprintf("invalid n: %d", n))
+	}
+	return bs.DropFirst64(uint64(n))
+}
+
+// DropFirst64 is equivalent to DropFirst but takes an uint64.
+func (bs BlockSeq) DropFirst64(n uint64) BlockSeq {
+	if n >= bs.limit {
+		return BlockSeq{}
+	}
+	for {
+		// Calling bs.Head() here is surprisingly expensive, so inline getting
+		// the head's length.
+		var headLen uint64
+		if bs.length < 0 {
+			headLen = bs.limit
+		} else {
+			headLen = uint64((*Block)(bs.data).Len() - bs.offset)
+		}
+		if n < headLen {
+			// Dropping ends partway through the head Block.
+			if bs.length < 0 {
+				return BlockSeqOf(bs.internalBlock().DropFirst64(n))
+			}
+			bs.offset += int(n)
+			bs.limit -= n
+			return bs
+		}
+		n -= headLen
+		bs = bs.Tail()
+	}
+}
+
+// TakeFirst returns a BlockSeq equivalent to the first n bytes of bs. If n >
+// bs.NumBytes(), TakeFirst returns a BlockSeq equivalent to bs.
+//
+// Preconditions: n >= 0.
+func (bs BlockSeq) TakeFirst(n int) BlockSeq {
+	if n < 0 {
+		panic(fmt.Sprintf("invalid n: %d", n))
+	}
+	return bs.TakeFirst64(uint64(n))
+}
+
+// TakeFirst64 is equivalent to TakeFirst but takes a uint64.
+func (bs BlockSeq) TakeFirst64(n uint64) BlockSeq {
+	if n == 0 {
+		return BlockSeq{}
+	}
+	if bs.limit > n {
+		bs.limit = n
+	}
+	return bs
+}
+
+// String implements fmt.Stringer.String.
+func (bs BlockSeq) String() string {
+	var buf bytes.Buffer
+	buf.WriteByte('[')
+	var sep string
+	for !bs.IsEmpty() {
+		buf.WriteString(sep)
+		sep = " "
+		buf.WriteString(bs.Head().String())
+		bs = bs.Tail()
+	}
+	buf.WriteByte(']')
+	return buf.String()
+}
+
+// CopySeq copies srcs.NumBytes() or dsts.NumBytes() bytes, whichever is less,
+// from srcs to dsts and returns the number of bytes copied.
+//
+// If srcs and dsts overlap, the data stored in dsts is unspecified.
+func CopySeq(dsts, srcs BlockSeq) (uint64, error) {
+	var done uint64
+	for !dsts.IsEmpty() && !srcs.IsEmpty() {
+		dst := dsts.Head()
+		src := srcs.Head()
+		n, err := Copy(dst, src)
+		done += uint64(n)
+		if err != nil {
+			return done, err
+		}
+		dsts = dsts.DropFirst(n)
+		srcs = srcs.DropFirst(n)
+	}
+	return done, nil
+}
+
+// ZeroSeq sets all bytes in dsts to 0 and returns the number of bytes zeroed.
+func ZeroSeq(dsts BlockSeq) (uint64, error) {
+	var done uint64
+	for !dsts.IsEmpty() {
+		n, err := Zero(dsts.Head())
+		done += uint64(n)
+		if err != nil {
+			return done, err
+		}
+		dsts = dsts.DropFirst(n)
+	}
+	return done, nil
+}