path: root/pkg/sentry/kernel/pipe/pipe.go

// 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 pipe provides a pipe implementation.
package pipe

import (
	"fmt"
	"io"
	"sync/atomic"
	"syscall"

	"gvisor.dev/gvisor/pkg/buffer"
	"gvisor.dev/gvisor/pkg/context"
	"gvisor.dev/gvisor/pkg/sentry/fs"
	"gvisor.dev/gvisor/pkg/sync"
	"gvisor.dev/gvisor/pkg/syserror"
	"gvisor.dev/gvisor/pkg/usermem"
	"gvisor.dev/gvisor/pkg/waiter"
)

const (
	// MinimumPipeSize is a hard limit of the minimum size of a pipe.
	// It corresponds to fs/pipe.c:pipe_min_size.
	MinimumPipeSize = usermem.PageSize

	// MaximumPipeSize is a hard limit on the maximum size of a pipe.
	// It corresponds to fs/pipe.c:pipe_max_size.
	MaximumPipeSize = 1048576

	// DefaultPipeSize is the system-wide default size of a pipe in bytes.
	// It corresponds to pipe_fs_i.h:PIPE_DEF_BUFFERS.
	DefaultPipeSize = 16 * usermem.PageSize

	// atomicIOBytes is the maximum number of bytes that the pipe will
	// guarantee atomic reads or writes atomically.
	// It corresponds to limits.h:PIPE_BUF.
	atomicIOBytes = 4096
)

// Pipe is an encapsulation of a platform-independent pipe.
// It manages a buffered byte queue shared between a reader/writer
// pair.
//
// +stateify savable
type Pipe struct {
	waiter.Queue `state:"nosave"`

	// isNamed indicates whether this is a named pipe.
	//
	// This value is immutable.
	isNamed bool

	// The number of active readers for this pipe.
	//
	// Access atomically.
	readers int32

	// The number of active writes for this pipe.
	//
	// Access atomically.
	writers int32

	// mu protects all pipe internal state below.
	mu sync.Mutex `state:"nosave"`

	// view is the underlying set of buffers.
	//
	// This is protected by mu.
	view buffer.View

	// max is the maximum size of the pipe in bytes. When this max has been
	// reached, writers will get EWOULDBLOCK.
	//
	// This is protected by mu.
	max int64

	// hadWriter indicates if this pipe ever had a writer. Note that this
	// does not necessarily indicate there is *currently* a writer, just
	// that there has been a writer at some point since the pipe was
	// created.
	//
	// This is protected by mu.
	hadWriter bool
}

// NewPipe initializes and returns a pipe.
//
// N.B. The size will be bounded.
func NewPipe(isNamed bool, sizeBytes int64) *Pipe {
	if sizeBytes < MinimumPipeSize {
		sizeBytes = MinimumPipeSize
	}
	if sizeBytes > MaximumPipeSize {
		sizeBytes = MaximumPipeSize
	}
	var p Pipe
	initPipe(&p, isNamed, sizeBytes)
	return &p
}

func initPipe(pipe *Pipe, isNamed bool, sizeBytes int64) {
	if sizeBytes < MinimumPipeSize {
		sizeBytes = MinimumPipeSize
	}
	if sizeBytes > MaximumPipeSize {
		sizeBytes = MaximumPipeSize
	}
	pipe.isNamed = isNamed
	pipe.max = sizeBytes
}

// NewConnectedPipe initializes a pipe and returns a pair of objects
// representing the read and write ends of the pipe.
func NewConnectedPipe(ctx context.Context, sizeBytes int64) (*fs.File, *fs.File) {
	p := NewPipe(false /* isNamed */, sizeBytes)

	// Build an fs.Dirent for the pipe which will be shared by both
	// returned files.
	perms := fs.FilePermissions{
		User: fs.PermMask{Read: true, Write: true},
	}
	iops := NewInodeOperations(ctx, perms, p)
	ino := pipeDevice.NextIno()
	sattr := fs.StableAttr{
		Type:      fs.Pipe,
		DeviceID:  pipeDevice.DeviceID(),
		InodeID:   ino,
		BlockSize: int64(atomicIOBytes),
	}
	ms := fs.NewPseudoMountSource(ctx)
	d := fs.NewDirent(ctx, fs.NewInode(ctx, iops, ms, sattr), fmt.Sprintf("pipe:[%d]", ino))
	// The p.Open calls below will each take a reference on the Dirent. We
	// must drop the one we already have.
	defer d.DecRef(ctx)
	return p.Open(ctx, d, fs.FileFlags{Read: true}), p.Open(ctx, d, fs.FileFlags{Write: true})
}

// Open opens the pipe and returns a new file.
//
// Precondition: at least one of flags.Read or flags.Write must be set.
func (p *Pipe) Open(ctx context.Context, d *fs.Dirent, flags fs.FileFlags) *fs.File {
	flags.NonSeekable = true
	switch {
	case flags.Read && flags.Write:
		p.rOpen()
		p.wOpen()
		return fs.NewFile(ctx, d, flags, &ReaderWriter{
			Pipe: p,
		})
	case flags.Read:
		p.rOpen()
		return fs.NewFile(ctx, d, flags, &Reader{
			ReaderWriter: ReaderWriter{Pipe: p},
		})
	case flags.Write:
		p.wOpen()
		return fs.NewFile(ctx, d, flags, &Writer{
			ReaderWriter: ReaderWriter{Pipe: p},
		})
	default:
		// Precondition violated.
		panic("invalid pipe flags")
	}
}

type readOps struct {
	// left returns the bytes remaining.
	left func() int64

	// limit limits subsequence reads.
	limit func(int64)

	// read performs the actual read operation.
	read func(*buffer.View) (int64, error)
}

// read reads data from the pipe into dst and returns the number of bytes
// read, or returns ErrWouldBlock if the pipe is empty.
//
// Precondition: this pipe must have readers.
func (p *Pipe) read(ctx context.Context, ops readOps) (int64, error) {
	p.mu.Lock()
	defer p.mu.Unlock()
	return p.readLocked(ctx, ops)
}

func (p *Pipe) readLocked(ctx context.Context, ops readOps) (int64, error) {
	// Don't block for a zero-length read even if the pipe is empty.
	if ops.left() == 0 {
		return 0, nil
	}

	// Is the pipe empty?
	if p.view.Size() == 0 {
		if !p.HasWriters() {
			// There are no writers, return EOF.
			return 0, io.EOF
		}
		return 0, syserror.ErrWouldBlock
	}

	// Limit how much we consume.
	if ops.left() > p.view.Size() {
		ops.limit(p.view.Size())
	}

	// Copy user data; the read op is responsible for trimming.
	done, err := ops.read(&p.view)
	return done, err
}

type writeOps struct {
	// left returns the bytes remaining.
	left func() int64

	// limit should limit subsequent writes.
	limit func(int64)

	// write should write to the provided buffer.
	write func(*buffer.View) (int64, error)
}

// write writes data from sv into the pipe and returns the number of bytes
// written. If no bytes are written because the pipe is full (or has less than
// atomicIOBytes free capacity), write returns ErrWouldBlock.
//
// Precondition: this pipe must have writers.
func (p *Pipe) write(ctx context.Context, ops writeOps) (int64, error) {
	p.mu.Lock()
	defer p.mu.Unlock()
	return p.writeLocked(ctx, ops)
}

func (p *Pipe) writeLocked(ctx context.Context, ops writeOps) (int64, error) {
	// Can't write to a pipe with no readers.
	if !p.HasReaders() {
		return 0, syscall.EPIPE
	}

	// POSIX requires that a write smaller than atomicIOBytes (PIPE_BUF) be
	// atomic, but requires no atomicity for writes larger than this.
	wanted := ops.left()
	avail := p.max - p.view.Size()
	if wanted > avail {
		if wanted <= atomicIOBytes {
			return 0, syserror.ErrWouldBlock
		}
		ops.limit(avail)
	}

	// Copy user data.
	done, err := ops.write(&p.view)
	if err != nil {
		return done, err
	}

	if done < avail {
		// Non-failure, but short write.
		return done, nil
	}
	if done < wanted {
		// Partial write due to full pipe. Note that this could also be
		// the short write case above, we would expect a second call
		// and the write to return zero bytes in this case.
		return done, syserror.ErrWouldBlock
	}

	return done, nil
}

// rOpen signals a new reader of the pipe.
func (p *Pipe) rOpen() {
	atomic.AddInt32(&p.readers, 1)
}

// wOpen signals a new writer of the pipe.
func (p *Pipe) wOpen() {
	p.mu.Lock()
	defer p.mu.Unlock()
	p.hadWriter = true
	atomic.AddInt32(&p.writers, 1)
}

// rClose signals that a reader has closed their end of the pipe.
func (p *Pipe) rClose() {
	newReaders := atomic.AddInt32(&p.readers, -1)
	if newReaders < 0 {
		panic(fmt.Sprintf("Refcounting bug, pipe has negative readers: %v", newReaders))
	}
}

// wClose signals that a writer has closed their end of the pipe.
func (p *Pipe) wClose() {
	newWriters := atomic.AddInt32(&p.writers, -1)
	if newWriters < 0 {
		panic(fmt.Sprintf("Refcounting bug, pipe has negative writers: %v.", newWriters))
	}
}

// HasReaders returns whether the pipe has any active readers.
func (p *Pipe) HasReaders() bool {
	return atomic.LoadInt32(&p.readers) > 0
}

// HasWriters returns whether the pipe has any active writers.
func (p *Pipe) HasWriters() bool {
	return atomic.LoadInt32(&p.writers) > 0
}

// rReadinessLocked calculates the read readiness.
//
// Precondition: mu must be held.
func (p *Pipe) rReadinessLocked() waiter.EventMask {
	ready := waiter.EventMask(0)
	if p.HasReaders() && p.view.Size() != 0 {
		ready |= waiter.EventIn
	}
	if !p.HasWriters() && p.hadWriter {
		// POLLHUP must be suppressed until the pipe has had at least one writer
		// at some point. Otherwise a reader thread may poll and immediately get
		// a POLLHUP before the writer ever opens the pipe, which the reader may
		// interpret as the writer opening then closing the pipe.
		ready |= waiter.EventHUp
	}
	return ready
}

// rReadiness returns a mask that states whether the read end of the pipe is
// ready for reading.
func (p *Pipe) rReadiness() waiter.EventMask {
	p.mu.Lock()
	defer p.mu.Unlock()
	return p.rReadinessLocked()
}

// wReadinessLocked calculates the write readiness.
//
// Precondition: mu must be held.
func (p *Pipe) wReadinessLocked() waiter.EventMask {
	ready := waiter.EventMask(0)
	if p.HasWriters() && p.view.Size() < p.max {
		ready |= waiter.EventOut
	}
	if !p.HasReaders() {
		ready |= waiter.EventErr
	}
	return ready
}

// wReadiness returns a mask that states whether the write end of the pipe
// is ready for writing.
func (p *Pipe) wReadiness() waiter.EventMask {
	p.mu.Lock()
	defer p.mu.Unlock()
	return p.wReadinessLocked()
}

// rwReadiness returns a mask that states whether a read-write handle to the
// pipe is ready for IO.
func (p *Pipe) rwReadiness() waiter.EventMask {
	p.mu.Lock()
	defer p.mu.Unlock()
	return p.rReadinessLocked() | p.wReadinessLocked()
}

// queued returns the amount of queued data.
func (p *Pipe) queued() int64 {
	p.mu.Lock()
	defer p.mu.Unlock()
	return p.queuedLocked()
}

func (p *Pipe) queuedLocked() int64 {
	return p.view.Size()
}

// FifoSize implements fs.FifoSizer.FifoSize.
func (p *Pipe) FifoSize(context.Context, *fs.File) (int64, error) {
	p.mu.Lock()
	defer p.mu.Unlock()
	return p.max, nil
}

// SetFifoSize implements fs.FifoSizer.SetFifoSize.
func (p *Pipe) SetFifoSize(size int64) (int64, error) {
	if size < 0 {
		return 0, syserror.EINVAL
	}
	if size < MinimumPipeSize {
		size = MinimumPipeSize // Per spec.
	}
	if size > MaximumPipeSize {
		return 0, syserror.EPERM
	}
	p.mu.Lock()
	defer p.mu.Unlock()
	if size < p.view.Size() {
		return 0, syserror.EBUSY
	}
	p.max = size
	return size, nil
}