Add basic plumbing for splice and stub implementation.

This does not actually implement an efficient splice or sendfile. Rather, it
adds a generic plumbing to the file internals so that this can be added. All
file implementations use the stub fileutil.NoSplice implementation, which
causes sendfile and splice to fall back to an internal copy.

A basic splice system call interface is added, along with a test.

PiperOrigin-RevId: 249335960
Change-Id: Ic5568be2af0a505c19e7aec66d5af2480ab0939b

1 files changed, 187 insertions, 0 deletions

diff --git a/pkg/sentry/fs/splice.go b/pkg/sentry/fs/splice.go
new file mode 100644
index 000000000..65937f44d
--- /dev/null
+++ b/pkg/sentry/fs/splice.go
@@ -0,0 +1,187 @@
+// Copyright 2019 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 fs
+
+import (
+	"io"
+	"sync/atomic"
+
+	"gvisor.googlesource.com/gvisor/pkg/secio"
+	"gvisor.googlesource.com/gvisor/pkg/sentry/context"
+	"gvisor.googlesource.com/gvisor/pkg/syserror"
+)
+
+// Splice moves data to this file, directly from another.
+//
+// Offsets are updated only if DstOffset and SrcOffset are set.
+func Splice(ctx context.Context, dst *File, src *File, opts SpliceOpts) (int64, error) {
+	// Verify basic file flag permissions.
+	if !dst.Flags().Write || !src.Flags().Read {
+		return 0, syserror.EBADF
+	}
+
+	// Check whether or not the objects being sliced are stream-oriented
+	// (i.e. pipes or sockets). If yes, we elide checks and offset locks.
+	srcPipe := IsPipe(src.Dirent.Inode.StableAttr) || IsSocket(src.Dirent.Inode.StableAttr)
+	dstPipe := IsPipe(dst.Dirent.Inode.StableAttr) || IsSocket(dst.Dirent.Inode.StableAttr)
+
+	if !dstPipe && !opts.DstOffset && !srcPipe && !opts.SrcOffset {
+		switch {
+		case dst.UniqueID < src.UniqueID:
+			// Acquire dst first.
+			if !dst.mu.Lock(ctx) {
+				return 0, syserror.ErrInterrupted
+			}
+			defer dst.mu.Unlock()
+			if !src.mu.Lock(ctx) {
+				return 0, syserror.ErrInterrupted
+			}
+			defer src.mu.Unlock()
+		case dst.UniqueID > src.UniqueID:
+			// Acquire src first.
+			if !src.mu.Lock(ctx) {
+				return 0, syserror.ErrInterrupted
+			}
+			defer src.mu.Unlock()
+			if !dst.mu.Lock(ctx) {
+				return 0, syserror.ErrInterrupted
+			}
+			defer dst.mu.Unlock()
+		case dst.UniqueID == src.UniqueID:
+			// Acquire only one lock; it's the same file. This is a
+			// bit of a edge case, but presumably it's possible.
+			if !dst.mu.Lock(ctx) {
+				return 0, syserror.ErrInterrupted
+			}
+			defer dst.mu.Unlock()
+		}
+		// Use both offsets (locked).
+		opts.DstStart = dst.offset
+		opts.SrcStart = src.offset
+	} else if !dstPipe && !opts.DstOffset {
+		// Acquire only dst.
+		if !dst.mu.Lock(ctx) {
+			return 0, syserror.ErrInterrupted
+		}
+		defer dst.mu.Unlock()
+		opts.DstStart = dst.offset // Safe: locked.
+	} else if !srcPipe && !opts.SrcOffset {
+		// Acquire only src.
+		if !src.mu.Lock(ctx) {
+			return 0, syserror.ErrInterrupted
+		}
+		defer src.mu.Unlock()
+		opts.SrcStart = src.offset // Safe: locked.
+	}
+
+	// Check append-only mode and the limit.
+	if !dstPipe {
+		if dst.Flags().Append {
+			if opts.DstOffset {
+				// We need to acquire the lock.
+				if !dst.mu.Lock(ctx) {
+					return 0, syserror.ErrInterrupted
+				}
+				defer dst.mu.Unlock()
+			}
+			// Figure out the appropriate offset to use.
+			if err := dst.offsetForAppend(ctx, &opts.DstStart); err != nil {
+				return 0, err
+			}
+		}
+
+		// Enforce file limits.
+		limit, ok := dst.checkLimit(ctx, opts.DstStart)
+		switch {
+		case ok && limit == 0:
+			return 0, syserror.ErrExceedsFileSizeLimit
+		case ok && limit < opts.Length:
+			opts.Length = limit // Cap the write.
+		}
+	}
+
+	// Attempt to do a WriteTo; this is likely the most efficient.
+	//
+	// The underlying implementation may be able to donate buffers.
+	newOpts := SpliceOpts{
+		Length:    opts.Length,
+		SrcStart:  opts.SrcStart,
+		SrcOffset: !srcPipe,
+		Dup:       opts.Dup,
+		DstStart:  opts.DstStart,
+		DstOffset: !dstPipe,
+	}
+	n, err := src.FileOperations.WriteTo(ctx, src, dst, newOpts)
+	if n == 0 && err != nil {
+		// Attempt as a ReadFrom. If a WriteTo, a ReadFrom may also
+		// be more efficient than a copy if buffers are cached or readily
+		// available. (It's unlikely that they can actually be donate
+		n, err = dst.FileOperations.ReadFrom(ctx, dst, src, newOpts)
+	}
+	if n == 0 && err != nil {
+		// If we've failed up to here, and at least one of the sources
+		// is a pipe or socket, then we can't properly support dup.
+		// Return an error indicating that this operation is not
+		// supported.
+		if (srcPipe || dstPipe) && newOpts.Dup {
+			return 0, syserror.EINVAL
+		}
+
+		// We failed to splice the files. But that's fine; we just fall
+		// back to a slow path in this case. This copies without doing
+		// any mode changes, so should still be more efficient.
+		var (
+			r io.Reader
+			w io.Writer
+		)
+		fw := &lockedWriter{
+			Ctx:  ctx,
+			File: dst,
+		}
+		if newOpts.DstOffset {
+			// Use the provided offset.
+			w = secio.NewOffsetWriter(fw, newOpts.DstStart)
+		} else {
+			// Writes will proceed with no offset.
+			w = fw
+		}
+		fr := &lockedReader{
+			Ctx:  ctx,
+			File: src,
+		}
+		if newOpts.SrcOffset {
+			// Limit to the given offset and length.
+			r = io.NewSectionReader(fr, opts.SrcStart, opts.Length)
+		} else {
+			// Limit just to the given length.
+			r = &io.LimitedReader{fr, opts.Length}
+		}
+
+		// Copy between the two.
+		n, err = io.Copy(w, r)
+	}
+
+	// Update offsets, if required.
+	if n > 0 {
+		if !dstPipe && !opts.DstOffset {
+			atomic.StoreInt64(&dst.offset, dst.offset+n)
+		}
+		if !srcPipe && !opts.SrcOffset {
+			atomic.StoreInt64(&src.offset, src.offset+n)
+		}
+	}
+
+	return n, err
+}