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
+}