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// 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.dev/gvisor/pkg/context"
"gvisor.dev/gvisor/pkg/errors/linuxerr"
)
// 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, linuxerr.EBADF
}
// Check whether or not the objects being sliced are stream-oriented
// (i.e. pipes or sockets). For all stream-oriented files and files
// where a specific offiset is not request, we acquire the file mutex.
// This has two important side effects. First, it provides the standard
// protection against concurrent writes that would mutate the offset.
// Second, it prevents Splice deadlocks. Only internal anonymous files
// implement the ReadFrom and WriteTo methods directly, and since such
// anonymous files are referred to by a unique fs.File object, we know
// that the file mutex takes strict precedence over internal locks.
// Since we enforce lock ordering here, we can't deadlock by using
// using a file in two different splice operations simultaneously.
srcPipe := !IsRegular(src.Dirent.Inode.StableAttr)
dstPipe := !IsRegular(dst.Dirent.Inode.StableAttr)
dstAppend := !dstPipe && dst.Flags().Append
srcLock := srcPipe || !opts.SrcOffset
dstLock := dstPipe || !opts.DstOffset || dstAppend
switch {
case srcLock && dstLock:
switch {
case dst.UniqueID < src.UniqueID:
// Acquire dst first.
if !dst.mu.Lock(ctx) {
return 0, linuxerr.ErrInterrupted
}
if !src.mu.Lock(ctx) {
dst.mu.Unlock()
return 0, linuxerr.ErrInterrupted
}
case dst.UniqueID > src.UniqueID:
// Acquire src first.
if !src.mu.Lock(ctx) {
return 0, linuxerr.ErrInterrupted
}
if !dst.mu.Lock(ctx) {
src.mu.Unlock()
return 0, linuxerr.ErrInterrupted
}
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, linuxerr.ErrInterrupted
}
srcLock = false // Only need one unlock.
}
// Use both offsets (locked).
opts.DstStart = dst.offset
opts.SrcStart = src.offset
case dstLock:
// Acquire only dst.
if !dst.mu.Lock(ctx) {
return 0, linuxerr.ErrInterrupted
}
opts.DstStart = dst.offset // Safe: locked.
case srcLock:
// Acquire only src.
if !src.mu.Lock(ctx) {
return 0, linuxerr.ErrInterrupted
}
opts.SrcStart = src.offset // Safe: locked.
}
var err error
if dstAppend {
unlock := dst.Dirent.Inode.lockAppendMu(dst.Flags().Append)
defer unlock()
// Figure out the appropriate offset to use.
err = dst.offsetForAppend(ctx, &opts.DstStart)
}
if err == nil && !dstPipe {
// Enforce file limits.
limit, ok := dst.checkLimit(ctx, opts.DstStart)
switch {
case ok && limit == 0:
err = linuxerr.ErrExceedsFileSizeLimit
case ok && limit < opts.Length:
opts.Length = limit // Cap the write.
}
}
if err != nil {
if dstLock {
dst.mu.Unlock()
}
if srcLock {
src.mu.Unlock()
}
return 0, err
}
// Construct readers and writers for the splice. This is used to
// provide a safer locking path for the WriteTo/ReadFrom operations
// (since they will otherwise go through public interface methods which
// conflict with locking done above), and simplifies the fallback path.
w := &lockedWriter{
Ctx: ctx,
File: dst,
Offset: opts.DstStart,
}
r := &lockedReader{
Ctx: ctx,
File: src,
Offset: opts.SrcStart,
}
// Attempt to do a WriteTo; this is likely the most efficient.
n, err := src.FileOperations.WriteTo(ctx, src, w, opts.Length, opts.Dup)
if n == 0 && linuxerr.Equals(linuxerr.ENOSYS, err) && !opts.Dup {
// 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 donated).
n, err = dst.FileOperations.ReadFrom(ctx, dst, r, opts.Length)
}
// Support one last fallback option, but only if at least one of
// the source and destination are regular files. This is because
// if we block at some point, we could lose data. If the source is
// not a pipe then reading is not destructive; if the destination
// is a regular file, then it is guaranteed not to block writing.
if n == 0 && linuxerr.Equals(linuxerr.ENOSYS, err) && !opts.Dup && (!dstPipe || !srcPipe) {
// Fallback to an in-kernel copy.
n, err = io.Copy(w, &io.LimitedReader{
R: r,
N: opts.Length,
})
}
// 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)
}
}
// Drop locks.
if dstLock {
dst.mu.Unlock()
}
if srcLock {
src.mu.Unlock()
}
return n, err
}
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