gvisor/fs: don't update file.offset for sockets, pipes, etc

sockets, pipes and other non-seekable file descriptors don't
use file.offset, so we don't need to update it.

With this change, we will be able to call file operations
without locking the file.mu mutex. This is already used for
pipes in the splice system call.

PiperOrigin-RevId: 253746644

3 files changed, 6 insertions, 2 deletions

diff --git a/pkg/sentry/fs/dev/null.go b/pkg/sentry/fs/dev/null.go
index 18dd08515..4404b97ef 100644
--- a/pkg/sentry/fs/dev/null.go
+++ b/pkg/sentry/fs/dev/null.go
@@ -97,6 +97,7 @@ func newZeroDevice(ctx context.Context, owner fs.FileOwner, mode linux.FileMode)
 func (zd *zeroDevice) GetFile(ctx context.Context, dirent *fs.Dirent, flags fs.FileFlags) (*fs.File, error) {
 	flags.Pread = true
 	flags.Pwrite = true
+	flags.NonSeekable = true
 
 	return fs.NewFile(ctx, dirent, flags, &zeroFileOperations{}), nil
 }
diff --git a/pkg/sentry/fs/file.go b/pkg/sentry/fs/file.go
index 6df7d7b4f..55ffe6c0c 100644
--- a/pkg/sentry/fs/file.go
+++ b/pkg/sentry/fs/file.go
@@ -267,7 +267,7 @@ func (f *File) Readv(ctx context.Context, dst usermem.IOSequence) (int64, error)
 
 	reads.Increment()
 	n, err := f.FileOperations.Read(ctx, f, dst, f.offset)
-	if n > 0 {
+	if n > 0 && !f.flags.NonSeekable {
 		atomic.AddInt64(&f.offset, n)
 	}
 	f.mu.Unlock()
@@ -330,7 +330,7 @@ func (f *File) Writev(ctx context.Context, src usermem.IOSequence) (int64, error
 
 	// We must hold the lock during the write.
 	n, err := f.FileOperations.Write(ctx, f, src, f.offset)
-	if n >= 0 {
+	if n >= 0 && !f.flags.NonSeekable {
 		atomic.StoreInt64(&f.offset, f.offset+n)
 	}
 	f.mu.Unlock()
diff --git a/pkg/sentry/fs/flags.go b/pkg/sentry/fs/flags.go
index f31bfa0db..1278f9c78 100644
--- a/pkg/sentry/fs/flags.go
+++ b/pkg/sentry/fs/flags.go
@@ -57,6 +57,9 @@ type FileFlags struct {
 	// Linux sets this flag for all files. Since gVisor is only compatible
 	// with 64-bit Linux, it also sets this flag for all files.
 	LargeFile bool
+
+	// NonSeekable indicates that file.offset isn't used.
+	NonSeekable bool
 }
 
 // SettableFileFlags is a subset of FileFlags above that can be changed