Port vfs1 implementation of sync_file_range to vfs2.

Currently, we always perform a full-file sync which could be extremely
expensive for some applications. Although vfs1 did not fully support
sync_file_range, there were some optimizations that allowed us skip some
unnecessary write-outs.

Updates #2923, #1897.

PiperOrigin-RevId: 319324213

3 files changed, 37 insertions, 11 deletions

diff --git a/pkg/sentry/fsimpl/gofer/special_file.go b/pkg/sentry/fsimpl/gofer/special_file.go
index a016cbae1..2b381af05 100644
--- a/pkg/sentry/fsimpl/gofer/special_file.go
+++ b/pkg/sentry/fsimpl/gofer/special_file.go
@@ -235,8 +235,5 @@ func (fd *specialFileFD) Seek(ctx context.Context, offset int64, whence int32) (
 
 // Sync implements vfs.FileDescriptionImpl.Sync.
 func (fd *specialFileFD) Sync(ctx context.Context) error {
-	if !fd.vfsfd.IsWritable() {
-		return nil
-	}
-	return fd.handle.sync(ctx)
+	return fd.dentry().syncSharedHandle(ctx)
 }
diff --git a/pkg/sentry/syscalls/linux/vfs2/sync.go b/pkg/sentry/syscalls/linux/vfs2/sync.go
index 365250b0b..0d0ebf46a 100644
--- a/pkg/sentry/syscalls/linux/vfs2/sync.go
+++ b/pkg/sentry/syscalls/linux/vfs2/sync.go
@@ -65,10 +65,8 @@ func SyncFileRange(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel
 	nbytes := args[2].Int64()
 	flags := args[3].Uint()
 
-	if offset < 0 {
-		return 0, nil, syserror.EINVAL
-	}
-	if nbytes < 0 {
+	// Check for negative values and overflow.
+	if offset < 0 || offset+nbytes < 0 {
 		return 0, nil, syserror.EINVAL
 	}
 	if flags&^(linux.SYNC_FILE_RANGE_WAIT_BEFORE|linux.SYNC_FILE_RANGE_WRITE|linux.SYNC_FILE_RANGE_WAIT_AFTER) != 0 {
@@ -81,7 +79,37 @@ func SyncFileRange(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel
 	}
 	defer file.DecRef()
 
-	// TODO(gvisor.dev/issue/1897): Avoid writeback of data ranges outside of
-	// [offset, offset+nbytes).
-	return 0, nil, file.Sync(t)
+	// TODO(gvisor.dev/issue/1897): Currently, the only file syncing we support
+	// is a full-file sync, i.e. fsync(2). As a result, there are severe
+	// limitations on how much we support sync_file_range:
+	// - In Linux, sync_file_range(2) doesn't write out the file's metadata, even
+	//   if the file size is changed. We do.
+	// - We always sync the entire file instead of [offset, offset+nbytes).
+	// - We do not support the use of WAIT_BEFORE without WAIT_AFTER. For
+	//   correctness, we would have to perform a write-out every time WAIT_BEFORE
+	//   was used, but this would be much more expensive than expected if there
+	//   were no write-out operations in progress.
+	// - Whenever WAIT_AFTER is used, we sync the file.
+	// - Ignore WRITE. If this flag is used with WAIT_AFTER, then the file will
+	//   be synced anyway. If this flag is used without WAIT_AFTER, then it is
+	//   safe (and less expensive) to do nothing, because the syscall will not
+	//   wait for the write-out to complete--we only need to make sure that the
+	//   next time WAIT_BEFORE or WAIT_AFTER are used, the write-out completes.
+	// - According to fs/sync.c, WAIT_BEFORE|WAIT_AFTER "will detect any I/O
+	//   errors or ENOSPC conditions and will return those to the caller, after
+	//   clearing the EIO and ENOSPC flags in the address_space." We don't do
+	//   this.
+
+	if flags&linux.SYNC_FILE_RANGE_WAIT_BEFORE != 0 &&
+		flags&linux.SYNC_FILE_RANGE_WAIT_AFTER == 0 {
+		t.Kernel().EmitUnimplementedEvent(t)
+		return 0, nil, syserror.ENOSYS
+	}
+
+	if flags&linux.SYNC_FILE_RANGE_WAIT_AFTER != 0 {
+		if err := file.Sync(t); err != nil {
+			return 0, nil, syserror.ConvertIntr(err, kernel.ERESTARTSYS)
+		}
+	}
+	return 0, nil, nil
 }
diff --git a/test/syscalls/BUILD b/test/syscalls/BUILD
index 67645cc83..790190273 100644
--- a/test/syscalls/BUILD
+++ b/test/syscalls/BUILD
@@ -962,6 +962,7 @@ syscall_test(
 syscall_test(
     add_overlay = True,
     test = "//test/syscalls/linux:sync_file_range_test",
+    vfs2 = "True",
 )
 
 syscall_test(