fusefrontend: sharedstorage: use byte-range lock on file header creation

Multiple host writing to the same empty file at the same time
could have overwritten each other's newly created file header,
leading to data corruption.

Fix the race by placing a byte-range lock on the file when
creating the file header.

1 files changed, 29 insertions, 0 deletions

diff --git a/internal/fusefrontend/file_lock.go b/internal/fusefrontend/file_lock.go
new file mode 100644
index 0000000..6f92cfe
--- /dev/null
+++ b/internal/fusefrontend/file_lock.go
@@ -0,0 +1,29 @@
+package fusefrontend
+
+import (
+	"golang.org/x/sys/unix"
+
+	"github.com/rfjakob/gocryptfs/v2/internal/syscallcompat"
+)
+
+// SharedStorageLock conveniently wraps F_OFD_SETLKW
+// See https://man7.org/linux/man-pages/man2/fcntl.2.html -> "Open file description locks (non-POSIX)"
+//
+// lkType is one of:
+// * unix.F_RDLCK (shared read lock)
+// * unix.F_WRLCK (exclusive write lock)
+// * unix.F_UNLCK (unlock)
+//
+// This function is a no-op if args.SharedStorage == false.
+func (f *File) LockSharedStorage(lkType int16, lkStart int64, lkLen int64) error {
+	if !f.rootNode.args.SharedStorage {
+		return nil
+	}
+	lk := unix.Flock_t{
+		Type:   lkType,
+		Whence: unix.SEEK_SET,
+		Start:  lkStart,
+		Len:    lkLen,
+	}
+	return unix.FcntlFlock(uintptr(f.intFd()), syscallcompat.F_OFD_SETLKW, &lk)
+}