Add inode number to synthetic dentries

Reserve the MSB from ino for synthetic dentries to prevent
conflict with regular dentries. Log warning in case MSB is
set for regular dentries.

Updates #1487

PiperOrigin-RevId: 319869858

1 files changed, 27 insertions, 7 deletions

diff --git a/pkg/sentry/fsimpl/gofer/gofer.go b/pkg/sentry/fsimpl/gofer/gofer.go
index 8e74e60a5..2b83094cd 100644
--- a/pkg/sentry/fsimpl/gofer/gofer.go
+++ b/pkg/sentry/fsimpl/gofer/gofer.go
@@ -110,6 +110,26 @@ type filesystem struct {
 	syncMu           sync.Mutex
 	syncableDentries map[*dentry]struct{}
 	specialFileFDs   map[*specialFileFD]struct{}
+
+	// syntheticSeq stores a counter to used to generate unique inodeNumber for
+	// synthetic dentries.
+	syntheticSeq uint64
+}
+
+// inodeNumber represents inode number reported in Dirent.Ino. For regular
+// dentries, it comes from QID.Path from the 9P server. Synthetic dentries
+// have have their inodeNumber generated sequentially, with the MSB reserved to
+// prevent conflicts with regular dentries.
+type inodeNumber uint64
+
+// Reserve MSB for synthetic mounts.
+const syntheticInoMask = uint64(1) << 63
+
+func inoFromPath(path uint64) inodeNumber {
+	if path&syntheticInoMask != 0 {
+		log.Warningf("Dropping MSB from ino, collision is possible. Original: %d, new: %d", path, path&^syntheticInoMask)
+	}
+	return inodeNumber(path &^ syntheticInoMask)
 }
 
 type filesystemOptions struct {
@@ -585,11 +605,11 @@ type dentry struct {
 	// Cached metadata; protected by metadataMu and accessed using atomic
 	// memory operations unless otherwise specified.
 	metadataMu sync.Mutex
-	ino        uint64 // immutable
-	mode       uint32 // type is immutable, perms are mutable
-	uid        uint32 // auth.KUID, but stored as raw uint32 for sync/atomic
-	gid        uint32 // auth.KGID, but ...
-	blockSize  uint32 // 0 if unknown
+	ino        inodeNumber // immutable
+	mode       uint32      // type is immutable, perms are mutable
+	uid        uint32      // auth.KUID, but stored as raw uint32 for sync/atomic
+	gid        uint32      // auth.KGID, but ...
+	blockSize  uint32      // 0 if unknown
 	// Timestamps, all nsecs from the Unix epoch.
 	atime int64
 	mtime int64
@@ -704,7 +724,7 @@ func (fs *filesystem) newDentry(ctx context.Context, file p9file, qid p9.QID, ma
 	d := &dentry{
 		fs:        fs,
 		file:      file,
-		ino:       qid.Path,
+		ino:       inoFromPath(qid.Path),
 		mode:      uint32(attr.Mode),
 		uid:       uint32(fs.opts.dfltuid),
 		gid:       uint32(fs.opts.dfltgid),
@@ -846,7 +866,7 @@ func (d *dentry) statTo(stat *linux.Statx) {
 	stat.UID = atomic.LoadUint32(&d.uid)
 	stat.GID = atomic.LoadUint32(&d.gid)
 	stat.Mode = uint16(atomic.LoadUint32(&d.mode))
-	stat.Ino = d.ino
+	stat.Ino = uint64(d.ino)
 	stat.Size = atomic.LoadUint64(&d.size)
 	// This is consistent with regularFileFD.Seek(), which treats regular files
 	// as having no holes.