1 files changed, 140 insertions, 0 deletions
diff --git a/pkg/sentry/vfs/filesystem_refs.go b/pkg/sentry/vfs/filesystem_refs.go
new file mode 100644
index 000000000..fc47919d0
--- /dev/null
+++ b/pkg/sentry/vfs/filesystem_refs.go
@@ -0,0 +1,140 @@
+package vfs
+
+import (
+	"fmt"
+	"sync/atomic"
+
+	"gvisor.dev/gvisor/pkg/refsvfs2"
+)
+
+// enableLogging indicates whether reference-related events should be logged (with
+// stack traces). This is false by default and should only be set to true for
+// debugging purposes, as it can generate an extremely large amount of output
+// and drastically degrade performance.
+const FilesystemenableLogging = false
+
+// obj is used to customize logging. Note that we use a pointer to T so that
+// we do not copy the entire object when passed as a format parameter.
+var Filesystemobj *Filesystem
+
+// Refs implements refs.RefCounter. It keeps a reference count using atomic
+// operations and calls the destructor when the count reaches zero.
+//
+// NOTE: Do not introduce additional fields to the Refs struct. It is used by
+// many filesystem objects, and we want to keep it as small as possible (i.e.,
+// the same size as using an int64 directly) to avoid taking up extra cache
+// space. In general, this template should not be extended at the cost of
+// performance. If it does not offer enough flexibility for a particular object
+// (example: b/187877947), we should implement the RefCounter/CheckedObject
+// interfaces manually.
+//
+// +stateify savable
+type FilesystemRefs struct {
+	// refCount is composed of two fields:
+	//
+	//	[32-bit speculative references]:[32-bit real references]
+	//
+	// Speculative references are used for TryIncRef, to avoid a CompareAndSwap
+	// loop. See IncRef, DecRef and TryIncRef for details of how these fields are
+	// used.
+	refCount int64
+}
+
+// InitRefs initializes r with one reference and, if enabled, activates leak
+// checking.
+func (r *FilesystemRefs) InitRefs() {
+	atomic.StoreInt64(&r.refCount, 1)
+	refsvfs2.Register(r)
+}
+
+// RefType implements refsvfs2.CheckedObject.RefType.
+func (r *FilesystemRefs) RefType() string {
+	return fmt.Sprintf("%T", Filesystemobj)[1:]
+}
+
+// LeakMessage implements refsvfs2.CheckedObject.LeakMessage.
+func (r *FilesystemRefs) LeakMessage() string {
+	return fmt.Sprintf("[%s %p] reference count of %d instead of 0", r.RefType(), r, r.ReadRefs())
+}
+
+// LogRefs implements refsvfs2.CheckedObject.LogRefs.
+func (r *FilesystemRefs) LogRefs() bool {
+	return FilesystemenableLogging
+}
+
+// ReadRefs returns the current number of references. The returned count is
+// inherently racy and is unsafe to use without external synchronization.
+func (r *FilesystemRefs) ReadRefs() int64 {
+	return atomic.LoadInt64(&r.refCount)
+}
+
+// IncRef implements refs.RefCounter.IncRef.
+//
+//go:nosplit
+func (r *FilesystemRefs) IncRef() {
+	v := atomic.AddInt64(&r.refCount, 1)
+	if FilesystemenableLogging {
+		refsvfs2.LogIncRef(r, v)
+	}
+	if v <= 1 {
+		panic(fmt.Sprintf("Incrementing non-positive count %p on %s", r, r.RefType()))
+	}
+}
+
+// TryIncRef implements refs.TryRefCounter.TryIncRef.
+//
+// To do this safely without a loop, a speculative reference is first acquired
+// on the object. This allows multiple concurrent TryIncRef calls to distinguish
+// other TryIncRef calls from genuine references held.
+//
+//go:nosplit
+func (r *FilesystemRefs) TryIncRef() bool {
+	const speculativeRef = 1 << 32
+	if v := atomic.AddInt64(&r.refCount, speculativeRef); int32(v) == 0 {
+
+		atomic.AddInt64(&r.refCount, -speculativeRef)
+		return false
+	}
+
+	v := atomic.AddInt64(&r.refCount, -speculativeRef+1)
+	if FilesystemenableLogging {
+		refsvfs2.LogTryIncRef(r, v)
+	}
+	return true
+}
+
+// DecRef implements refs.RefCounter.DecRef.
+//
+// Note that speculative references are counted here. Since they were added
+// prior to real references reaching zero, they will successfully convert to
+// real references. In other words, we see speculative references only in the
+// following case:
+//
+//	A: TryIncRef [speculative increase => sees non-negative references]
+//	B: DecRef [real decrease]
+//	A: TryIncRef [transform speculative to real]
+//
+//go:nosplit
+func (r *FilesystemRefs) DecRef(destroy func()) {
+	v := atomic.AddInt64(&r.refCount, -1)
+	if FilesystemenableLogging {
+		refsvfs2.LogDecRef(r, v)
+	}
+	switch {
+	case v < 0:
+		panic(fmt.Sprintf("Decrementing non-positive ref count %p, owned by %s", r, r.RefType()))
+
+	case v == 0:
+		refsvfs2.Unregister(r)
+
+		if destroy != nil {
+			destroy()
+		}
+	}
+}
+
+func (r *FilesystemRefs) afterLoad() {
+	if r.ReadRefs() > 0 {
+		refsvfs2.Register(r)
+	}
+}