Add reference counting utility to VFS2.

The utility has several differences from the VFS1 equivalent:
- There are no weak references, which have a significant overhead
- In order to print useful debug messages with the type of the reference-
  counted object, we use a generic Refs object with the owner type as a
  template parameter. In vfs1, this was accomplished by storing a type name
  and caller stack directly in the ref count (as in vfs1), which increases the
  struct size by 6x. (Note that the caller stack was needed because fs types
  like Dirent were shared by all fs implementations; in vfs2, each impl has
  its own data structures, so this is no longer necessary.)

As an example, the utility is added to tmpfs.inode.

Updates #1486.

PiperOrigin-RevId: 324906582

2 files changed, 19 insertions, 21 deletions

diff --git a/pkg/sentry/fsimpl/tmpfs/BUILD b/pkg/sentry/fsimpl/tmpfs/BUILD
index e73732a6b..5cd428d64 100644
--- a/pkg/sentry/fsimpl/tmpfs/BUILD
+++ b/pkg/sentry/fsimpl/tmpfs/BUILD
@@ -26,6 +26,17 @@ go_template_instance(
     },
 )
 
+go_template_instance(
+    name = "inode_refs",
+    out = "inode_refs.go",
+    package = "tmpfs",
+    prefix = "inode",
+    template = "//pkg/refs_vfs2:refs_template",
+    types = {
+        "T": "inode",
+    },
+)
+
 go_library(
     name = "tmpfs",
     srcs = [
@@ -34,6 +45,7 @@ go_library(
         "directory.go",
         "filesystem.go",
         "fstree.go",
+        "inode_refs.go",
         "named_pipe.go",
         "regular_file.go",
         "socket_file.go",
@@ -47,6 +59,7 @@ go_library(
         "//pkg/context",
         "//pkg/fspath",
         "//pkg/log",
+        "//pkg/refs",
         "//pkg/safemem",
         "//pkg/sentry/arch",
         "//pkg/sentry/fs",
diff --git a/pkg/sentry/fsimpl/tmpfs/tmpfs.go b/pkg/sentry/fsimpl/tmpfs/tmpfs.go
index 68e615e8b..5640380dc 100644
--- a/pkg/sentry/fsimpl/tmpfs/tmpfs.go
+++ b/pkg/sentry/fsimpl/tmpfs/tmpfs.go
@@ -285,13 +285,10 @@ type inode struct {
 	// fs is the owning filesystem. fs is immutable.
 	fs *filesystem
 
-	// refs is a reference count. refs is accessed using atomic memory
-	// operations.
-	//
 	// A reference is held on all inodes as long as they are reachable in the
 	// filesystem tree, i.e. nlink is nonzero. This reference is dropped when
 	// nlink reaches 0.
-	refs int64
+	refs inodeRefs
 
 	// xattrs implements extended attributes.
 	//
@@ -327,7 +324,6 @@ func (i *inode) init(impl interface{}, fs *filesystem, kuid auth.KUID, kgid auth
 		panic("file type is required in FileMode")
 	}
 	i.fs = fs
-	i.refs = 1
 	i.mode = uint32(mode)
 	i.uid = uint32(kuid)
 	i.gid = uint32(kgid)
@@ -339,6 +335,7 @@ func (i *inode) init(impl interface{}, fs *filesystem, kuid auth.KUID, kgid auth
 	i.mtime = now
 	// i.nlink initialized by caller
 	i.impl = impl
+	i.refs.EnableLeakCheck()
 }
 
 // incLinksLocked increments i's link count.
@@ -369,25 +366,15 @@ func (i *inode) decLinksLocked(ctx context.Context) {
 }
 
 func (i *inode) incRef() {
-	if atomic.AddInt64(&i.refs, 1) <= 1 {
-		panic("tmpfs.inode.incRef() called without holding a reference")
-	}
+	i.refs.IncRef()
 }
 
 func (i *inode) tryIncRef() bool {
-	for {
-		refs := atomic.LoadInt64(&i.refs)
-		if refs == 0 {
-			return false
-		}
-		if atomic.CompareAndSwapInt64(&i.refs, refs, refs+1) {
-			return true
-		}
-	}
+	return i.refs.TryIncRef()
 }
 
 func (i *inode) decRef(ctx context.Context) {
-	if refs := atomic.AddInt64(&i.refs, -1); refs == 0 {
+	i.refs.DecRef(func() {
 		i.watches.HandleDeletion(ctx)
 		if regFile, ok := i.impl.(*regularFile); ok {
 			// Release memory used by regFile to store data. Since regFile is
@@ -395,9 +382,7 @@ func (i *inode) decRef(ctx context.Context) {
 			// metadata.
 			regFile.data.DropAll(regFile.memFile)
 		}
-	} else if refs < 0 {
-		panic("tmpfs.inode.decRef() called without holding a reference")
-	}
+	})
 }
 
 func (i *inode) checkPermissions(creds *auth.Credentials, ats vfs.AccessTypes) error {