From ae7ab0a330aaa1676d1fe066e3f5ac5fe805ec1c Mon Sep 17 00:00:00 2001
From: Jamie Liu <jamieliu@google.com>
Date: Thu, 12 Nov 2020 16:54:22 -0800
Subject: Filter dentries with non-zero refs in VFS2 gofer/overlay checks.

PiperOrigin-RevId: 342161204
---
 pkg/sentry/fsimpl/gofer/filesystem.go | 80 ++++++++++++++++++++---------------
 pkg/sentry/fsimpl/gofer/gofer.go      |  5 +++
 2 files changed, 50 insertions(+), 35 deletions(-)

(limited to 'pkg/sentry/fsimpl/gofer')

diff --git a/pkg/sentry/fsimpl/gofer/filesystem.go b/pkg/sentry/fsimpl/gofer/filesystem.go
index 7ab298019..2294c490e 100644
--- a/pkg/sentry/fsimpl/gofer/filesystem.go
+++ b/pkg/sentry/fsimpl/gofer/filesystem.go
@@ -114,6 +114,51 @@ func putDentrySlice(ds *[]*dentry) {
 	dentrySlicePool.Put(ds)
 }
 
+// renameMuRUnlockAndCheckCaching calls fs.renameMu.RUnlock(), then calls
+// dentry.checkCachingLocked on all dentries in *dsp with fs.renameMu locked
+// for writing.
+//
+// dsp is a pointer-to-pointer since defer evaluates its arguments immediately,
+// but dentry slices are allocated lazily, and it's much easier to say "defer
+// fs.renameMuRUnlockAndCheckCaching(&ds)" than "defer func() {
+// fs.renameMuRUnlockAndCheckCaching(ds) }()" to work around this.
+func (fs *filesystem) renameMuRUnlockAndCheckCaching(ctx context.Context, dsp **[]*dentry) {
+	fs.renameMu.RUnlock()
+	if *dsp == nil {
+		return
+	}
+	ds := **dsp
+	// Only go through calling dentry.checkCachingLocked() (which requires
+	// re-locking renameMu) if we actually have any dentries with zero refs.
+	checkAny := false
+	for i := range ds {
+		if atomic.LoadInt64(&ds[i].refs) == 0 {
+			checkAny = true
+			break
+		}
+	}
+	if checkAny {
+		fs.renameMu.Lock()
+		for _, d := range ds {
+			d.checkCachingLocked(ctx)
+		}
+		fs.renameMu.Unlock()
+	}
+	putDentrySlice(*dsp)
+}
+
+func (fs *filesystem) renameMuUnlockAndCheckCaching(ctx context.Context, ds **[]*dentry) {
+	if *ds == nil {
+		fs.renameMu.Unlock()
+		return
+	}
+	for _, d := range **ds {
+		d.checkCachingLocked(ctx)
+	}
+	fs.renameMu.Unlock()
+	putDentrySlice(*ds)
+}
+
 // stepLocked resolves rp.Component() to an existing file, starting from the
 // given directory.
 //
@@ -651,41 +696,6 @@ func (fs *filesystem) unlinkAt(ctx context.Context, rp *vfs.ResolvingPath, dir b
 	return nil
 }
 
-// renameMuRUnlockAndCheckCaching calls fs.renameMu.RUnlock(), then calls
-// dentry.checkCachingLocked on all dentries in *ds with fs.renameMu locked for
-// writing.
-//
-// ds is a pointer-to-pointer since defer evaluates its arguments immediately,
-// but dentry slices are allocated lazily, and it's much easier to say "defer
-// fs.renameMuRUnlockAndCheckCaching(&ds)" than "defer func() {
-// fs.renameMuRUnlockAndCheckCaching(ds) }()" to work around this.
-func (fs *filesystem) renameMuRUnlockAndCheckCaching(ctx context.Context, ds **[]*dentry) {
-	fs.renameMu.RUnlock()
-	if *ds == nil {
-		return
-	}
-	if len(**ds) != 0 {
-		fs.renameMu.Lock()
-		for _, d := range **ds {
-			d.checkCachingLocked(ctx)
-		}
-		fs.renameMu.Unlock()
-	}
-	putDentrySlice(*ds)
-}
-
-func (fs *filesystem) renameMuUnlockAndCheckCaching(ctx context.Context, ds **[]*dentry) {
-	if *ds == nil {
-		fs.renameMu.Unlock()
-		return
-	}
-	for _, d := range **ds {
-		d.checkCachingLocked(ctx)
-	}
-	fs.renameMu.Unlock()
-	putDentrySlice(*ds)
-}
-
 // AccessAt implements vfs.Filesystem.Impl.AccessAt.
 func (fs *filesystem) AccessAt(ctx context.Context, rp *vfs.ResolvingPath, creds *auth.Credentials, ats vfs.AccessTypes) error {
 	var ds *[]*dentry
diff --git a/pkg/sentry/fsimpl/gofer/gofer.go b/pkg/sentry/fsimpl/gofer/gofer.go
index 53bcc9986..75a836899 100644
--- a/pkg/sentry/fsimpl/gofer/gofer.go
+++ b/pkg/sentry/fsimpl/gofer/gofer.go
@@ -1352,6 +1352,11 @@ func (d *dentry) checkCachingLocked(ctx context.Context) {
 	}
 	if refs > 0 {
 		if d.cached {
+			// This isn't strictly necessary (fs.cachedDentries is permitted to
+			// contain dentries with non-zero refs, which are skipped by
+			// fs.evictCachedDentryLocked() upon reaching the end of the LRU),
+			// but since we are already holding fs.renameMu for writing we may
+			// as well.
 			d.fs.cachedDentries.Remove(d)
 			d.fs.cachedDentriesLen--
 			d.cached = false
-- 
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