Remove dentry_cache_limit mount option from mqfs.

The dentry cache limit should only be set at filesystem creation.
Mqfs filesystems are created during IPC namespace creation and
mount(2)s return a reference to the shared filesystem
object. Modifying the cache limit once the filesystem is in use can
cause the cache to exceed the limit. Since this mount option is rarely
(never?) used, use a static cache size and remove the mount option.

Reported-by: syzbot+e89efb5faa374468b6bb@syzkaller.appspotmail.com
PiperOrigin-RevId: 425436253

pkg/sentry/fsimpl/mqfs/mqfs.go

@@ -18,10 +18,8 @@ package mqfs
 
 import (
 	"fmt"
-	"strconv"
 
 	"gvisor.dev/gvisor/pkg/context"
-	"gvisor.dev/gvisor/pkg/errors/linuxerr"
 	"gvisor.dev/gvisor/pkg/sentry/fsimpl/kernfs"
 	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
 	"gvisor.dev/gvisor/pkg/sentry/kernel/ipc"
@@ -31,8 +29,7 @@ import (
 
 const (
 	// Name is the user-visible filesystem name.
-	Name                     = "mqueue"
-	defaultMaxCachedDentries = uint64(1000)
+	Name = "mqueue"
 )
 
 // FilesystemType implements vfs.FilesystemType.
@@ -67,34 +64,11 @@ func (ft FilesystemType) GetFilesystem(ctx context.Context, vfsObj *vfs.VirtualF
 		return nil, nil, fmt.Errorf("mqfs.FilesystemType.GetFilesystem: ipc namespace doesn't have a POSIX registry")
 	}
 	impl := registry.Impl().(*RegistryImpl)
-
-	maxCachedDentries, err := maxCachedDentries(ctx, vfs.GenericParseMountOptions(opts.Data))
-	if err != nil {
-		return nil, nil, err
-	}
-	impl.fs.MaxCachedDentries = maxCachedDentries
-
 	impl.fs.VFSFilesystem().IncRef()
 	impl.root.IncRef()
 	return impl.fs.VFSFilesystem(), impl.root.VFSDentry(), nil
 }
 
-// maxCachedDentries checks mopts for dentry_cache_limit. If a value is
-// specified, parse it into uint64 and return it. Otherwise, return the default
-// value. An error is returned if a value is found but can't be parsed.
-func maxCachedDentries(ctx context.Context, mopts map[string]string) (_ uint64, err error) {
-	max := defaultMaxCachedDentries
-	if str, ok := mopts["dentry_cache_limit"]; ok {
-		delete(mopts, "dentry_cache_limit")
-		max, err = strconv.ParseUint(str, 10, 64)
-		if err != nil {
-			ctx.Warningf("mqfs.FilesystemType.GetFilesystem: invalid dentry cache limit: dentry_cache_limit=%s", str)
-			return 0, linuxerr.EINVAL
-		}
-	}
-	return max, nil
-}
-
 // filesystem implements kernfs.Filesystem.
 //
 // +stateify savable
@@ -111,7 +85,7 @@ func (fs *filesystem) Release(ctx context.Context) {
 
 // MountOptions implements vfs.FilesystemImpl.MountOptions.
 func (fs *filesystem) MountOptions() string {
-	return fmt.Sprintf("dentry_cache_limit=%d", fs.MaxCachedDentries)
+	return ""
 }
 
 // ipcNamespace defines functions we need from kernel.IPCNamespace. We redefine

pkg/sentry/fsimpl/mqfs/registry.go

@@ -25,6 +25,10 @@ import (
 	"gvisor.dev/gvisor/pkg/sentry/vfs"
 )
 
+const (
+	maxCachedDentries = 1000
+)
+
 // RegistryImpl implements mq.RegistryImpl. It implements the interface using
 // the message queue filesystem, and is provided to mq.Registry at
 // initialization.
@@ -58,7 +62,8 @@ func NewRegistryImpl(ctx context.Context, vfsObj *vfs.VirtualFilesystem, creds *
 	}
 
 	fs := &filesystem{
-		devMinor: devMinor,
+		devMinor:   devMinor,
+		Filesystem: kernfs.Filesystem{MaxCachedDentries: maxCachedDentries},
 	}
 	fs.VFSFilesystem().Init(vfsObj, &FilesystemType{}, fs)
 	vfsfs := fs.VFSFilesystem()