Ensure fsimpl/gofer.dentryPlatformFile.hostFileMapper is initialized.

Fixes #1812. (The more direct cause of the deadlock is panic unsafety because
the historically high cost of defer means that we avoid it in hot paths,
including much of MM; defer is much cheaper as of Go 1.14, but still a
measurable overhead.)

PiperOrigin-RevId: 294560316

pkg/sentry/fs/fsutil/host_file_mapper.go

@@ -65,13 +65,18 @@ type mapping struct {
 	writable bool
 }
 
-// NewHostFileMapper returns a HostFileMapper with no references or cached
-// mappings.
+// Init must be called on zero-value HostFileMappers before first use.
+func (f *HostFileMapper) Init() {
+	f.refs = make(map[uint64]int32)
+	f.mappings = make(map[uint64]mapping)
+}
+
+// NewHostFileMapper returns an initialized HostFileMapper allocated on the
+// heap with no references or cached mappings.
 func NewHostFileMapper() *HostFileMapper {
-	return &HostFileMapper{
-		refs:     make(map[uint64]int32),
-		mappings: make(map[uint64]mapping),
-	}
+	f := &HostFileMapper{}
+	f.Init()
+	return f
 }
 
 // IncRefOn increments the reference count on all offsets in mr.

pkg/sentry/fsimpl/gofer/regular_file.go

@@ -571,6 +571,8 @@ func (fd *regularFileFD) ConfigureMMap(ctx context.Context, opts *memmap.MMapOpt
 	default:
 		panic(fmt.Sprintf("unknown InteropMode %v", d.fs.opts.interop))
 	}
+	// After this point, d may be used as a memmap.Mappable.
+	d.pf.hostFileMapperInitOnce.Do(d.pf.hostFileMapper.Init)
 	return vfs.GenericConfigureMMap(&fd.vfsfd, d, opts)
 }
 
@@ -799,6 +801,9 @@ type dentryPlatformFile struct {
 	// If this dentry represents a regular file, and handle.fd >= 0,
 	// hostFileMapper caches mappings of handle.fd.
 	hostFileMapper fsutil.HostFileMapper
+
+	// hostFileMapperInitOnce is used to lazily initialize hostFileMapper.
+	hostFileMapperInitOnce sync.Once
 }
 
 // IncRef implements platform.File.IncRef.