Implement memmap.MappingIdentity for vfs.FileDescription.

PiperOrigin-RevId: 285255855

pkg/sentry/memmap/BUILD

@@ -41,7 +41,6 @@ go_library(
     visibility = ["//pkg/sentry:internal"],
     deps = [
         "//pkg/log",
-        "//pkg/refs",
         "//pkg/sentry/context",
         "//pkg/sentry/platform",
         "//pkg/sentry/usermem",

pkg/sentry/memmap/memmap.go

@@ -18,7 +18,6 @@ package memmap
 import (
 	"fmt"
 
-	"gvisor.dev/gvisor/pkg/refs"
 	"gvisor.dev/gvisor/pkg/sentry/context"
 	"gvisor.dev/gvisor/pkg/sentry/platform"
 	"gvisor.dev/gvisor/pkg/sentry/usermem"
@@ -235,8 +234,11 @@ type InvalidateOpts struct {
 // coincidental; fs.File implements MappingIdentity, and some
 // fs.InodeOperations implement Mappable.)
 type MappingIdentity interface {
-	// MappingIdentity is reference-counted.
-	refs.RefCounter
+	// IncRef increments the MappingIdentity's reference count.
+	IncRef()
+
+	// DecRef decrements the MappingIdentity's reference count.
+	DecRef()
 
 	// MappedName returns the application-visible name shown in
 	// /proc/[pid]/maps.

pkg/sentry/vfs/context.go

@@ -24,6 +24,9 @@ type contextID int
 const (
 	// CtxMountNamespace is a Context.Value key for a MountNamespace.
 	CtxMountNamespace contextID = iota
+
+	// CtxRoot is a Context.Value key for a VFS root.
+	CtxRoot
 )
 
 // MountNamespaceFromContext returns the MountNamespace used by ctx. It does
@@ -35,3 +38,13 @@ func MountNamespaceFromContext(ctx context.Context) *MountNamespace {
 	}
 	return nil
 }
+
+// RootFromContext returns the VFS root used by ctx. It takes a reference on
+// the returned VirtualDentry. If ctx does not have a specific VFS root,
+// RootFromContext returns a zero-value VirtualDentry.
+func RootFromContext(ctx context.Context) VirtualDentry {
+	if v := ctx.Value(CtxRoot); v != nil {
+		return v.(VirtualDentry)
+	}
+	return VirtualDentry{}
+}

pkg/sentry/vfs/file_description.go

@@ -334,3 +334,47 @@ func (fd *FileDescription) Ioctl(ctx context.Context, uio usermem.IO, args arch.
 func (fd *FileDescription) SyncFS(ctx context.Context) error {
 	return fd.vd.mount.fs.impl.Sync(ctx)
 }
+
+// MappedName implements memmap.MappingIdentity.MappedName.
+func (fd *FileDescription) MappedName(ctx context.Context) string {
+	vfsroot := RootFromContext(ctx)
+	s, _ := fd.vd.mount.vfs.PathnameWithDeleted(ctx, vfsroot, fd.vd)
+	if vfsroot.Ok() {
+		vfsroot.DecRef()
+	}
+	return s
+}
+
+// DeviceID implements memmap.MappingIdentity.DeviceID.
+func (fd *FileDescription) DeviceID() uint64 {
+	stat, err := fd.impl.Stat(context.Background(), StatOptions{
+		// There is no STATX_DEV; we assume that Stat will return it if it's
+		// available regardless of mask.
+		Mask: 0,
+		// fs/proc/task_mmu.c:show_map_vma() just reads inode::i_sb->s_dev
+		// directly.
+		Sync: linux.AT_STATX_DONT_SYNC,
+	})
+	if err != nil {
+		return 0
+	}
+	return uint64(linux.MakeDeviceID(uint16(stat.DevMajor), stat.DevMinor))
+}
+
+// InodeID implements memmap.MappingIdentity.InodeID.
+func (fd *FileDescription) InodeID() uint64 {
+	stat, err := fd.impl.Stat(context.Background(), StatOptions{
+		Mask: linux.STATX_INO,
+		// fs/proc/task_mmu.c:show_map_vma() just reads inode::i_ino directly.
+		Sync: linux.AT_STATX_DONT_SYNC,
+	})
+	if err != nil || stat.Mask&linux.STATX_INO == 0 {
+		return 0
+	}
+	return stat.Ino
+}
+
+// Msync implements memmap.MappingIdentity.Msync.
+func (fd *FileDescription) Msync(ctx context.Context, mr memmap.MappableRange) error {
+	return fd.impl.Sync(ctx)
+}

pkg/sentry/vfs/file_description_impl_util.go

@@ -252,3 +252,12 @@ func (fd *DynamicBytesFileDescriptionImpl) Seek(ctx context.Context, offset int6
 	fd.off = offset
 	return offset, nil
 }
+
+// GenericConfigureMMap may be used by most implementations of
+// FileDescriptionImpl.ConfigureMMap.
+func GenericConfigureMMap(fd *FileDescription, m memmap.Mappable, opts *memmap.MMapOpts) error {
+	opts.Mappable = m
+	opts.MappingIdentity = fd
+	fd.IncRef()
+	return nil
+}