From 99cc6c2dea59ca51f7672359ed4321041fae02a4 Mon Sep 17 00:00:00 2001
From: Lucas Manning <lucasmanning@google.com>
Date: Thu, 1 Dec 2022 16:03:04 -0800
Subject: [PATCH] Display the proper root path in mountinfo.

PiperOrigin-RevId: 492323370
---
 pkg/sentry/fsimpl/tmpfs/filesystem.go |  2 +-
 pkg/sentry/vfs/mount.go               | 30 +++++++++++++++++----------
 pkg/sentry/vfs/pathname.go            | 23 ++++++++++++++++++++
 test/syscalls/linux/mount.cc          | 20 ++++++++++++++++++
 4 files changed, 63 insertions(+), 12 deletions(-)

diff --git a/pkg/sentry/fsimpl/tmpfs/filesystem.go b/pkg/sentry/fsimpl/tmpfs/filesystem.go
index fb398f499..59b2efe4e 100644
--- a/pkg/sentry/fsimpl/tmpfs/filesystem.go
+++ b/pkg/sentry/fsimpl/tmpfs/filesystem.go
@@ -920,7 +920,7 @@ func (fs *filesystem) PrependPath(ctx context.Context, vfsroot, vd vfs.VirtualDe
 		if mnt == vfsroot.Mount() && &d.vfsd == vfsroot.Dentry() {
 			return vfs.PrependPathAtVFSRootError{}
 		}
-		if &d.vfsd == mnt.Root() {
+		if mnt != nil && &d.vfsd == mnt.Root() {
 			return nil
 		}
 		if d.parent == nil {
diff --git a/pkg/sentry/vfs/mount.go b/pkg/sentry/vfs/mount.go
index 5904adb9e..3383fa8cc 100644
--- a/pkg/sentry/vfs/mount.go
+++ b/pkg/sentry/vfs/mount.go
@@ -1161,16 +1161,27 @@ func (vfs *VirtualFilesystem) GenerateProcMountInfo(ctx context.Context, taskRoo
 			mount:  mnt,
 			dentry: mnt.root,
 		}
-		path, err := vfs.PathnameReachable(ctx, taskRootDir, mntRootVD)
+		pathFromRoot, err := vfs.PathnameReachable(ctx, taskRootDir, mntRootVD)
 		if err != nil {
 			// For some reason we didn't get a path. Log a warning
 			// and run with empty path.
 			ctx.Warningf("VFS.GenerateProcMountInfo: error getting pathname for mount root %+v: %v", mnt.root, err)
-			path = ""
+			continue
 		}
-		if path == "" {
-			// Either an error occurred, or path is not reachable
-			// from root.
+		if pathFromRoot == "" {
+			// The path is not reachable from root.
+			continue
+		}
+		var pathFromFS string
+		pathFromFS, err = vfs.PathnameInFilesystem(ctx, mntRootVD)
+		if err != nil {
+			// For some reason we didn't get a path. Log a warning
+			// and run with empty path.
+			ctx.Warningf("VFS.GenerateProcMountInfo: error getting pathname for mount root %+v: %v", mnt.root, err)
+			continue
+		}
+		if pathFromFS == "" {
+			// The path is not reachable from root.
 			continue
 		}
 		// Stat the mount root to get the major/minor device numbers.
@@ -1208,13 +1219,10 @@ func (vfs *VirtualFilesystem) GenerateProcMountInfo(ctx context.Context, taskRoo
 
 		// (4) Root: the pathname of the directory in the filesystem
 		// which forms the root of this mount.
-		//
-		// NOTE(b/78135857): This will always be "/" until we implement
-		// bind mounts.
-		fmt.Fprintf(buf, "/ ")
+		fmt.Fprintf(buf, "%s ", manglePath(pathFromFS))
 
 		// (5) Mount point (relative to process root).
-		fmt.Fprintf(buf, "%s ", manglePath(path))
+		fmt.Fprintf(buf, "%s ", manglePath(pathFromRoot))
 
 		// (6) Mount options.
 		opts := "rw"
@@ -1241,7 +1249,7 @@ func (vfs *VirtualFilesystem) GenerateProcMountInfo(ctx context.Context, taskRoo
 		fmt.Fprintf(buf, "none ")
 
 		// (11) Superblock options, and final newline.
-		fmt.Fprintf(buf, "%s\n", superBlockOpts(path, mnt))
+		fmt.Fprintf(buf, "%s\n", superBlockOpts(pathFromRoot, mnt))
 	}
 }
 
diff --git a/pkg/sentry/vfs/pathname.go b/pkg/sentry/vfs/pathname.go
index 7efe72021..a06a69c59 100644
--- a/pkg/sentry/vfs/pathname.go
+++ b/pkg/sentry/vfs/pathname.go
@@ -133,6 +133,29 @@ loop:
 	return b.String(), nil
 }
 
+// PathnameInFilesystem returns an absolute path to vd relative to vd's
+// Filesystem root. It also appends //deleted to for disowned entries. It is
+// equivalent to Linux's dentry_path().
+func (vfs *VirtualFilesystem) PathnameInFilesystem(ctx context.Context, vd VirtualDentry) (string, error) {
+	b := getFSPathBuilder()
+	defer putFSPathBuilder(b)
+	if vd.dentry.IsDead() {
+		b.PrependString("//deleted")
+	}
+	if err := vd.mount.fs.impl.PrependPath(ctx, VirtualDentry{}, VirtualDentry{dentry: vd.dentry}, b); err != nil {
+		// PrependPath returns an error if it encounters a filesystem root before
+		// the provided vfsroot. We don't provide a vfsroot, so encountering this
+		// error is expected and can be ignored.
+		switch err.(type) {
+		case PrependPathAtNonMountRootError:
+		default:
+			return "", err
+		}
+	}
+	b.PrependByte('/')
+	return b.String(), nil
+}
+
 // PathnameForGetcwd returns an absolute pathname to vd, consistent with
 // Linux's sys_getcwd().
 func (vfs *VirtualFilesystem) PathnameForGetcwd(ctx context.Context, vfsroot, vd VirtualDentry) (string, error) {
diff --git a/test/syscalls/linux/mount.cc b/test/syscalls/linux/mount.cc
index 096e31a30..ea34c3477 100644
--- a/test/syscalls/linux/mount.cc
+++ b/test/syscalls/linux/mount.cc
@@ -1397,6 +1397,26 @@ TEST(MountTest, BindParentToChild) {
   ASSERT_EQ(opt2, opt3);
 }
 
+TEST(MountTest, MountInfoHasRoot) {
+  SKIP_IF(!ASSERT_NO_ERRNO_AND_VALUE(HaveCapability(CAP_SYS_ADMIN)));
+  auto const parent = ASSERT_NO_ERRNO_AND_VALUE(TempPath::CreateDir());
+  auto const mount = ASSERT_NO_ERRNO_AND_VALUE(
+      Mount("", parent.path(), "tmpfs", 0, "mode=0123", 0));
+  auto const child =
+      ASSERT_NO_ERRNO_AND_VALUE(TempPath::CreateDirIn(parent.path()));
+  auto const bind_mount = Mount(child.path(), child.path(), "", MS_BIND, "", 0);
+  std::vector<ProcMountInfoEntry> mounts =
+      ASSERT_NO_ERRNO_AND_VALUE(ProcSelfMountInfoEntries());
+  for (const auto& e : mounts) {
+    if (e.mount_point == child.path()) {
+      ASSERT_EQ(e.root, JoinPath("/", Basename(child.path())));
+    }
+    if (e.mount_point == parent.path()) {
+      ASSERT_EQ(e.root, "/");
+    }
+  }
+}
+
 }  // namespace
 
 }  // namespace testing