Correctly implement magic symlinks in VFS2 procfs.

Updates #1195

PiperOrigin-RevId: 305143567

4 files changed, 49 insertions, 13 deletions

diff --git a/pkg/sentry/fsimpl/kernfs/filesystem.go b/pkg/sentry/fsimpl/kernfs/filesystem.go
index 89f5da3d4..16a3c18ae 100644
--- a/pkg/sentry/fsimpl/kernfs/filesystem.go
+++ b/pkg/sentry/fsimpl/kernfs/filesystem.go
@@ -79,16 +79,22 @@ afterSymlink:
 	}
 	// Resolve any symlink at current path component.
 	if rp.ShouldFollowSymlink() && next.isSymlink() {
-		// TODO: VFS2 needs something extra for /proc/[pid]/fd/ "magic symlinks".
-		target, err := next.inode.Readlink(ctx)
+		targetVD, targetPathname, err := next.inode.Getlink(ctx)
 		if err != nil {
 			return nil, err
 		}
-		if err := rp.HandleSymlink(target); err != nil {
-			return nil, err
+		if targetVD.Ok() {
+			err := rp.HandleJump(targetVD)
+			targetVD.DecRef()
+			if err != nil {
+				return nil, err
+			}
+		} else {
+			if err := rp.HandleSymlink(targetPathname); err != nil {
+				return nil, err
+			}
 		}
 		goto afterSymlink
-
 	}
 	rp.Advance()
 	return &next.vfsd, nil
@@ -470,19 +476,25 @@ afterTrailingSymlink:
 	}
 	childDentry := childVFSD.Impl().(*Dentry)
 	childInode := childDentry.inode
-	if rp.ShouldFollowSymlink() {
-		if childDentry.isSymlink() {
-			target, err := childInode.Readlink(ctx)
+	if rp.ShouldFollowSymlink() && childDentry.isSymlink() {
+		targetVD, targetPathname, err := childInode.Getlink(ctx)
+		if err != nil {
+			return nil, err
+		}
+		if targetVD.Ok() {
+			err := rp.HandleJump(targetVD)
+			targetVD.DecRef()
 			if err != nil {
 				return nil, err
 			}
-			if err := rp.HandleSymlink(target); err != nil {
+		} else {
+			if err := rp.HandleSymlink(targetPathname); err != nil {
 				return nil, err
 			}
-			// rp.Final() may no longer be true since we now need to resolve the
-			// symlink target.
-			goto afterTrailingSymlink
 		}
+		// rp.Final() may no longer be true since we now need to resolve the
+		// symlink target.
+		goto afterTrailingSymlink
 	}
 	if err := childInode.CheckPermissions(ctx, rp.Credentials(), ats); err != nil {
 		return nil, err
diff --git a/pkg/sentry/fsimpl/kernfs/inode_impl_util.go b/pkg/sentry/fsimpl/kernfs/inode_impl_util.go
index 5c84b10c9..65f09af5d 100644
--- a/pkg/sentry/fsimpl/kernfs/inode_impl_util.go
+++ b/pkg/sentry/fsimpl/kernfs/inode_impl_util.go
@@ -181,6 +181,11 @@ func (InodeNotSymlink) Readlink(context.Context) (string, error) {
 	return "", syserror.EINVAL
 }
 
+// Getlink implements Inode.Getlink.
+func (InodeNotSymlink) Getlink(context.Context) (vfs.VirtualDentry, string, error) {
+	return vfs.VirtualDentry{}, "", syserror.EINVAL
+}
+
 // InodeAttrs partially implements the Inode interface, specifically the
 // inodeMetadata sub interface. InodeAttrs provides functionality related to
 // inode attributes.
diff --git a/pkg/sentry/fsimpl/kernfs/kernfs.go b/pkg/sentry/fsimpl/kernfs/kernfs.go
index 2cefef020..ad76b9f64 100644
--- a/pkg/sentry/fsimpl/kernfs/kernfs.go
+++ b/pkg/sentry/fsimpl/kernfs/kernfs.go
@@ -414,7 +414,21 @@ type inodeDynamicLookup interface {
 }
 
 type inodeSymlink interface {
-	// Readlink resolves the target of a symbolic link. If an inode is not a
+	// Readlink returns the target of a symbolic link. If an inode is not a
 	// symlink, the implementation should return EINVAL.
 	Readlink(ctx context.Context) (string, error)
+
+	// Getlink returns the target of a symbolic link, as used by path
+	// resolution:
+	//
+	// - If the inode is a "magic link" (a link whose target is most accurately
+	// represented as a VirtualDentry), Getlink returns (ok VirtualDentry, "",
+	// nil). A reference is taken on the returned VirtualDentry.
+	//
+	// - If the inode is an ordinary symlink, Getlink returns (zero-value
+	// VirtualDentry, symlink target, nil).
+	//
+	// - If the inode is not a symlink, Getlink returns (zero-value
+	// VirtualDentry, "", EINVAL).
+	Getlink(ctx context.Context) (vfs.VirtualDentry, string, error)
 }
diff --git a/pkg/sentry/fsimpl/kernfs/symlink.go b/pkg/sentry/fsimpl/kernfs/symlink.go
index 5918d3309..018aa503c 100644
--- a/pkg/sentry/fsimpl/kernfs/symlink.go
+++ b/pkg/sentry/fsimpl/kernfs/symlink.go
@@ -55,6 +55,11 @@ func (s *StaticSymlink) Readlink(_ context.Context) (string, error) {
 	return s.target, nil
 }
 
+// Getlink implements Inode.Getlink.
+func (s *StaticSymlink) Getlink(_ context.Context) (vfs.VirtualDentry, string, error) {
+	return vfs.VirtualDentry{}, s.target, nil
+}
+
 // SetStat implements Inode.SetStat not allowing inode attributes to be changed.
 func (*StaticSymlink) SetStat(context.Context, *vfs.Filesystem, *auth.Credentials, vfs.SetStatOptions) error {
 	return syserror.EPERM