Add LinkAt support to gofer

Updates #1198

PiperOrigin-RevId: 325350818

2 files changed, 24 insertions, 4 deletions

diff --git a/pkg/sentry/fsimpl/gofer/filesystem.go b/pkg/sentry/fsimpl/gofer/filesystem.go
index e6af37d0d..582b744bb 100644
--- a/pkg/sentry/fsimpl/gofer/filesystem.go
+++ b/pkg/sentry/fsimpl/gofer/filesystem.go
@@ -15,6 +15,7 @@
 package gofer
 
 import (
+	"math"
 	"sync"
 	"sync/atomic"
 
@@ -724,8 +725,29 @@ func (fs *filesystem) LinkAt(ctx context.Context, rp *vfs.ResolvingPath, vd vfs.
 		if rp.Mount() != vd.Mount() {
 			return syserror.EXDEV
 		}
-		// 9P2000.L supports hard links, but we don't.
-		return syserror.EPERM
+		d := vd.Dentry().Impl().(*dentry)
+		if d.isDir() {
+			return syserror.EPERM
+		}
+		gid := auth.KGID(atomic.LoadUint32(&d.gid))
+		uid := auth.KUID(atomic.LoadUint32(&d.uid))
+		mode := linux.FileMode(atomic.LoadUint32(&d.mode))
+		if err := vfs.MayLink(rp.Credentials(), mode, uid, gid); err != nil {
+			return err
+		}
+		if d.nlink == 0 {
+			return syserror.ENOENT
+		}
+		if d.nlink == math.MaxUint32 {
+			return syserror.EMLINK
+		}
+		if err := parent.file.link(ctx, d.file, childName); err != nil {
+			return err
+		}
+
+		// Success!
+		atomic.AddUint32(&d.nlink, 1)
+		return nil
 	}, nil)
 }
 
diff --git a/pkg/sentry/fsimpl/gofer/gofer.go b/pkg/sentry/fsimpl/gofer/gofer.go
index 2e5575d8d..6ae796c6d 100644
--- a/pkg/sentry/fsimpl/gofer/gofer.go
+++ b/pkg/sentry/fsimpl/gofer/gofer.go
@@ -558,8 +558,6 @@ type dentry struct {
 	// filesystem.renameMu.
 	name string
 
-	// We don't support hard links, so each dentry maps 1:1 to an inode.
-
 	// file is the unopened p9.File that backs this dentry. file is immutable.
 	//
 	// If file.isNil(), this dentry represents a synthetic file, i.e. a file