fusefronted: reject GETXATTR "security.capability"

Unless we are mounted with -suid, we can reject
these requests, and gain back some lost speed.

Closes https://github.com/rfjakob/gocryptfs/issues/515

2 files changed, 18 insertions, 0 deletions

diff --git a/internal/fusefrontend/args.go b/internal/fusefrontend/args.go
index 646d6c4..f822650 100644
--- a/internal/fusefrontend/args.go
+++ b/internal/fusefrontend/args.go
@@ -39,4 +39,9 @@ type Args struct {
 	// ExcludeFrom is a list of files from which to read exclusion patterns
 	// (with wildcard syntax)
 	ExcludeFrom []string
+	// Suid is true if the filesystem has been mounted with the "-suid" flag.
+	// If it is false, we can ignore the GETXATTR "security.capability" calls,
+	// which are a performance problem for writes. See
+	// https://github.com/rfjakob/gocryptfs/issues/515 for details.
+	Suid bool
 }
diff --git a/internal/fusefrontend/node_xattr.go b/internal/fusefrontend/node_xattr.go
index de40915..cbc5804 100644
--- a/internal/fusefrontend/node_xattr.go
+++ b/internal/fusefrontend/node_xattr.go
@@ -18,11 +18,24 @@ var xattrNameIV = []byte("xattr_name_iv_xx")
 // encrypted original name.
 var xattrStorePrefix = "user.gocryptfs."
 
+// We get one read of this xattr for each write -
+// see https://github.com/rfjakob/gocryptfs/issues/515 for details.
+var xattrCapability = "security.capability"
+
 // GetXAttr - FUSE call. Reads the value of extended attribute "attr".
 //
 // This function is symlink-safe through Fgetxattr.
 func (n *Node) Getxattr(ctx context.Context, attr string, dest []byte) (uint32, syscall.Errno) {
 	rn := n.rootNode()
+	// If we are not mounted with -suid, reading the capability xattr does not
+	// make a lot of sense, so reject the request and gain a massive speedup.
+	// See https://github.com/rfjakob/gocryptfs/issues/515 .
+	if !rn.args.Suid && attr == xattrCapability {
+		// Returning EOPNOTSUPP is what we did till
+		// ca9e912a28b901387e1dbb85f6c531119f2d5ef2 "fusefrontend: drop xattr user namespace restriction"
+		// and it did not cause trouble. Seems cleaner than saying ENODATA.
+		return 0, syscall.EOPNOTSUPP
+	}
 	cAttr := rn.encryptXattrName(attr)
 	cData, errno := n.getXAttr(cAttr)
 	if errno != 0 {