From d3ae87fa2b10269f9619f7a36fd4a01f35448fb6 Mon Sep 17 00:00:00 2001
From: Jakob Unterwurzacher <jakobunt@gmail.com>
Date: Sun, 11 Nov 2018 18:04:44 +0100
Subject: [PATCH] fusefrontend: make RemoveXAttr() symlink-safe

Uses /proc/self/fd on Linux.
---
 internal/fusefrontend/xattr.go        | 15 ++++++---------
 internal/fusefrontend/xattr_darwin.go | 16 +++++++++++++++-
 internal/fusefrontend/xattr_linux.go  | 15 +++++++++++++++
 3 files changed, 36 insertions(+), 10 deletions(-)

diff --git a/internal/fusefrontend/xattr.go b/internal/fusefrontend/xattr.go
index b511bcf..8e88a62 100644
--- a/internal/fusefrontend/xattr.go
+++ b/internal/fusefrontend/xattr.go
@@ -68,21 +68,18 @@ func (fs *FS) SetXAttr(relPath string, attr string, data []byte, flags int, cont
 
 // RemoveXAttr - FUSE call.
 //
-// TODO: Make symlink-safe. Blocker: package xattr does not provide
-// fremovexattr(2).
-func (fs *FS) RemoveXAttr(path string, attr string, context *fuse.Context) fuse.Status {
-	if fs.isFiltered(path) {
+// This function is symlink-safe on Linux.
+// Darwin does not have fremovexattr(2) nor /proc/self/fd. How to implement this
+// on Darwin in a symlink-safe way?
+func (fs *FS) RemoveXAttr(relPath string, attr string, context *fuse.Context) fuse.Status {
+	if fs.isFiltered(relPath) {
 		return fuse.EPERM
 	}
 	if disallowedXAttrName(attr) {
 		return _EOPNOTSUPP
 	}
-	cPath, err := fs.getBackingPath(path)
-	if err != nil {
-		return fuse.ToStatus(err)
-	}
 	cAttr := fs.encryptXattrName(attr)
-	return unpackXattrErr(xattr.LRemove(cPath, cAttr))
+	return fs.removeXAttr(relPath, cAttr, context)
 }
 
 // ListXAttr - FUSE call. Lists extended attributes on the file at "path".
diff --git a/internal/fusefrontend/xattr_darwin.go b/internal/fusefrontend/xattr_darwin.go
index e605f83..f0b755f 100644
--- a/internal/fusefrontend/xattr_darwin.go
+++ b/internal/fusefrontend/xattr_darwin.go
@@ -18,7 +18,8 @@ func filterXattrSetFlags(flags int) int {
 	return flags &^ xattr.XATTR_NOSECURITY
 }
 
-// This function is NOT symlink-safe because Darwin lacks fgetxattr().
+// This function is NOT symlink-safe because Darwin lacks
+// both fgetxattr() and /proc/self/fd.
 func (fs *FS) getXattr(relPath string, cAttr string, context *fuse.Context) ([]byte, fuse.Status) {
 	cPath, err := fs.getBackingPath(relPath)
 	if err != nil {
@@ -31,6 +32,8 @@ func (fs *FS) getXattr(relPath string, cAttr string, context *fuse.Context) ([]b
 	return cData, fuse.OK
 }
 
+// This function is NOT symlink-safe because Darwin lacks
+// both fsetxattr() and /proc/self/fd.
 func (fs *FS) setXattr(relPath string, cAttr string, cData []byte, flags int, context *fuse.Context) fuse.Status {
 	cPath, err := fs.getBackingPath(relPath)
 	if err != nil {
@@ -39,3 +42,14 @@ func (fs *FS) setXattr(relPath string, cAttr string, cData []byte, flags int, co
 	err = xattr.LSetWithFlags(cPath, cAttr, cData, flags)
 	return unpackXattrErr(err)
 }
+
+// This function is NOT symlink-safe because Darwin lacks
+// both fremovexattr() and /proc/self/fd.
+func (fs *FS) removeXAttr(relPath string, cAttr string, context *fuse.Context) fuse.Status {
+	cPath, err := fs.getBackingPath(relPath)
+	if err != nil {
+		return fuse.ToStatus(err)
+	}
+	err = xattr.LRemove(cPath, cAttr)
+	return unpackXattrErr(err)
+}
diff --git a/internal/fusefrontend/xattr_linux.go b/internal/fusefrontend/xattr_linux.go
index 659657d..a17b12c 100644
--- a/internal/fusefrontend/xattr_linux.go
+++ b/internal/fusefrontend/xattr_linux.go
@@ -84,3 +84,18 @@ func (fs *FS) setXattr(relPath string, cAttr string, cData []byte, flags int, co
 	err := xattr.SetWithFlags(procFd(fd), cAttr, cData, flags)
 	return unpackXattrErr(err)
 }
+
+// removeXAttr - remove encrypted xattr name "cAttr" from
+// plaintext path "relPath".
+//
+// This function is symlink-safe on Linux by using /proc/self/fd.
+func (fs *FS) removeXAttr(relPath string, cAttr string, context *fuse.Context) fuse.Status {
+	file, fd, status := fs.getFileFd(relPath, context)
+	if !status.Ok() {
+		return status
+	}
+	defer file.Release()
+
+	err := xattr.Remove(procFd(fd), cAttr)
+	return unpackXattrErr(err)
+}