From 810d2a8b474e0102a8be3f6b00a3855e182dbd43 Mon Sep 17 00:00:00 2001
From: Jakob Unterwurzacher <jakobunt@gmail.com>
Date: Sun, 11 Nov 2018 17:57:24 +0100
Subject: [PATCH] fusefrontend: make SetXAttr() symlink-safe on Linux

Uses the /proc/self/fd trick.
---
 internal/fusefrontend/xattr.go        | 25 ++++++++++---------------
 internal/fusefrontend/xattr_darwin.go |  9 +++++++++
 internal/fusefrontend/xattr_linux.go  | 20 ++++++++++++++++++--
 3 files changed, 37 insertions(+), 17 deletions(-)

diff --git a/internal/fusefrontend/xattr.go b/internal/fusefrontend/xattr.go
index caf1e15..b511bcf 100644
--- a/internal/fusefrontend/xattr.go
+++ b/internal/fusefrontend/xattr.go
@@ -26,8 +26,8 @@ var xattrStorePrefix = "user.gocryptfs."
 // GetXAttr - FUSE call. Reads the value of extended attribute "attr".
 //
 // This function is symlink-safe on Linux.
-// Darwin does not have fgetxattr(2) nor /proc. How to implement this on Darwin
-// in a symlink-safe way?
+// Darwin does not have fgetxattr(2) nor /proc/self/fd. How to implement this
+// on Darwin in a symlink-safe way?
 func (fs *FS) GetXAttr(relPath string, attr string, context *fuse.Context) ([]byte, fuse.Status) {
 	if fs.isFiltered(relPath) {
 		return nil, fuse.EPERM
@@ -48,27 +48,22 @@ func (fs *FS) GetXAttr(relPath string, attr string, context *fuse.Context) ([]by
 	return data, fuse.OK
 }
 
-// SetXAttr - FUSE call.
+// SetXAttr - FUSE call. Set extended attribute.
 //
-// TODO: Make symlink-safe. Currently blocked because the xattr package does
-// not provide fsetxattr.
-func (fs *FS) SetXAttr(path string, attr string, data []byte, flags int, context *fuse.Context) fuse.Status {
-	if fs.isFiltered(path) {
+// This function is symlink-safe on Linux.
+// Darwin does not have fsetxattr(2) nor /proc/self/fd. How to implement this
+// on Darwin in a symlink-safe way?
+func (fs *FS) SetXAttr(relPath string, attr string, data []byte, flags int, context *fuse.Context) fuse.Status {
+	if fs.isFiltered(relPath) {
 		return fuse.EPERM
 	}
 	if disallowedXAttrName(attr) {
 		return _EOPNOTSUPP
 	}
-
 	flags = filterXattrSetFlags(flags)
-
-	cPath, err := fs.getBackingPath(path)
-	if err != nil {
-		return fuse.ToStatus(err)
-	}
 	cAttr := fs.encryptXattrName(attr)
 	cData := fs.encryptXattrValue(data)
-	return unpackXattrErr(xattr.LSetWithFlags(cPath, cAttr, cData, flags))
+	return fs.setXattr(relPath, cAttr, cData, flags, context)
 }
 
 // RemoveXAttr - FUSE call.
@@ -175,7 +170,7 @@ func (fs *FS) decryptXattrValue(cData []byte) (data []byte, err error) {
 }
 
 // unpackXattrErr unpacks an error value that we got from xattr.LGet/LSet/etc
-// and converts it to a fuse status.
+// and converts it to a fuse status. If err == nil, it returns fuse.OK.
 func unpackXattrErr(err error) fuse.Status {
 	if err == nil {
 		return fuse.OK
diff --git a/internal/fusefrontend/xattr_darwin.go b/internal/fusefrontend/xattr_darwin.go
index cf48d13..e605f83 100644
--- a/internal/fusefrontend/xattr_darwin.go
+++ b/internal/fusefrontend/xattr_darwin.go
@@ -30,3 +30,12 @@ func (fs *FS) getXattr(relPath string, cAttr string, context *fuse.Context) ([]b
 	}
 	return cData, fuse.OK
 }
+
+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 {
+		return fuse.ToStatus(err)
+	}
+	err = xattr.LSetWithFlags(cPath, cAttr, cData, flags)
+	return unpackXattrErr(err)
+}
diff --git a/internal/fusefrontend/xattr_linux.go b/internal/fusefrontend/xattr_linux.go
index 5a189db..659657d 100644
--- a/internal/fusefrontend/xattr_linux.go
+++ b/internal/fusefrontend/xattr_linux.go
@@ -28,6 +28,7 @@ func filterXattrSetFlags(flags int) int {
 	return flags
 }
 
+// procFd returns the path to file descriptor "fd" in /proc/self/fd.
 func procFd(fd int) string {
 	return fmt.Sprintf("/proc/self/fd/%d", fd)
 }
@@ -51,10 +52,10 @@ func (fs *FS) getFileFd(relPath string, context *fuse.Context) (*File, int, fuse
 	return file, file.intFd(), fuse.OK
 }
 
-// getXattr - read encrypted xattr name "cAttr" from the file at relative
+// getXattr - read encrypted xattr name "cAttr" from relative
 // plaintext path "relPath". Returns the encrypted xattr value.
 //
-// This function is symlink-safe.
+// This function is symlink-safe by using /proc/self/fd.
 func (fs *FS) getXattr(relPath string, cAttr string, context *fuse.Context) ([]byte, fuse.Status) {
 	file, fd, status := fs.getFileFd(relPath, context)
 	if !status.Ok() {
@@ -68,3 +69,18 @@ func (fs *FS) getXattr(relPath string, cAttr string, context *fuse.Context) ([]b
 	}
 	return cData, fuse.OK
 }
+
+// setXattr - set encrypted xattr name "cAttr" to value "cData" on plaintext
+// path "relPath".
+//
+// This function is symlink-safe by using /proc/self/fd.
+func (fs *FS) setXattr(relPath string, cAttr string, cData []byte, flags int, context *fuse.Context) fuse.Status {
+	file, fd, status := fs.getFileFd(relPath, context)
+	if !status.Ok() {
+		return status
+	}
+	defer file.Release()
+
+	err := xattr.SetWithFlags(procFd(fd), cAttr, cData, flags)
+	return unpackXattrErr(err)
+}