fusefrontend: Allow to set/remove xattr on directory without read permission.

Setting/removing extended attributes on directories was partially fixed with
commit eff35e60b63331e3e10f921792baa10b236a721d. However, on most file systems
it is also possible to do these operations without read access (see tests).

Since we cannot open a write-access fd to a directory, we have to use the
/proc/self/fd trick (already used for ListXAttr) for the other operations aswell.
For simplicity, let's separate the Linux and Darwin code again (basically revert
commit f320b76fd189a363a34bffe981aa67ab97df3362), and always use the
/proc/self/fd trick on Linux. On Darwin we use the best-effort approach with
openBackingFile() as a fallback.

More discussion about the available options is available in
https://github.com/rfjakob/gocryptfs/issues/308.

1 files changed, 9 insertions, 74 deletions

diff --git a/internal/fusefrontend/xattr.go b/internal/fusefrontend/xattr.go
index 2248bad..1de9cac 100644
--- a/internal/fusefrontend/xattr.go
+++ b/internal/fusefrontend/xattr.go
@@ -2,16 +2,11 @@
 package fusefrontend
 
 import (
-	"fmt"
-	"runtime"
 	"strings"
 	"syscall"
 
-	"golang.org/x/sys/unix"
-
 	"github.com/hanwen/go-fuse/fuse"
 
-	"github.com/rfjakob/gocryptfs/internal/syscallcompat"
 	"github.com/rfjakob/gocryptfs/internal/tlog"
 )
 
@@ -36,18 +31,11 @@ func (fs *FS) GetXAttr(relPath string, attr string, context *fuse.Context) ([]by
 		return nil, _EOPNOTSUPP
 	}
 
-	// O_NONBLOCK to not block on FIFOs.
-	fd, err := fs.openBackingFile(relPath, syscall.O_RDONLY|syscall.O_NONBLOCK)
-	if err != nil {
-		return nil, fuse.ToStatus(err)
-	}
-	defer syscall.Close(fd)
-
 	cAttr := fs.encryptXattrName(attr)
 
-	cData, err := syscallcompat.Fgetxattr(fd, cAttr)
-	if err != nil {
-		return nil, fuse.ToStatus(err)
+	cData, status := fs.getXAttr(relPath, cAttr, context)
+	if !status.Ok() {
+		return nil, status
 	}
 
 	data, err := fs.decryptXattrValue(cData)
@@ -69,26 +57,10 @@ func (fs *FS) SetXAttr(relPath string, attr string, data []byte, flags int, cont
 		return _EOPNOTSUPP
 	}
 
-	// O_NONBLOCK to not block on FIFOs.
-	fd, err := fs.openBackingFile(relPath, syscall.O_WRONLY|syscall.O_NONBLOCK)
-	// Directories cannot be opened read-write. Retry.
-	if err == syscall.EISDIR {
-		fd, err = fs.openBackingFile(relPath, syscall.O_RDONLY|syscall.O_DIRECTORY|syscall.O_NONBLOCK)
-	}
-	if err != nil {
-		return fuse.ToStatus(err)
-	}
-	defer syscall.Close(fd)
-
 	flags = filterXattrSetFlags(flags)
 	cAttr := fs.encryptXattrName(attr)
 	cData := fs.encryptXattrValue(data)
-
-	err = unix.Fsetxattr(fd, cAttr, cData, flags)
-	if err != nil {
-		return fuse.ToStatus(err)
-	}
-	return fuse.OK
+	return fs.setXAttr(relPath, cAttr, cData, flags, context)
 }
 
 // RemoveXAttr - FUSE call.
@@ -102,23 +74,8 @@ func (fs *FS) RemoveXAttr(relPath string, attr string, context *fuse.Context) fu
 		return _EOPNOTSUPP
 	}
 
-	// O_NONBLOCK to not block on FIFOs.
-	fd, err := fs.openBackingFile(relPath, syscall.O_WRONLY|syscall.O_NONBLOCK)
-	// Directories cannot be opened read-write. Retry.
-	if err == syscall.EISDIR {
-		fd, err = fs.openBackingFile(relPath, syscall.O_RDONLY|syscall.O_DIRECTORY|syscall.O_NONBLOCK)
-	}
-	if err != nil {
-		return fuse.ToStatus(err)
-	}
-	defer syscall.Close(fd)
-
 	cAttr := fs.encryptXattrName(attr)
-	err = unix.Fremovexattr(fd, cAttr)
-	if err != nil {
-		return fuse.ToStatus(err)
-	}
-	return fuse.OK
+	return fs.removeXAttr(relPath, cAttr, context)
 }
 
 // ListXAttr - FUSE call. Lists extended attributes on the file at "relPath".
@@ -128,32 +85,10 @@ func (fs *FS) ListXAttr(relPath string, context *fuse.Context) ([]string, fuse.S
 	if fs.isFiltered(relPath) {
 		return nil, fuse.EPERM
 	}
-	var cNames []string
-	var err error
-	if runtime.GOOS == "linux" {
-		dirfd, cName, err2 := fs.openBackingDir(relPath)
-		if err2 != nil {
-			return nil, fuse.ToStatus(err2)
-		}
-		defer syscall.Close(dirfd)
-		procPath := fmt.Sprintf("/proc/self/fd/%d/%s", dirfd, cName)
-		cNames, err = syscallcompat.Llistxattr(procPath)
-	} else {
-		// O_NONBLOCK to not block on FIFOs.
-		fd, err2 := fs.openBackingFile(relPath, syscall.O_RDONLY|syscall.O_NONBLOCK)
-		// On a symlink, openBackingFile fails with ELOOP. Let's pretend there
-		// can be no xattrs on symlinks, and always return an empty result.
-		if err2 == syscall.ELOOP {
-			return nil, fuse.OK
-		}
-		if err2 != nil {
-			return nil, fuse.ToStatus(err2)
-		}
-		defer syscall.Close(fd)
-		cNames, err = syscallcompat.Flistxattr(fd)
-	}
-	if err != nil {
-		return nil, fuse.ToStatus(err)
+
+	cNames, status := fs.listXAttr(relPath, context)
+	if !status.Ok() {
+		return nil, status
 	}
 
 	names := make([]string, 0, len(cNames))