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.

4 files changed, 172 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))
diff --git a/internal/fusefrontend/xattr_darwin.go b/internal/fusefrontend/xattr_darwin.go
index 8760480..741eb6c 100644
--- a/internal/fusefrontend/xattr_darwin.go
+++ b/internal/fusefrontend/xattr_darwin.go
@@ -3,6 +3,16 @@
 // Package fusefrontend interfaces directly with the go-fuse library.
 package fusefrontend
 
+import (
+	"syscall"
+
+	"golang.org/x/sys/unix"
+
+	"github.com/hanwen/go-fuse/fuse"
+
+	"github.com/rfjakob/gocryptfs/internal/syscallcompat"
+)
+
 func disallowedXAttrName(attr string) bool {
 	return false
 }
@@ -14,3 +24,71 @@ func filterXattrSetFlags(flags int) int {
 
 	return flags &^ XATTR_NOSECURITY
 }
+
+func (fs *FS) getXAttr(relPath string, cAttr string, context *fuse.Context) ([]byte, fuse.Status) {
+	// 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)
+
+	cData, err := syscallcompat.Fgetxattr(fd, cAttr)
+	if err != nil {
+		return nil, fuse.ToStatus(err)
+	}
+
+	return cData, fuse.OK
+}
+
+func (fs *FS) setXAttr(relPath string, cAttr string, cData []byte, flags int, context *fuse.Context) fuse.Status {
+	// 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)
+
+	err = unix.Fsetxattr(fd, cAttr, cData, flags)
+	return fuse.ToStatus(err)
+}
+
+func (fs *FS) removeXAttr(relPath string, cAttr string, context *fuse.Context) fuse.Status {
+	// 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)
+
+	err = unix.Fremovexattr(fd, cAttr)
+	return fuse.ToStatus(err)
+}
+
+func (fs *FS) listXAttr(relPath string, context *fuse.Context) ([]string, fuse.Status) {
+	// O_NONBLOCK to not block on FIFOs.
+	fd, err := 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 err == syscall.ELOOP {
+		return nil, fuse.OK
+	}
+	if err != nil {
+		return nil, fuse.ToStatus(err)
+	}
+	defer syscall.Close(fd)
+
+	cNames, err := syscallcompat.Flistxattr(fd)
+	if err != nil {
+		return nil, fuse.ToStatus(err)
+	}
+	return cNames, fuse.OK
+}
diff --git a/internal/fusefrontend/xattr_linux.go b/internal/fusefrontend/xattr_linux.go
index fd3b7e5..3a64412 100644
--- a/internal/fusefrontend/xattr_linux.go
+++ b/internal/fusefrontend/xattr_linux.go
@@ -4,7 +4,15 @@
 package fusefrontend
 
 import (
+	"fmt"
 	"strings"
+	"syscall"
+
+	"golang.org/x/sys/unix"
+
+	"github.com/hanwen/go-fuse/fuse"
+
+	"github.com/rfjakob/gocryptfs/internal/syscallcompat"
 )
 
 // Only allow the "user" namespace, block "trusted" and "security", as
@@ -19,3 +27,57 @@ func disallowedXAttrName(attr string) bool {
 func filterXattrSetFlags(flags int) int {
 	return flags
 }
+
+func (fs *FS) getXAttr(relPath string, cAttr string, context *fuse.Context) ([]byte, fuse.Status) {
+	dirfd, cName, err := fs.openBackingDir(relPath)
+	if err != nil {
+		return nil, fuse.ToStatus(err)
+	}
+	defer syscall.Close(dirfd)
+
+	procPath := fmt.Sprintf("/proc/self/fd/%d/%s", dirfd, cName)
+	cData, err := syscallcompat.Lgetxattr(procPath, cAttr)
+	if err != nil {
+		return nil, fuse.ToStatus(err)
+	}
+	return cData, fuse.OK
+}
+
+func (fs *FS) setXAttr(relPath string, cAttr string, cData []byte, flags int, context *fuse.Context) fuse.Status {
+	dirfd, cName, err := fs.openBackingDir(relPath)
+	if err != nil {
+		return fuse.ToStatus(err)
+	}
+	defer syscall.Close(dirfd)
+
+	procPath := fmt.Sprintf("/proc/self/fd/%d/%s", dirfd, cName)
+	err = unix.Lsetxattr(procPath, cAttr, cData, flags)
+	return fuse.ToStatus(err)
+}
+
+func (fs *FS) removeXAttr(relPath string, cAttr string, context *fuse.Context) fuse.Status {
+	dirfd, cName, err := fs.openBackingDir(relPath)
+	if err != nil {
+		return fuse.ToStatus(err)
+	}
+	defer syscall.Close(dirfd)
+
+	procPath := fmt.Sprintf("/proc/self/fd/%d/%s", dirfd, cName)
+	err = unix.Lremovexattr(procPath, cAttr)
+	return fuse.ToStatus(err)
+}
+
+func (fs *FS) listXAttr(relPath string, context *fuse.Context) ([]string, fuse.Status) {
+	dirfd, cName, err := fs.openBackingDir(relPath)
+	if err != nil {
+		return nil, fuse.ToStatus(err)
+	}
+	defer syscall.Close(dirfd)
+
+	procPath := fmt.Sprintf("/proc/self/fd/%d/%s", dirfd, cName)
+	cNames, err := syscallcompat.Llistxattr(procPath)
+	if err != nil {
+		return nil, fuse.ToStatus(err)
+	}
+	return cNames, fuse.OK
+}
diff --git a/internal/syscallcompat/sys_common.go b/internal/syscallcompat/sys_common.go
index 4ce0208..a1aa66d 100644
--- a/internal/syscallcompat/sys_common.go
+++ b/internal/syscallcompat/sys_common.go
@@ -84,6 +84,29 @@ func Fgetxattr(fd int, attr string) (val []byte, err error) {
 	return val, nil
 }
 
+// Lgetxattr is a wrapper around unix.Lgetxattr that handles the buffer sizing.
+func Lgetxattr(path string, attr string) (val []byte, err error) {
+	// See the buffer sizing comments in Fgetxattr.
+	// TODO: smarter buffer sizing?
+	buf := make([]byte, XATTR_BUFSZ)
+	sz, err := unix.Lgetxattr(path, attr, buf)
+	if err == syscall.ERANGE {
+		// Do NOT return ERANGE - the user might retry ad inifinitum!
+		return nil, syscall.EOVERFLOW
+	}
+	if err != nil {
+		return nil, err
+	}
+	if sz >= XATTR_SIZE_MAX {
+		return nil, syscall.EOVERFLOW
+	}
+	// Copy only the actually used bytes to a new (smaller) buffer
+	// so "buf" never leaves the function and can be allocated on the stack.
+	val = make([]byte, sz)
+	copy(val, buf)
+	return val, nil
+}
+
 // Flistxattr is a wrapper for unix.Flistxattr that handles buffer sizing and
 // parsing the returned blob to a string slice.
 func Flistxattr(fd int) (attrs []string, err error) {