fusefrontend: make GetXAttr() symlink-safe on Linux

Uses the /proc/self/fd trick, which does not work
on Darwin.

1 files changed, 52 insertions, 1 deletions

diff --git a/internal/fusefrontend/xattr_linux.go b/internal/fusefrontend/xattr_linux.go
index 61b90e3..5a189db 100644
--- a/internal/fusefrontend/xattr_linux.go
+++ b/internal/fusefrontend/xattr_linux.go
@@ -3,7 +3,17 @@
 // Package fusefrontend interfaces directly with the go-fuse library.
 package fusefrontend
 
-import "strings"
+import (
+	"fmt"
+	"strings"
+	"syscall"
+
+	"github.com/hanwen/go-fuse/fuse"
+
+	"github.com/pkg/xattr"
+
+	"github.com/rfjakob/gocryptfs/internal/tlog"
+)
 
 // Only allow the "user" namespace, block "trusted" and "security", as
 // these may be interpreted by the system, and we don't want to cause
@@ -17,3 +27,44 @@ func disallowedXAttrName(attr string) bool {
 func filterXattrSetFlags(flags int) int {
 	return flags
 }
+
+func procFd(fd int) string {
+	return fmt.Sprintf("/proc/self/fd/%d", fd)
+}
+
+// getFileFd calls fs.Open() on relative plaintext path "relPath" and returns
+// the resulting fusefrontend.*File along with the underlying fd. The caller
+// MUST call file.Release() when done with the file.
+//
+// Used by xattrGet() and friends.
+func (fs *FS) getFileFd(relPath string, context *fuse.Context) (*File, int, fuse.Status) {
+	fuseFile, status := fs.Open(relPath, syscall.O_RDONLY, context)
+	if !status.Ok() {
+		return nil, -1, status
+	}
+	file, ok := fuseFile.(*File)
+	if !ok {
+		tlog.Warn.Printf("BUG: xattrGet: cast to *File failed")
+		fuseFile.Release()
+		return nil, -1, fuse.EIO
+	}
+	return file, file.intFd(), fuse.OK
+}
+
+// getXattr - read encrypted xattr name "cAttr" from the file at relative
+// plaintext path "relPath". Returns the encrypted xattr value.
+//
+// This function is symlink-safe.
+func (fs *FS) getXattr(relPath string, cAttr string, context *fuse.Context) ([]byte, fuse.Status) {
+	file, fd, status := fs.getFileFd(relPath, context)
+	if !status.Ok() {
+		return nil, status
+	}
+	defer file.Release()
+
+	cData, err := xattr.Get(procFd(fd), cAttr)
+	if err != nil {
+		return nil, unpackXattrErr(err)
+	}
+	return cData, fuse.OK
+}