Update system/xattrs_linux.go

- Add Llistxattr() support
- Additionally cleanup Lgetxattr() and implement it properly
  in accordance with getxattr() syscall behavior.

Signed-off-by: Harshavardhana <fharshav@redhat.com>

system/xattrs_linux.go

@@ -5,8 +5,35 @@ import (
 	"unsafe"
 )
 
-// Returns a nil slice and nil error if the xattr is not set
+var _zero uintptr
+
+// Returns the size of xattrs and nil error
+// Requires path, takes allocated []byte or nil as last argument
+func Llistxattr(path string, dest []byte) (size int, err error) {
+	pathBytes, err := syscall.BytePtrFromString(path)
+	if err != nil {
+		return -1, err
+	}
+	var newpathBytes unsafe.Pointer
+	if len(dest) > 0 {
+		newpathBytes = unsafe.Pointer(&dest[0])
+	} else {
+		newpathBytes = unsafe.Pointer(&_zero)
+	}
+
+	_size, _, errno := syscall.Syscall6(syscall.SYS_LLISTXATTR, uintptr(unsafe.Pointer(pathBytes)), uintptr(newpathBytes), uintptr(len(dest)), 0, 0, 0)
+	size = int(_size)
+	if errno != 0 {
+		return -1, errno
+	}
+
+	return size, nil
+}
+
+// Returns a []byte slice if the xattr is set and nil otherwise
+// Requires path and its attribute as arguments
 func Lgetxattr(path string, attr string) ([]byte, error) {
+	var sz int
 	pathBytes, err := syscall.BytePtrFromString(path)
 	if err != nil {
 		return nil, err
@@ -16,26 +43,39 @@ func Lgetxattr(path string, attr string) ([]byte, error) {
 		return nil, err
 	}
 
-	dest := make([]byte, 128)
+	// Start with a 128 length byte array
+	sz = 128
+	dest := make([]byte, sz)
 	destBytes := unsafe.Pointer(&dest[0])
-	sz, _, errno := syscall.Syscall6(syscall.SYS_LGETXATTR, uintptr(unsafe.Pointer(pathBytes)), uintptr(unsafe.Pointer(attrBytes)), uintptr(destBytes), uintptr(len(dest)), 0, 0)
-	if errno == syscall.ENODATA {
-		return nil, nil
-	}
-	if errno == syscall.ERANGE {
+	_sz, _, errno := syscall.Syscall6(syscall.SYS_LGETXATTR, uintptr(unsafe.Pointer(pathBytes)), uintptr(unsafe.Pointer(attrBytes)), uintptr(destBytes), uintptr(len(dest)), 0, 0)
+
+	switch {
+	case errno == syscall.ENODATA:
+		return nil, errno
+	case errno == syscall.ENOTSUP:
+		return nil, errno
+	case errno == syscall.ERANGE:
+		// 128 byte array might just not be good enough,
+		// A dummy buffer is used ``uintptr(0)`` to get real size
+		// of the xattrs on disk
+		_sz, _, errno = syscall.Syscall6(syscall.SYS_LGETXATTR, uintptr(unsafe.Pointer(pathBytes)), uintptr(unsafe.Pointer(attrBytes)), uintptr(unsafe.Pointer(nil)), uintptr(0), 0, 0)
+		sz = int(_sz)
+		if sz < 0 {
+			return nil, errno
+		}
 		dest = make([]byte, sz)
 		destBytes := unsafe.Pointer(&dest[0])
-		sz, _, errno = syscall.Syscall6(syscall.SYS_LGETXATTR, uintptr(unsafe.Pointer(pathBytes)), uintptr(unsafe.Pointer(attrBytes)), uintptr(destBytes), uintptr(len(dest)), 0, 0)
-	}
-	if errno != 0 {
+		_sz, _, errno = syscall.Syscall6(syscall.SYS_LGETXATTR, uintptr(unsafe.Pointer(pathBytes)), uintptr(unsafe.Pointer(attrBytes)), uintptr(destBytes), uintptr(len(dest)), 0, 0)
+		if errno != 0 {
+			return nil, errno
+		}
+	case errno != 0:
 		return nil, errno
 	}
-
+	sz = int(_sz)
 	return dest[:sz], nil
 }
 
-var _zero uintptr
-
 func Lsetxattr(path string, attr string, data []byte, flags int) error {
 	pathBytes, err := syscall.BytePtrFromString(path)
 	if err != nil {