[PATCH v4 05/17] ctype: Work around Clang -mbranch-protection=none bug

Kees Cook
2020-06-29T19:13:48Z

In preparation for building efi/libstub with -mbranch-protection=none
(EFI does not support branch protection features[1]), add no-op code
to work around a Clang bug that emits an unwanted .note.gnu.property
section for object files without code[2].

[1] https://lore.kernel.org/lkml/CAMj1kXHck12juGi=E=P4hWP_8vQhQ+-x3vBMc3TGeRWdQ-XkxQ@mail.gmail.com
[2] https://bugs.llvm.org/show_bug.cgi?id=46480

Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Will Deacon <will@kernel.org>
Cc: Dave Martin <Dave.Martin@arm.com>
Cc: clang-built-linux@googlegroups.com
Signed-off-by: Kees Cook <keescook@chromium.org>
---
lib/ctype.c | 10 ++++++++++
1 file changed, 10 insertions(+)

diff --git a/lib/ctype.c b/lib/ctype.c
index c819fe269eb2..21245ed57d90 100644
--- a/lib/ctype.c
+++ b/lib/ctype.c
@@ -36,3 +36,13 @@ _L,_L,_L,_L,_L,_L,_L,_L,_L,_L,_L,_L,_L,_L,_L,_L, /* 224-239 */
_L,_L,_L,_L,_L,_L,_L,_P,_L,_L,_L,_L,_L,_L,_L,_L}; /* 240-255 */

EXPORT_SYMBOL(_ctype);
+
+/*
+ * Clang will generate .note.gnu.property sections for object files
+ * without code, even in the presence of -mbranch-protection=none.
+ * To work around this, define an unused static function.
+ * https://bugs.llvm.org/show_bug.cgi?id=46480
+ */
+#ifdef CONFIG_CC_IS_CLANG
+void __maybe_unused __clang_needs_code_here(void) { }
+#endif
--
2.25.1