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

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Message-Id: <20200629061840.4065483-6-keescook@chromium.org>
Date:   Sun, 28 Jun 2020 23:18:28 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Will Deacon <will@...nel.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Dave Martin <Dave.Martin@....com>,
        clang-built-linux@...glegroups.com,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        Peter Collingbourne <pcc@...gle.com>,
        James Morse <james.morse@....com>,
        Borislav Petkov <bp@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Russell King <linux@...linux.org.uk>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Arnd Bergmann <arnd@...db.de>, x86@...nel.org,
        linux-arch@...r.kernel.org, linux-efi@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: [PATCH v4 05/17] ctype: Work around Clang -mbranch-protection=none bug

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@...nel.org>
Cc: Will Deacon <will@...nel.org>
Cc: Dave Martin <Dave.Martin@....com>
Cc: clang-built-linux@...glegroups.com
Signed-off-by: Kees Cook <keescook@...omium.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