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Date:   Wed, 13 Oct 2021 11:16:46 -0700
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Subject: [PATCH v5 03/15] linkage: Add DECLARE_NOT_CALLED_FROM_C
From:   Sami Tolvanen <samitolvanen@google.com>
To:     x86@kernel.org
Cc:     Kees Cook <keescook@chromium.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Sedat Dilek <sedat.dilek@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        linux-hardening@vger.kernel.org, linux-kernel@vger.kernel.org,
        llvm@lists.linux.dev, Sami Tolvanen <samitolvanen@google.com>
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The kernel has several assembly functions, which are not directly
callable from C but need to be referred to from C code. This change adds
the DECLARE_NOT_CALLED_FROM_C macro, which allows us to declare these
symbols using an opaque type, which makes misuse harder, and avoids the
need to annotate references to the functions for Clang's Control-Flow
Integrity (CFI).

Suggested-by: Andy Lutomirski <luto@amacapital.net>
Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
---
 include/linux/linkage.h | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/include/linux/linkage.h b/include/linux/linkage.h
index dbf8506decca..f982d5f550ac 100644
--- a/include/linux/linkage.h
+++ b/include/linux/linkage.h
@@ -48,6 +48,19 @@
 #define __PAGE_ALIGNED_DATA	.section ".data..page_aligned", "aw"
 #define __PAGE_ALIGNED_BSS	.section ".bss..page_aligned", "aw"
 
+/*
+ * Declares a function not callable from C using an opaque type. Defined as
+ * an array to allow the address of the symbol to be taken without '&'.
+ */
+#ifndef DECLARE_NOT_CALLED_FROM_C
+#define DECLARE_NOT_CALLED_FROM_C(sym) \
+	extern const u8 sym[]
+#endif
+
+#ifndef __ASSEMBLY__
+typedef const u8 *asm_func_ptr;
+#endif
+
 /*
  * This is used by architectures to keep arguments on the stack
  * untouched by the compiler by keeping them live until the end.
-- 
2.33.0.1079.g6e70778dc9-goog