linux-hardening - [PATCH v5 06/15] ftrace: Use an opaque type for functions not callable from C

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Message-Id: <20211013181658.1020262-7-samitolvanen@google.com>
Date:   Wed, 13 Oct 2021 11:16:49 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     x86@...nel.org
Cc:     Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev, Sami Tolvanen <samitolvanen@...gle.com>
Subject: [PATCH v5 06/15] ftrace: Use an opaque type for functions not
 callable from C

With CONFIG_CFI_CLANG, the compiler changes function references to
point to the CFI jump table. As ftrace_call, ftrace_regs_call, and
mcount_call are not called by C code, but are trampolines injected
as calls replacing the nops at the start of functions added by the
compiler, use DECLARE_NOT_CALLED_FROM_C to declare them.

Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
Tested-by: Nick Desaulniers <ndesaulniers@...gle.com>
Tested-by: Sedat Dilek <sedat.dilek@...il.com>
---
 include/linux/ftrace.h | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index 832e65f06754..c53a00b96ba9 100644
--- a/include/linux/ftrace.h
+++ b/include/linux/ftrace.h
@@ -578,9 +578,10 @@ extern void ftrace_replace_code(int enable);
 extern int ftrace_update_ftrace_func(ftrace_func_t func);
 extern void ftrace_caller(void);
 extern void ftrace_regs_caller(void);
-extern void ftrace_call(void);
-extern void ftrace_regs_call(void);
-extern void mcount_call(void);
+
+DECLARE_NOT_CALLED_FROM_C(ftrace_call);
+DECLARE_NOT_CALLED_FROM_C(ftrace_regs_call);
+DECLARE_NOT_CALLED_FROM_C(mcount_call);
 
 void ftrace_modify_all_code(int command);
 
-- 
2.33.0.1079.g6e70778dc9-goog