linux-hardening - [RFC PATCH v2 15/21] objtool: Don't warn about __cfi

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Date:   Fri, 13 May 2022 13:21:53 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     linux-kernel@...r.kernel.org
Cc:     Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Joao Moreira <joao@...rdrivepizza.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-hardening@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, llvm@...ts.linux.dev,
        Sami Tolvanen <samitolvanen@...gle.com>
Subject: [RFC PATCH v2 15/21] objtool: Don't warn about __cfi_ preambles
 falling through

The __cfi_ preambles contain valid instructions, which embed KCFI
type information in the following format:

  __cfi_function:
    int3
    int3
    mov <id>, %eax
    int3
    int3
  function:
    ...

While the preambles are STT_FUNC and contain valid instructions,
they are not executed and always fall through. Skip the warning for
them.

Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
---
 tools/objtool/check.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index ca5b74603008..88f005ae6dcc 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -3311,6 +3311,10 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
 		next_insn = next_insn_to_validate(file, insn);
 
 		if (func && insn->func && func != insn->func->pfunc) {
+			/* Ignore KCFI type preambles, which always fall through */
+			if (!strncmp(func->name, "__cfi_", 6))
+				return 0;
+
 			WARN("%s() falls through to next function %s()",
 			     func->name, insn->func->name);
 			return 1;
-- 
2.36.0.550.gb090851708-goog