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From:   Josh Poimboeuf <jpoimboe@kernel.org>
To:     x86@kernel.org
Cc:     linux-kernel@vger.kernel.org,
        Peter Zijlstra <peterz@infradead.org>,
        Miroslav Benes <mbenes@suse.cz>,
        Nick Desaulniers <ndesaulniers@google.com>
Subject: [PATCH v3 5/8] objtool: Ignore exc_double_fault() __noreturn warnings
Date:   Tue, 18 Apr 2023 14:27:51 -0700
Message-Id: <a45b085071d3a7d049a20f9e78754452336ecbe8.1681853186.git.jpoimboe@kernel.org>
In-Reply-To: <cover.1681853186.git.jpoimboe@kernel.org>
References: <cover.1681853186.git.jpoimboe@kernel.org>
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This is a hack, but it works for now.

Problem is, exc_double_fault() may or may not return, depending on
whether CONFIG_X86_ESPFIX64 is set.  But objtool has no visibility to
the kernel config.

"Fix" it by silencing the exc_double_fault() __noreturn warning.

This removes the following warning:

  vmlinux.o: warning: objtool: xenpv_exc_double_fault+0xd: exc_double_fault() is missing a __noreturn annotation

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
---
 tools/objtool/check.c | 31 ++++++++++++++++++++++++++++++-
 1 file changed, 30 insertions(+), 1 deletion(-)

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index dd10128ab013..1cace399d32a 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -4505,6 +4505,35 @@ static int validate_sls(struct objtool_file *file)
 	return warnings;
 }
 
+static bool ignore_noreturn_call(struct instruction *insn)
+{
+	struct symbol *call_dest = insn_call_dest(insn);
+
+	/*
+	 * FIXME: hack, we need a real noreturn solution
+	 *
+	 * Problem is, exc_double_fault() may or may not return, depending on
+	 * whether CONFIG_X86_ESPFIX64 is set.  But objtool has no visibility
+	 * to the kernel config.
+	 *
+	 * Other potential ways to fix it:
+	 *
+	 *   - have compiler communicate __noreturn functions somehow
+	 *   - remove CONFIG_X86_ESPFIX64
+	 *   - read the .config file
+	 *   - add a cmdline option
+	 *   - create a generic objtool annotation format (vs a bunch of custom
+	 *     formats) and annotate it
+	 */
+	if (!strcmp(call_dest->name, "exc_double_fault")) {
+		/* prevent further unreachable warnings for the caller */
+		insn->sym->warned = 1;
+		return true;
+	}
+
+	return false;
+}
+
 static int validate_reachable_instructions(struct objtool_file *file)
 {
 	struct instruction *insn, *prev_insn;
@@ -4521,7 +4550,7 @@ static int validate_reachable_instructions(struct objtool_file *file)
 		prev_insn = prev_insn_same_sec(file, insn);
 		if (prev_insn && prev_insn->dead_end) {
 			call_dest = insn_call_dest(prev_insn);
-			if (call_dest) {
+			if (call_dest && !ignore_noreturn_call(prev_insn)) {
 				WARN_INSN(insn, "%s() is missing a __noreturn annotation",
 					  call_dest->name);
 				warnings++;
-- 
2.39.2