linux-kernel - [PATCH v2 01/24] x86/xen: Mark cpu_bringup_and_idle() as dead_end

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Date:   Thu, 24 Jun 2021 11:41:00 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     jpoimboe@...hat.com, tglx@...utronix.de
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        joro@...tes.org, boris.ostrovsky@...cle.com, jgross@...e.com,
        x86@...nel.org, mbenes@...e.com, rostedt@...dmis.org,
        dvyukov@...gle.com, elver@...gle.com
Subject: [PATCH v2 01/24] x86/xen: Mark cpu_bringup_and_idle() as dead_end_function

The asm_cpu_bringup_and_idle() function is required to push the return
value on the stack in order to make ORC happy, but the only reason
objtool doesn't complain is because of a happy accident.

The thing is that asm_cpu_bringup_and_idle() doesn't return, so
validate_branch() never terminates and falls through to the next
function, which in the normal case is the hypercall_page. And that, as
it happens, is 4095 NOPs and a RET.

Make asm_cpu_bringup_and_idle() terminate on it's own, by making the
function it calls as a dead-end. This way we no longer rely on what
code happens to come after.

Fixes: c3881eb58d56 ("x86/xen: Make the secondary CPU idle tasks reliable")
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 tools/objtool/check.c |    1 +
 1 file changed, 1 insertion(+)

--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -177,6 +177,7 @@ static bool __dead_end_function(struct o
 		"rewind_stack_do_exit",
 		"kunit_try_catch_throw",
 		"xen_start_kernel",
+		"cpu_bringup_and_idle",
 	};

 	if (!func)