linux-kernel - [tip: objtool/core] x86/xen: Mark cpu_bringup_and_idle() as dead_end

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Date:   Wed, 15 Sep 2021 15:49:35 -0000
From:   "tip-bot2 for Peter Zijlstra" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Juergen Gross <jgross@...e.com>,
        Miroslav Benes <mbenes@...e.cz>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: objtool/core] x86/xen: Mark cpu_bringup_and_idle() as dead_end_function

The following commit has been merged into the objtool/core branch of tip:

Commit-ID:     9af9dcf11bda3e2c0e24c1acaacb8685ad974e93
Gitweb:        https://git.kernel.org/tip/9af9dcf11bda3e2c0e24c1acaacb8685ad974e93
Author:        Peter Zijlstra <peterz@...radead.org>
AuthorDate:    Thu, 24 Jun 2021 11:41:00 +02:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Wed, 15 Sep 2021 15:51:44 +02:00

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>
Reviewed-by: Juergen Gross <jgross@...e.com>
Reviewed-by: Miroslav Benes <mbenes@...e.cz>
Link: https://lore.kernel.org/r/20210624095147.693801717@infradead.org
---
 tools/objtool/check.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index e5947fb..0e3981d 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -173,6 +173,7 @@ static bool __dead_end_function(struct objtool_file *file, struct symbol *func,
 		"rewind_stack_do_exit",
 		"kunit_try_catch_throw",
 		"xen_start_kernel",
+		"cpu_bringup_and_idle",
 	};
 
 	if (!func)