linux-kernel - [PATCH 5.15 334/917] x86/xen: Mark cpu_bringup_and_idle() as dead_end

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Message-Id: <20211115165440.083725329@linuxfoundation.org>
Date:   Mon, 15 Nov 2021 17:57:09 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Juergen Gross <jgross@...e.com>,
        Miroslav Benes <mbenes@...e.cz>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 5.15 334/917] x86/xen: Mark cpu_bringup_and_idle() as dead_end_function

From: Peter Zijlstra <peterz@...radead.org>

[ Upstream commit 9af9dcf11bda3e2c0e24c1acaacb8685ad974e93 ]

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
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 tools/objtool/check.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 06b5c164ae931..8bffc004f4e53 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)
-- 
2.33.0