Re: [PATCH v2] x86: fix early boot crash on gcc-10

From: Borislav Petkov <bp@alien8.de>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Michael Matz <matz@suse.de>, Jakub Jelinek <jakub@redhat.com>,
	Sergei Trofimovich <slyfox@gentoo.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
	Andy Lutomirski <luto@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
	<x86@kernel.org>,
	clang-built-linux <clang-built-linux@googlegroups.com>
Subject: Re: [PATCH v2] x86: fix early boot crash on gcc-10
Date: Wed, 22 Apr 2020 12:23:09 +0200
Message-ID: <20200422102309.GA26846@zn.tnic> (raw)
In-Reply-To: <alpine.LSU.2.21.2004201401120.11688@wotan.suse.de>

Ok,

let's try the simple and clean fix first. Nick, would that work on LLVM
too?

And I hope this will remain working and the compiler won't jump over an
inline asm and go nuts.

Thx.

---

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 3b9bf8c7e29d..06d2e16bedbb 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -266,6 +266,13 @@ static void notrace start_secondary(void *unused)
 
 	wmb();
 	cpu_startup_entry(CPUHP_AP_ONLINE_IDLE);
+
+	/*
+	 * Prevent tail call to cpu_startup_entry() because the stack protector
+	 * guard has been changed in the middle of this function and must not be
+	 * checked before tail calling another function.
+	 */
+        asm ("");
 }
 
 /**

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette
