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Date:   Wed, 22 Apr 2020 12:23:09 +0200
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
Message-ID: <20200422102309.GA26846@zn.tnic>
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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