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Date:   Sat, 25 Apr 2020 19:52:49 +0200
From:   Borislav Petkov <bp@alien8.de>
To:     Arvind Sankar <nivedita@alum.mit.edu>
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        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>, x86@kernel.org,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Kees Cook <keescook@chromium.org>,
        Martin =?utf-8?B?TGnFoWth?= <mliska@suse.cz>,
        =?utf-8?Q?Fr=C3=A9d=C3=A9ric_Pierret_=28fepitre=29?= 
        <frederic.pierret@qubes-os.org>, boris.ostrovsky@oracle.com,
        jgross@suse.com, linuxppc-dev@lists.ozlabs.org,
        Michael Ellerman <mpe@ellerman.id.au>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>
Subject: Re: [PATCH] x86: Fix early boot crash on gcc-10, next try
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On Sat, Apr 25, 2020 at 07:31:40PM +0200, Borislav Petkov wrote:
> Hmm, that's what I was afraid of - having to sprinkle this around. Yah, let's
> wait for compiler guys to have a look here and then maybe I'll convert that
> thing to a macro called
> 
> 	compiler_prevent_tail_call_opt()
> 
> or so, so that it can be sprinkled around. ;-\

IOW, something like this (ontop) which takes care of the xen case too.
If it needs to be used by all arches, then I'll split the patch:

---
diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 73bf8450afa1..4f275ac7830b 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -273,7 +273,7 @@ static void notrace start_secondary(void *unused)
 	 * boot_init_stack_canary() and must not be checked before tail calling
 	 * another function.
 	 */
-	asm ("");
+	prevent_tail_call_optimization();
 }
 
 /**
diff --git a/arch/x86/xen/smp_pv.c b/arch/x86/xen/smp_pv.c
index 8fb8a50a28b4..f2adb63b2d7c 100644
--- a/arch/x86/xen/smp_pv.c
+++ b/arch/x86/xen/smp_pv.c
@@ -93,6 +93,7 @@ asmlinkage __visible void cpu_bringup_and_idle(void)
 	cpu_bringup();
 	boot_init_stack_canary();
 	cpu_startup_entry(CPUHP_AP_ONLINE_IDLE);
+	prevent_tail_call_optimization();
 }
 
 void xen_smp_intr_free_pv(unsigned int cpu)
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 034b0a644efc..73f889f64513 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -356,4 +356,7 @@ static inline void *offset_to_ptr(const int *off)
 /* &a[0] degrades to a pointer: a different type from an array */
 #define __must_be_array(a)	BUILD_BUG_ON_ZERO(__same_type((a), &(a)[0]))
 
+
+#define prevent_tail_call_optimization()	asm("")
+
 #endif /* __LINUX_COMPILER_H */


-- 
Regards/Gruss,
    Boris.

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