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Date:   Mon, 16 Mar 2020 21:08:55 +0100
From:   Jakub Jelinek <jakub@redhat.com>
To:     Arvind Sankar <nivedita@alum.mit.edu>
Cc:     Borislav Petkov <bp@alien8.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Sergei Trofimovich <slyfox@gentoo.org>,
        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>, x86@kernel.org
Subject: Re: [PATCH] x86: fix early boot crash on gcc-10
Message-ID: <20200316200855.GS2156@tucnak>
Reply-To: Jakub Jelinek <jakub@redhat.com>
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On Mon, Mar 16, 2020 at 03:53:41PM -0400, Arvind Sankar wrote:
> > /*
> >  * Initialize the stackprotector canary value.
> >  *
> >  * NOTE: this must only be called from functions that never return,
> >  * and it must always be inlined.
> >  */
> > static __always_inline void boot_init_stack_canary(void)
> 
> Ugh, gcc10 tail-call optimizes the cpu_startup_entry call, and so checks
> the canary before jumping out. The xen one will need to have stack
> protector disabled too. It doesn't optimize the arch_call_rest_init call
> in start_kernel for some reason, but we should probably disable it there
> too.

If you mark cpu_startup_entry with __attribute__((noreturn)), then gcc won't
tail call it.
If you don't, you could add asm (""); after the call to avoid the tail call
too.
> 
>      a06:       0f ae f8                sfence
>      a09:       48 8b 44 24 08          mov    0x8(%rsp),%rax
>      a0e:       65 48 2b 04 25 28 00    sub    %gs:0x28,%rax
>      a15:       00 00
>      a17:       75 1b                   jne    a34 <start_secondary+0x164>
>      a19:       48 83 c4 10             add    $0x10,%rsp
>      a1d:       bf 8d 00 00 00          mov    $0x8d,%edi
>      a22:       5b                      pop    %rbx
>      a23:       e9 00 00 00 00          jmpq   a28 <start_secondary+0x158>
>                         a24: R_X86_64_PLT32     cpu_startup_entry-0x4
>      a28:       0f 01 1d 00 00 00 00    lidt   0x0(%rip)        # a2f <start_secondary+0x15f>
>                         a2b: R_X86_64_PC32      debug_idt_descr-0x4
>      a2f:       e9 cc fe ff ff          jmpq   900 <start_secondary+0x30>
>      a34:       e8 00 00 00 00          callq  a39 <start_secondary+0x169>
>                         a35: R_X86_64_PLT32     __stack_chk_fail-0x4

	Jakub