linux-kernel - Re: [PATCH v10 00/18] Enable FSGSBASE instructions

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Date:   Sun, 10 May 2020 10:17:22 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Vegard Nossum <vegard.nossum@...cle.com>, tony.luck@...el.com,
        ak@...ux.intel.com, chang.seok.bae@...el.com, x86@...nel.org,
        linux-kernel@...r.kernel.org, luto@...nel.org
Subject: Re: [PATCH v10 00/18] Enable FSGSBASE instructions

On Sun, May 10, 2020 at 12:15:34PM +0200, Thomas Gleixner wrote:
>Vegard Nossum <vegard.nossum@...cle.com> writes:
>> On 5/10/20 10:09 AM, Vegard Nossum wrote:
>>
>> I spoke a few minutes too soon. Just hit this, if anybody wants to have
>> a look:
>>
>> [ 6402.786418] ------------[ cut here ]------------
>> [ 6402.787769] WARNING: CPU: 0 PID: 13802 at arch/x86/kernel/traps.c:811
>> do_debug+0x16c/0x210
>
>> [ 6402.820353] Call Trace:
>> [ 6402.821043]  <#DB>
>> [ 6402.821622]  debug+0x37/0x70
>> [ 6402.822449] RIP: 0010:arch_stack_walk_user+0x79/0x110
>
>That's a cute way to trigger that WARN_ON in the #DB handler.
>
>> [ 6402.816468] DR0: 0000000000000001 DR1: 0000000040006070 DR2: 00007ffff7ffd000
>> [ 6402.818406] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000003b3062a
>
>#DB recursion
>
>  [ 6402.832288] RDX: 0000000040006073
>
>27:	48 8b 1a      mov    (%rdx),%rbx
>
>Breakpoint on user space stack, #DB triggers and the low level ASM
>irqflags tracepoint has stacktrace enabled which unwinds into the user
>stack and triggers #DB again.
>
>Bah. I know why I want to ban all that tracing muck from low level entry code.
>
>> It might not be related to the patch set, mind.
>
>It's unrelated.

Thanks for testing Vegard!

-- 
Thanks,
Sasha