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Subject: Re: [patch 15/14] x86/dumpstack/64: Speedup in_exception_stack()
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <alpine.DEB.2.21.1904022127210.1812@nanos.tec.linutronix.de>
Date:   Tue, 2 Apr 2019 18:36:00 -0600
Cc:     Josh Poimboeuf <jpoimboe@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>, x86@kernel.org,
        Andy Lutomirski <luto@kernel.org>
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Message-Id: <6205D576-694A-4C7D-B1B7-A9FF2E1F9E77@amacapital.net>
References: <20190331214020.836098943@linutronix.de> <20190331215136.039902969@linutronix.de> <alpine.DEB.2.21.1904021214340.1676@nanos.tec.linutronix.de> <20190402154329.scp7i7uqevubgwrz@treble> <alpine.DEB.2.21.1904021746080.1676@nanos.tec.linutronix.de> <7789E14E-C623-4DB7-B076-76B931957C9C@amacapital.net> <alpine.DEB.2.21.1904022027170.1812@nanos.tec.linutronix.de> <alpine.DEB.2.21.1904022127210.1812@nanos.tec.linutronix.de>
To:     Thomas Gleixner <tglx@linutronix.de>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On Apr 2, 2019, at 1:29 PM, Thomas Gleixner <tglx@linutronix.de> wrote:
>=20
>> On Tue, 2 Apr 2019, Thomas Gleixner wrote:
>> On Tue, 2 Apr 2019, Andy Lutomirski wrote:
>>>> On Apr 2, 2019, at 9:48 AM, Thomas Gleixner <tglx@linutronix.de> wrote:=

>>>>=20
>>>>>> On Tue, 2 Apr 2019, Josh Poimboeuf wrote:
>>>>>> On Tue, Apr 02, 2019 at 12:19:46PM +0200, Thomas Gleixner wrote:
>>>>>> +/*
>>>>>> + * Array of exception stack page descriptors. If the stack is larger=
 than
>>>>>> + * PAGE_SIZE, all pages covering a particular stack will have the sa=
me
>>>>>> + * info.
>>>>>> + */
>>>>>> +static const struct estack_pages estack_pages[ESTACK_PAGES] ____cach=
eline_aligned =3D {
>>>>>> +    [CONDRANGE(DF)]        =3D ESTACK_PAGE(DOUBLEFAULT_IST, DF),
>>>>>> +    [CONDRANGE(NMI)]    =3D ESTACK_PAGE(NMI_IST, NMI),
>>>>>> +    [PAGERANGE(DB)]        =3D ESTACK_PAGE(DEBUG_IST, DB),
>>>>>> +    [CONDRANGE(MCE)]    =3D ESTACK_PAGE(MCE_IST, MCE),
>>>>>=20
>>>>> It would be nice if the *_IST macro naming aligned with the struct
>>>>> cea_exception_stacks field naming.  Then you could just do, e.g.
>>>>> ESTACKPAGE(DF).
>>>>=20
>>>> Yes, lemme fix that up.
>>>>=20
>>>>> Also it's a bit unfortunate that some of the stack size knowledge is
>>>>> hard-coded here, i.e #DB always being > 1 page and non-#DB being
>>>>> sometimes 1 page.
>>>>=20
>>>> The problem is that there is no way to make this macro maze conditional=
 on
>>>> sizeof(). But my macro foo is rusty.
>>>=20
>>> How about a much better fix: make the DB stack be the same size as all
>>> the others and just have 4 of them (DB0, DB1, DB2, and DB3.  After all,
>>> overflowing from one debug stack into another is just as much of a bug a=
s
>>> overflowing into a different IST stack.
>>=20
>> That makes sense.
>=20
> Except that we just have two not four.
>=20
> It needs some tweaking of the ist_shift stuff in entry_64.S but that's not=

> rocket science. Famous last words....
>=20

The ist_shift mess should probably be in C, but that=E2=80=99s a big can of w=
orms. That being said, why do we have it at all?  Once upon a time, we=E2=80=
=99d do ICEBP from user mode (or a legit breakpoint), then send a signal and=
 hit a data breakpoint, and we=E2=80=99d recurse.  But we don=E2=80=99t run u=
ser debug handlers on the IST stack at all anymore.

Maybe we can convince ourselves it=E2=80=99s safe?

What we should do is check, on IST return, that we=E2=80=99re not about to r=
eturn to our own stack.  Then we can at least properly panic.=