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Subject: Re: [PATCH 1/4] x86/thread_info: introduce ->ftrace_int3_stack member
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <87k1fdygcx.fsf@suse.de>
Date:   Sun, 28 Apr 2019 16:27:21 -0700
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Jiri Kosina <jikos@kernel.org>,
        Miroslav Benes <mbenes@suse.cz>,
        Petr Mladek <pmladek@suse.com>,
        Joe Lawrence <joe.lawrence@redhat.com>,
        Shuah Khan <shuah@kernel.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Mimi Zohar <zohar@linux.ibm.com>,
        Juergen Gross <jgross@suse.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Nayna Jain <nayna@linux.ibm.com>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        Andy Lutomirski <luto@kernel.org>,
        Joerg Roedel <jroedel@suse.de>, linux-kernel@vger.kernel.org,
        live-patching@vger.kernel.org, linux-kselftest@vger.kernel.org
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Message-Id: <C55DED25-C60D-4731-9A6B-92BDA8771766@amacapital.net>
References: <20190427100639.15074-1-nstange@suse.de> <20190427100639.15074-2-nstange@suse.de> <F6268634-96D3-4267-B739-6D375FAB26A8@amacapital.net> <20190428135143.09d35bb6@oasis.local.home> <87k1fdygcx.fsf@suse.de>
To:     Nicolai Stange <nstange@suse.de>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On Apr 28, 2019, at 2:22 PM, Nicolai Stange <nstange@suse.de> wrote:
>=20
> Steven Rostedt <rostedt@goodmis.org> writes:
>=20
>> On Sun, 28 Apr 2019 10:41:10 -0700
>> Andy Lutomirski <luto@amacapital.net> wrote:
>>=20
>>=20
>>>> Note that at any given point
>>>> in time, there can be at most four such call insn emulations pending:
>>>> namely at most one per "process", "irq", "softirq" and "nmi" context.
>>>>=20
>>>=20
>>> That=E2=80=99s quite an assumption. I think your list should also contai=
n
>>> exception, exceptions nested inside that exception, and machine
>>> check, at the very least.  I=E2=80=99m also wondering why irq and softir=
q are
>>> treated separately.
>=20
> You're right, I missed the machine check case.
>=20
>=20
>> 4 has usually been the context count we choose. But I guess in theory,
>> if we get exceptions then I could potentially be more.
>=20
> After having seen the static_call discussion, I'm in no way defending
> this limited approach here, but out of curiosity: can the code between
> the push onto the stack from ftrace_int3_handler() and the subsequent
> pop from the stub actually trigger an (non-#MC) exception? There's an
> iret inbetween, but that can fault only when returning to user space,
> correct?

IRET doesn=E2=80=99t do any fancy masking, so #DB, NMI and regular IRQs shou=
ld all be possible.

>=20
>=20
>> As for irq vs softirq, an interrupt can preempt a softirq. Interrupts
>> are enabled while softirqs are running. When sofirqs run, softirqs are
>> disabled to prevent nested softirqs. But interrupts are enabled again,
>> and another interrupt may come in while a softirq is executing.
>>=20
>=20
> Thanks,
>=20
> Nicolai
>=20
>=20
> --=20
> SUSE Linux GmbH, GF: Felix Imend=C3=B6rffer, Jane Smithard, Graham Norton,=

> HRB 21284 (AG N=C3=BCrnberg)