linux-kernel - Re: [PATCH 1/4] x86/thread_info: introduce ->ftrace_int3

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Date:   Sun, 28 Apr 2019 16:27:21 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Nicolai Stange <nstange@...e.de>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>,
        Petr Mladek <pmladek@...e.com>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Shuah Khan <shuah@...nel.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Juergen Gross <jgross@...e.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nayna Jain <nayna@...ux.ibm.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Andy Lutomirski <luto@...nel.org>,
        Joerg Roedel <jroedel@...e.de>, linux-kernel@...r.kernel.org,
        live-patching@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 1/4] x86/thread_info: introduce ->ftrace_int3_stack member


> On Apr 28, 2019, at 2:22 PM, Nicolai Stange <nstange@...e.de> wrote:
> 
> Steven Rostedt <rostedt@...dmis.org> writes:
> 
>> On Sun, 28 Apr 2019 10:41:10 -0700
>> Andy Lutomirski <luto@...capital.net> wrote:
>> 
>> 
>>>> 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.
>>>> 
>>> 
>>> That’s quite an assumption. I think your list should also contain
>>> exception, exceptions nested inside that exception, and machine
>>> check, at the very least.  I’m also wondering why irq and softirq are
>>> treated separately.
> 
> You're right, I missed the machine check case.
> 
> 
>> 4 has usually been the context count we choose. But I guess in theory,
>> if we get exceptions then I could potentially be more.
> 
> 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’t do any fancy masking, so #DB, NMI and regular IRQs should all be possible.

> 
> 
>> 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.
>> 
> 
> Thanks,
> 
> Nicolai
> 
> 
> -- 
> SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton,
> HRB 21284 (AG Nürnberg)