Subject: [PATCH 3/9] arm64: mm: install SError abort handler

From: [email-masked] (Mark Rutland)
Date: Fri, 24 Mar 2017 18:31:48 +0000

Hi Florian,

On Fri, Mar 24, 2017 at 10:53:48AM -0700, Florian Fainelli wrote:
> On 03/24/2017 10:35 AM, Mark Rutland wrote:
> > On Fri, Mar 24, 2017 at 09:48:40AM -0700, Doug Berger wrote:
> >> On 03/24/2017 08:16 AM, Mark Rutland wrote:
> >>> On Fri, Mar 24, 2017 at 07:46:26AM -0700, Doug Berger wrote:
> > 
> >> If you would consider an alternative implementation where we scrap
> >> the SError handler (i.e. maintain the ugliness in our downstream
> >> kernel) in favor of a more gentle user mode crash on SError that
> >> allows the kernel the opportunity to service the interrupt for
> >> diagnostic purposes I could try to repackage that.
> > 
> > If this is just for diagnostic purposes, I believe you can register a
> > panic notifier, which can then read from the bus. The panic will occur,
> > but you'll have the opportunity to log some information to dmesg.
> 
> And crash the kernel? That sounds awful, FWIW the ARM/Linux kernel is
> able to recover just fine from user-space accessing e.g: invalid
> physical addresses in the GISB register space, bringing the same level
> of functionality to ARM64/Linux sounds reasonable to me.

I disagree, given that:

(a) You cannot determine the (HW) origin of the SError in an
    architecturally portable way. i.e. when you take an SError, you have
    no way of determining what asynchronous event caused it.

(b) SError is effectively an edge-triggered interrupt for fatal system
    errors (e.g. it may be triggered in resonse to ECC errors,
    corruption detected in caches, etc). Even if you can determine that
    the GISB triggered *an* SError, this does not tell you that this was
    the *only* SError.

    If you take an SError, something bad has already happened. Your data
    may already have been corrupted, and worse, you don't know when or
    where specifically this occurred (nor how many times).
    
(c) You cannot determine the (SW) origin of an SError without relying
    upon implementation details. This cannot be written in a way that
    does not rely on microarchitecture, integration, etc, and would need
    to be updated for every future system with this misfeature.

(d) Even if you can determine the (SW) origin of an SError by relying on
    IMPLEMENTATION DEFINED details, your handler needs to be intimately
    familiar with the arch in question in order to attempt to recover.

    For example, the existing code tries to skip an ARM instruction in
    some cases. For arm64 there are three cases that would need to be
    handled (AArch64 A64, AArch32 A32/ARM, AArch32 T32/Thumb).

    Further, it appears to me that the existing code is broken given
    that it doesn't handle Thumb, and given that it's skipping an
    instruction in response to an asynchronous event -- i.e. some
    arbitrary instruction after the one which triggered the abort.

For better or worse, SError *must* be treated as fatal.

As Doug stated:

    The main benefit is to help debug user mode code that accidentally
    maps a bad address since we would never make such an egregious error
    in the kernel ;)

This is just one of many ways a userspace application with direct HW
access can bring down the system. I see no reason to treat it any
differently, especially given the above points.

Thanks,
Mark.