linux-kernel - Re: [PATCH] ARM: Fix null die() string for unhandled data and prefetch abort cases

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Message-ID: <20190725223248.GO1330@shell.armlinux.org.uk>
Date:   Thu, 25 Jul 2019 23:32:49 +0100
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     "George G. Davis" <george_davis@...tor.com>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Florian Fainelli <f.fainelli@...il.com>,
        Allison Randal <allison@...utok.net>,
        Souptick Joarder <jrdr.linux@...il.com>,
        "moderated list:ARM PORT" <linux-arm-kernel@...ts.infradead.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ARM: Fix null die() string for unhandled data and
 prefetch abort cases

On Thu, Jul 25, 2019 at 06:24:01PM -0400, George G. Davis wrote:
> Hello Russell,
> 
> On Thu, Jul 25, 2019 at 10:55:40PM +0100, Russell King - ARM Linux admin wrote:
> > On Thu, Jul 25, 2019 at 05:37:54PM -0400, George G. Davis wrote:
> > > Hello Russell,
> > > 
> > > Thanks for your prompt reply!
> > > 
> > > On Sat, Jul 20, 2019 at 01:30:23PM +0100, Russell King - ARM Linux admin wrote:
> > > > On Fri, Jul 19, 2019 at 10:32:55PM -0400, George G. Davis wrote:
> > > > > When an unhandled data or prefetch abort occurs, the die() string
> > > > > is empty resulting in backtrace messages similar to the following:
> > > > > 
> > > > > 	Internal error: : 1 [#1] PREEMPT SMP ARM
> > > > > 
> > > > > Replace the null string with the name of the abort handler in order
> > > > > to provide more meaningful hints as to the cause of the fault.
> > > > 
> > > > NAK.
> > > > 
> > > > We already print the cause of the abort earlier in the dump, and we've
> > > > also added a "cut here" marker to help people include all the necessary
> > > > information when reporting a problem.
> > > 
> > > For what it's worth, I often receive crash dumps which lack the pr_alert
> > > messages and only include the pr_emerg messages which this change would at
> > > least provide extra hints, since the "Internal error" as at EMERG level
> > > wereas the initial messages are only at ALERT level. It's subtle but for
> > > cases where the end user has set loglevel such that they only see EMERG
> > > messages, the change is helpful, to me at least.
> > > 
> > > > It's unfortunate that we have the additional colon in the oops dump,
> > > 
> > > Agreed, it's rather unfortunate that the string is NULL in these cases.
> > > 
> > > > but repeating the information that we've printed on one of the previous
> > > > two lines is really not necessary.
> > > 
> > > It depends on the loglevel the user has set. So perhaps it's not such a
> > > bad thing to repeat the information?
> > 
> > Or maybe we should arrange for consistent usage of the log levels?
> 
> Unfortunately, some of the users that I work with have very specific limits
> and requirements for kernel error message logging which are driven by
> performance and/or storage limitations. So it's not always possible to "arrange
> for consistent usage of the log levels" with some users. Meanwhile, these
> messages do show up in logs without the pre-able headers, lacking the string
> which is already available. It's hardly a big deal to re-use the same string,
> especially for the !user_mode(regs) case, where the kernel will oops at
> EMERG loglevel, leaving the NULL string as the reason. I can assure you that
> I've tried to convince these users to change the loglevel but they have their
> reasons for keeping it as they do and I'm unable to convince them otherwise.

Sorry, but I really don't buy this.

By your argument, we should get rid of the pre-amble headers because
they're "not useful" in your eyes...

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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