linux-kernel - Re: [RFC][PATCH 1/8] x86/mm: clarify hardware vs. software "error

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Message-Id: <E480E807-701D-43C6-A0AF-DF7829A701BC@amacapital.net>
Date:   Mon, 10 Sep 2018 14:17:56 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org, sean.j.christopherson@...el.com,
        peterz@...radead.org, tglx@...utronix.de, x86@...nel.org,
        luto@...nel.org
Subject: Re: [RFC][PATCH 1/8] x86/mm: clarify hardware vs. software "error_code"



> On Sep 10, 2018, at 1:07 PM, Dave Hansen <dave.hansen@...ux.intel.com> wrote:
> 
> On 09/07/2018 03:48 PM, Andy Lutomirski wrote:
>>> 
>>> For part of the page fault handler, "error_code" does exactly
>>> match PFEC.  But, during later parts, it diverges and starts to
>>> mean something a bit different.
>>> 
>>> Give it two names for its two jobs.
>> How hard would it be to just remove sw_error_code instead?  It seems
>> like it adds little value and much confusion.
> 
> I think it would be really nice to have hw_error_code stand by itself
> and be limited in scope to just __do_page_fault() and then have
> FAULT_FLAG_* for everything else.
> 
> But, I was a little scared off of that.  For one, I think we fill in
> signal info with error_code, which makes it nominally part of the ABI.
> So, I wanted to muck with it as little as possible in this set.
> 
> But, if we just said that
> 1. hw_error_code goes out to userspace, always, and

Nope, it’s an info leak. If the address is in kernel land (and not vsyscall), we must (and do, I believe) fake it.

> 2. We drive all kernel behavior off of FAULT_FLAG_*, not error_code,
>   I think we can get away with it.
> 
>> I’m also unconvinced that the warning is terribly useful. We’re going
>> to oops when this happens anyway.
> 
> One thing I wanted to get out of the warning was the contents of
> hw_error_code before we go screwing with it.  I also don't mind a nice,
> clarifying warning showing up just before an oops.  Maybe it could be a
> pr_warn/err() instead of a full warning?

Sure.