linux-kernel - Re: [PATCH] x86/mpx: Correctly report do_mpx_bt

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Date:   Thu, 20 Apr 2017 08:45:28 -0700
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Joerg Roedel <jroedel@...e.de>
Cc:     Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/mpx: Correctly report do_mpx_bt_fault() failures to
 user-space

On 04/20/2017 05:08 AM, Joerg Roedel wrote:
>> do_mpx_bt_fault() can fail for a bunch of reasons:
>>  * unexpected or invalid value in BNDCSR
>>  * out of memory (physical or virtual)
>>  * unresolvable fault walking/filling bounds tables
>>  * !valid and non-empty bad entry in the bounds tables
>>
>> This will end up sending a signal that *looks* like a X86_TRAP_BR for
>> all of those, including those that are not really bounds-related, like
>> unresolvable faults.  We also don't populate enough information in the
>> siginfo that gets delivered for userspace to resolve the fault.
>>
>> I'm not sure this patch is the right thing.
> 
> The problem is, without this patch the trap_nr reported to user-space is
> 0, which maps to divide-by-zero. I think this is wrong, and since all
> failure cases from do_mpx_bt_fault() can only happen in the #BR
> exception handler, I think that reporting X86_TRAP_BR for all failure
> cases is the right thing to do.

Urg, that does sound bogus.

How about doing X86_TRAP_PF?  That would at least be consistent with
SIGBUS, which is probably the closest thing to a generic error code that
we have.