linux-kernel - Re: [RFC PATCH v3 0/4] x86: Add exception fixup for SGX ENCLU

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Date:   Tue, 11 Dec 2018 07:41:27 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Josh Triplett <josh@...htriplett.org>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        linux-sgx@...r.kernel.org,
        Haitao Huang <haitao.huang@...ux.intel.com>,
        Jethro Beekman <jethro@...tanix.com>,
        "Dr . Greg Wettstein" <greg@...ellic.com>
Subject: Re: [RFC PATCH v3 0/4] x86: Add exception fixup for SGX ENCLU



> On Dec 10, 2018, at 3:24 PM, Josh Triplett <josh@...htriplett.org> wrote:
> 
>> On Mon, Dec 10, 2018 at 03:21:37PM -0800, Sean Christopherson wrote:
>> At that point I realized it's a hell of a lot easier to simply provide
>> an IOCTL via /dev/sgx that allows userspace to register a per-process
>> ENCLU exception handler.  At a high level, the basic idea is the same
>> as the vDSO approach: provide a hardcoded fixup handler for ENCLU and
>> attempt to fixup select unhandled exceptions that occurred in user code.
> 
> So, on the one hand, this is *absolutely* much cleaner than the VDSO
> approach. On the other hand, this is global process state and has some
> of the same problems as a signal handler as a result.

I liked the old version better for this reason and for another reason: while this new one looks very very simple, it still has the hidden complexity that the magic values written to registers in the event of an exception are very much Linux specific.

OTOH, the old approach clobbered more regs than needed, but that’s a easy fix.