linux-kernel - Re: RFC: userspace exception fixups

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Message-ID: <20181102223757.GB24373@linux.intel.com>
Date:   Sat, 3 Nov 2018 00:37:57 +0200
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Rich Felker <dalias@...c.org>, Jann Horn <jannh@...gle.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Christopherson, Sean J" <sean.j.christopherson@...el.com>,
        Jethro Beekman <jethro@...tanix.com>,
        Florian Weimer <fweimer@...hat.com>,
        Linux API <linux-api@...r.kernel.org>, X86 ML <x86@...nel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>, nhorman@...hat.com,
        npmccallum@...hat.com, "Ayoun, Serge" <serge.ayoun@...el.com>,
        shay.katz-zamir@...el.com, linux-sgx@...r.kernel.org,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Carlos O'Donell <carlos@...hat.com>,
        adhemerval.zanella@...aro.org
Subject: Re: RFC: userspace exception fixups

On Thu, Nov 01, 2018 at 04:22:55PM -0700, Andy Lutomirski wrote:
> On Thu, Nov 1, 2018 at 2:24 PM Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > On Thu, Nov 1, 2018 at 12:31 PM Rich Felker <dalias@...c.org> wrote:
> > >
> > > See my other emails in this thread. You would register the *address*
> > > (in TLS) of a function pointer object pointing to the handler, rather
> > > than the function address of the handler. Then switching handler is
> > > just a single store in userspace, no syscalls involved.
> >
> > Yes.
> >
> > And for just EENTER, maybe that's the right model.
> >
> > If we want to generalize it to other thread-synchronous faults, it
> > needs way more information and a list of handlers, but if we limit the
> > thing to _only_ EENTER getting an SGX fault, then a single "this is
> > the fault handler" address is probably the right thing to do.
> 
> It sounds like you're saying that the kernel should know, *before*
> running any user fixup code, whether the fault in question is one that
> wants a fixup.  Sounds reasonable.
> 
> I think it would be nice, but not absolutely necessary, if user code
> didn't need to poke some value into TLS each time it ran a function
> that had a fixup.  With the poke-into-TLS approach, it looks a lot
> like rseq, and rseq doesn't nest very nicely.  I think we really want
> this mechanism to Just Work.  So we could maybe have a syscall that
> associates a list of fixups with a given range of text addresses.  We
> might want the kernel to automatically zap the fixups when the text in
> question is unmapped.

If we would have a syscall to specify a list fixups that would do the
job. Now essentially the only reason we require a vDSO is to implement
a single fixup for EENTER.

If this fixup stuff makes sense for other parts of the kernel,
introducing a vDSO for EENTER means essentially adding ABI to the kernel
that might possibly become legacy fast.

/Jarkko