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From:   Andy Lutomirski <luto@kernel.org>
Date:   Thu, 1 Nov 2018 16:22:55 -0700
Message-ID: <CALCETrWe4+apXJNswHAKVVqajGS3jTEKxdd2r3iu-MzGK1v0DA@mail.gmail.com>
Subject: Re: RFC: userspace exception fixups
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Rich Felker <dalias@libc.org>, Jann Horn <jannh@google.com>,
        Andrew Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "Christopherson, Sean J" <sean.j.christopherson@intel.com>,
        Jethro Beekman <jethro@fortanix.com>,
        Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>,
        Florian Weimer <fweimer@redhat.com>,
        Linux API <linux-api@vger.kernel.org>, X86 ML <x86@kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>, nhorman@redhat.com,
        npmccallum@redhat.com, "Ayoun, Serge" <serge.ayoun@intel.com>,
        shay.katz-zamir@intel.com, linux-sgx@vger.kernel.org,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "Carlos O'Donell" <carlos@redhat.com>,
        adhemerval.zanella@linaro.org
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On Thu, Nov 1, 2018 at 2:24 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Thu, Nov 1, 2018 at 12:31 PM Rich Felker <dalias@libc.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.