linux-kernel - Re: RFC: userspace exception fixups

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Date:   Thu, 1 Nov 2018 15:31:07 -0400
From:   Rich Felker <dalias@...c.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Jann Horn <jannh@...gle.com>, luto@...nel.org,
        dave.hansen@...ux.intel.com, sean.j.christopherson@...el.com,
        jethro@...tanix.com, jarkko.sakkinen@...ux.intel.com,
        fweimer@...hat.com, linux-api@...r.kernel.org, x86@...nel.org,
        linux-arch@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>, nhorman@...hat.com,
        npmccallum@...hat.com, serge.ayoun@...el.com,
        shay.katz-zamir@...el.com, linux-sgx@...r.kernel.org,
        andriy.shevchenko@...ux.intel.com, tglx@...utronix.de,
        Ingo Molnar <mingo@...hat.com>, bp@...en8.de,
        carlos@...hat.com, adhemerval.zanella@...aro.org
Subject: Re: RFC: userspace exception fixups

On Thu, Nov 01, 2018 at 12:10:35PM -0700, Linus Torvalds wrote:
> On Thu, Nov 1, 2018 at 11:52 AM Rich Felker <dalias@...c.org> wrote:
> >
> > There's no need to chain if the handler is specific to the context
> > where the fault happens. You just replace the handler with the one
> > relevant to the code you're about to run before you run it.
> 
> That's much too expensive to do as a system call.

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.

Rich