linux-kernel - Re: [PATCH v20 22/28] x86/traps: Attempt to fixup exceptions in vDSO before signaling

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Message-ID: <20190625154341.GA7046@linux.intel.com>
Date:   Tue, 25 Jun 2019 18:43:41 +0300
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     linux-kernel@...r.kernel.org, x86@...nel.org,
        linux-sgx@...r.kernel.org, sean.j.christopherson@...el.com
Cc:     akpm@...ux-foundation.org, dave.hansen@...el.com,
        nhorman@...hat.com, npmccallum@...hat.com, serge.ayoun@...el.com,
        shay.katz-zamir@...el.com, haitao.huang@...el.com,
        andriy.shevchenko@...ux.intel.com, tglx@...utronix.de,
        kai.svahn@...el.com, bp@...en8.de, josh@...htriplett.org,
        luto@...nel.org, kai.huang@...el.com, rientjes@...gle.com,
        Andy Lutomirski <luto@...capital.net>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH v20 22/28] x86/traps: Attempt to fixup exceptions in vDSO
 before signaling

On Wed, Apr 17, 2019 at 01:39:32PM +0300, Jarkko Sakkinen wrote:
> From: Sean Christopherson <sean.j.christopherson@...el.com>
> 
> vDSO functions can now leverage an exception fixup mechanism similar to
> kernel exception fixup.  For vDSO exception fixup, the initial user is
> Intel's Software Guard Extensions (SGX), which will wrap the low-level
> transitions to/from the enclave, i.e. EENTER and ERESUME instructions,
> in a vDSO function and leverage fixup to intercept exceptions that would
> otherwise generate a signal.  This allows the vDSO wrapper to return the
> fault information directly to its caller, obviating the need for SGX
> applications and libraries to juggle signal handlers.
> 
> Attempt to fixup vDSO exceptions immediately prior to populating and
> sending signal information.  Except for the delivery mechanism, an
> exception in a vDSO function should be treated like any other exception
> in userspace, e.g. any fault that is successfully handled by the kernel
> should not be directly visible to userspace.
> 
> Although it's debatable whether or not all exceptions are of interest to
> enclaves, defer to the vDSO fixup to decide whether to do fixup or
> generate a signal.  Future users of vDSO fixup, if there ever are any,
> will undoubtedly have different requirements than SGX enclaves, e.g. the
> fixup vs. signal logic can be made function specific if/when necessary.
> 
> Suggested-by: Andy Lutomirski <luto@...capital.net>
> Cc: Andy Lutomirski <luto@...capital.net>
> Cc: Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: Josh Triplett <josh@...htriplett.org>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>

I went through the vDSO changes just to revisit couple of details that I
had forgotten. Sean, if you don't mind I'd squash this and prepending
patch.

Is there any obvious reason why #PF fixup is in its own patch and the
rest are collected to the same patch? I would not find it confusing if
there was one patch per exception but really don't get this division.

/Jarkko