[Vm-dev] Robust FFI with Memory Protection Keys

Ben Coman btc at openinworld.com
Fri Jun 28 05:31:31 UTC 2019

Hi Eliot,

On Tue, 7 Aug 2018 at 03:32, Eliot Miranda <eliot.miranda at gmail.com> wrote:

> Hi Ben,
> > On Aug 5, 2018, at 6:34 PM, Ben Coman <btc at openinworld.com> wrote:
> >
> >
> >> On 5 August 2018 at 23:10, Eliot Miranda <eliot.miranda at gmail.com>
> wrote:
> >>
> >> Hi Ben,
> >>
> >>
> >>> On Aug 4, 2018, at 8:40 AM, Ben Coman <btc at openinworld.com> wrote:
> >>>
> >>>
> >>> A problem with FFI is that if a callout segfaults, all of memory
> >>> including that of the Image is suspect, and execution of the Image
> terminates.
> >>>
> >>> Occasionally I hunt around hoping to find technology to mitigate that
> problem.
> >>> Maybe this time in I found something... Memory Protection Keys [1]
> >>> Perhaps these could ensure Image memory safe when an FFI callout
> segfaults.
> >>>
> >>> IIUC the main problem with protecting Image memory on every FFI callout
> >>> is the time it would take update the flags on every page of Image
> memory.
> >>> Would being able to change the protection of a massive number of pages
> >>> with one syscall make it feasible to wrap them around FFI callouts?
> >>>
> >>> This may be useful at least where the FFI use is more about reuse of
> >>> existing functionality than about performance.
> >>> Or at least useful while someone is learning/experimenting with FFI for
> >>> the first time or while becoming familiar with some external library.
> >>> Further info at [2] & [3].
> >>
> >> I think there’s a much simpler improvement that doesn’t go this far.  I
> implemented it in VisualWorks and it’s been in production for more than a
> decade.  It should be easy to add to Cog.
> >> The idea is simply to add a flag that tracks if the VM is in an FFI
> call or not and to test this flag in the VM’s exception handlers for
> SIGBUS, SIGILL, SIGSEGV and their equivalents on Windows.  The exception
> handlers then respond when in an FFI call by failing the FFI call
> primitive, answering a primitive fail code that includes the exception
> information.  Recently we extended Cog’s failure codes to allow a
> structured object (I font have the details handy; I’ll check soon).  In
> this case we need a pc and/or address and an exception code.
> >> Would this approach satisfy you?
> >
> > That sounds good.  Although the argument I've seen is that a memory
> > access error means you "cant recover because you don't know what may
> have been corrupted"
> > I think its worthwhile to be optimistic that the Image may last a bit
> > longer to get more information about what call from the Image
> invoked the FFI failure.
> > And if you've been notified (e.g. via Growl message) you can still
> > take steps to move to a new Image if the current one is suspect.
> >> While it’s possible that an FFI call could damage the Smalltalk heap
> and VM state it’s often not the case do one wants to be able to at least
> reap the error code and hence identify where the error occurred, inspect
> the arguments to the call, etc.  It’s hence about having enough
> functionality to gather what information is available from outside the
> call, not about being able to continue for a long time after.
> > Also, the approach you suggest would be a pre-requisite for what I
> > suggested anyway, and make it easier to later experiment with MPKs.
> Cool.
> > Let me know what I can do to help (probably more capable on the testing
> side).
> Will do.  If you’re happy with C programming and the simulator part of it
> is adding a global flag variable, setting and u setting it in FFI calls and
> callbacks, and then responding to the flag in the exception handler.  The
> tricky bit is arranging failure.  That I can work on when time allows.

I believe you did some work on this catching of segfaults in FFI callouts
to return a primitive failure.
Where did it get up to?  Can you point me at the code that sets/tests this
flag and sets up the primitive failure?

cheers -ben
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