Re: RFC: userspace exception fixups

[Jarkko Sakkinen]

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@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > On Thu, Nov 1, 2018 at 12:31 PM Rich Felker <dalias@xxxxxxxx> 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