Re: RFC: userspace exception fixups

[Jarkko Sakkinen]

On Mon, Nov 19, 2018 at 05:17:26AM +0000, Jethro Beekman wrote:
> On 2018-11-18 18:32, Jarkko Sakkinen wrote:
> > On Sun, Nov 18, 2018 at 09:15:48AM +0200, Jarkko Sakkinen wrote:
> > > On Thu, Nov 01, 2018 at 10:53:40AM -0700, Andy Lutomirski wrote:
> > > > Hi all-
> > > > 
> > > > The people working on SGX enablement are grappling with a somewhat
> > > > annoying issue: the x86 EENTER instruction is used from user code and
> > > > can, as part of its normal-ish operation, raise an exception.  It is
> > > > also highly likely to be used from a library, and signal handling in
> > > > libraries is unpleasant at best.
> > > > 
> > > > There's been some discussion of adding a vDSO entry point to wrap
> > > > EENTER and do something sensible with the exceptions, but I'm
> > > > wondering if a more general mechanism would be helpful.
> > > 
> > > I haven't really followed all of this discussion because I've been busy
> > > working on the patch set but for me all of these approaches look awfully
> > > complicated.
> > > 
> > > I'll throw my own suggestion and apologize if this has been already
> > > suggested and discarded: return-to-AEP.
> > > 
> > > My idea is to do just a small extension to SGX AEX handling. At the
> > > moment hardware will RAX, RBX and RCX with ERESUME parameters. We can
> > > fill extend this by filling other three spare registers with exception
> > > information.
> > > 
> > > AEP handler can then do whatever it wants to do with this information
> > > or just do ERESUME.
> > 
> > A correction here. In practice this will add a requirement to have a bit
> > more complicated AEP code (check the regs for exceptions) than before
> > and not just bytes for ENCLU.
> > 
> > e.g. AEP handler should be along the lines
> > 
> > 1. #PF (or #UD or) happens. Kernel fills the registers when it cannot
> >     handle the exception and returns back to user space i.e. to the
> >     AEP handler.
> > 2. Check the registers containing exception information. If they have
> >     been filled, take whatever actions user space wants to take.
> > 3. Otherwise, just ERESUME.
> > 
> >  From my point of view this is making the AEP parameter useful. Its
> > standard use is just weird (always point to a place just containing
> > ENCLU bytes, why the heck it even exists).
> 
> I like this solution. Keeps things simple. One question: when an exception
> occurs, how does the kernel know whether to set special registers or send a
> signal?

Yes, and AFAIK people do in many cases people want to do something else
than just direct ERESUME in AEP handler so would neither be a major
bummer for user space. If I remember correctly you have such?

You can check the cases that we have for SIGSEGV (namely EPCM conflict)
from Sean's patch 08/23.

I'm open for expanding the scope. It is the easy part after there is
consensus for the handling mechanism :-)

/Jarkko