Re: [PATCH v2 23/39] x86: Introduce userspace API for CET enabling

[Edgecombe, Rick P]

On Wed, 2022-10-12 at 14:18 +0200, Florian Weimer wrote:
> * Rick P. Edgecombe:
> 
> > On Mon, 2022-10-10 at 12:56 +0200, Florian Weimer wrote:
> > > > +     /* Only support enabling/disabling one feature at a time.
> > > > */
> > > > +     if (hweight_long(features) > 1)
> > > > +             return -EINVAL;
> > > 
> > > This means we'll soon need three extra system calls for x86-64
> > > process
> > > start: SHSTK, IBT, and switching off vsyscall emulation.  (The
> > > latter
> > > does not need any special CPU support.)
> > > 
> > > Maybe we can do something else instead to make the strace output
> > > a
> > > little bit cleaner?
> > 
> > In previous versions it supported enabling multiple features in a
> > single syscall. Thomas Gleixner pointed out that (this was on the
> > LAM
> > patchset that shared the interface at the time) it makes the
> > behavior
> > of what to do when one feature fails to enable complicated:
> > 
> > https://lore.kernel.org/lkml/87zgjjqico.ffs@tglx/
> 
> Can we return the bits for the features that were actually enabled?

Actually that specific option is covered in that thread as well. I was
thinking we would need to pass a struct in an out to do a batch
operation. Thomas suggested it could be added later and to start with a
simpler option. Is an extra syscall or two at startup really a big
problem?

> Those three don't have cross-dependencies in the sense that you would
> only use X & Y together, but not X or Y alone.

I don't fully follow this, but WRSS does actually depend on SHSTK.