Re: [PATCH v10 26/26] x86/cet/shstk: Add arch_prctl functions for shadow stack

[Yu-cheng Yu]

On Fri, 2020-05-22 at 19:29 +0200, Eugene Syromiatnikov wrote:
> On Fri, May 22, 2020 at 10:17:43AM -0700, Yu-cheng Yu wrote:
> > On Thu, 2020-05-21 at 15:42 -0700, Kees Cook wrote:
> > > On Wed, Apr 29, 2020 at 03:07:32PM -0700, Yu-cheng Yu wrote:
> > [...]
> > > > +
> > > > +int prctl_cet(int option, u64 arg2)
> > > > +{
> > > > +	struct cet_status *cet;
> > > > +
> > > > +	if (!IS_ENABLED(CONFIG_X86_INTEL_CET))
> > > > +		return -EINVAL;
> > > 
> > > Using -EINVAL here means userspace can't tell the difference between an
> > > old kernel and a kernel not built with CONFIG_X86_INTEL_CET. Perhaps
> > > -ENOTSUPP?
> > 
> > Looked into this.  The kernel and GLIBC are not in sync.  So maybe we still use
> > EINVAL here?
> > 
> > Yu-cheng
> > 
> > 
> > 
> > In kernel:
> > ----------
> > 
> > #define EOPNOTSUPP	95
> > #define ENOTSUPP 	524
> > 
> > In GLIBC:
> > ---------
> > 
> > printf("ENOTSUP=%d\n", ENOTSUP);
> > printf("EOPNOTSUPP=%d\n", EOPNOTSUPP);
> > printf("%s=524\n", strerror(524));
> >  
> > ENOTSUP=95
> > EOPNOTSUPP=95
> > Unknown error 524=524
> 
> EOPNOTSUPP/ENOTSUP/ENOTSUPP is actually a mess, it's summarized recently
> by Michael Kerrisk[1].  From the kernel's point of view, I think it
> would be reasonable to return EOPNOTSUPP, and expect that the userspace
> would use ENOTSUP to match against it.

Ok, use EOPNOTSUPP and add a comment why.

Yu-cheng