Re: [PATCH v18 24/25] x86/cet/shstk: Add arch_prctl functions for shadow stack

[Yu, Yu-cheng]

On 1/29/2021 10:56 AM, Yu, Yu-cheng wrote:
> On 1/29/2021 9:07 AM, Dave Hansen wrote:
> > On 1/27/21 1:25 PM, Yu-cheng Yu wrote:
> > > arch_prctl(ARCH_X86_CET_STATUS, u64 *args)
> > >      Get CET feature status.
> > >
> > >      The parameter 'args' is a pointer to a user buffer.  The kernel 
> > > returns
> > >      the following information:
> > >
> > >      *args = shadow stack/IBT status
> > >      *(args + 1) = shadow stack base address
> > >      *(args + 2) = shadow stack size

[...]

> > > +int prctl_cet(int option, u64 arg2)
> > > +{
> > > +    struct cet_status *cet;
> > > +    unsigned int features;
> > > +
> > > +    /*
> > > +     * GLIBC's ENOTSUPP == EOPNOTSUPP == 95, and it does not recognize
> > > +     * the kernel's ENOTSUPP (524).  So return EOPNOTSUPP here.
> > > +     */
> > > +    if (!IS_ENABLED(CONFIG_X86_CET))
> > > +        return -EOPNOTSUPP;
> >
> > Let's ignore glibc for a moment.  What error code *should* the kernel be
> > returning here?  errno(3) says:
> >
> >         EOPNOTSUPP      Operation not supported on socket (POSIX.1)
> > ...
> >         ENOTSUP         Operation not supported (POSIX.1)
>
> Yeah, other places in kernel use ENOTSUPP.  This seems to be out of 
> line.  And since the issue is long-existing, applications already know 
> how to deal with it.  I should have made that argument.  Change it to 
> ENOTSUPP.

When I make the change, checkpatch says...

WARNING: ENOTSUPP is not a SUSV4 error code, prefer EOPNOTSUPP
#128: FILE: arch/x86/kernel/cet_prctl.c:33:
+		return -ENOTSUPP;

Do we want to reconsider?

[...]