Re: [RFC PATCH v4 6/9] x86/cet/ibt: Add arch_prctl functions for IBT

[Yu-cheng Yu]

On Thu, 2018-10-04 at 09:12 -0700, Andy Lutomirski wrote:
> On Thu, Oct 4, 2018 at 9:08 AM Florian Weimer <fw@xxxxxxxxxxxxx> wrote:
> > 
> > * Yu-cheng Yu:
> > 
> > > On Thu, 2018-10-04 at 15:28 +0200, Eugene Syromiatnikov wrote:
> > > > On Fri, Sep 21, 2018 at 08:05:50AM -0700, Yu-cheng Yu wrote:
> > > > > Update ARCH_CET_STATUS and ARCH_CET_DISABLE to include Indirect
> > > > > Branch Tracking features.
> > > > > 
> > > > > Introduce:
> > > > > 
> > > > > arch_prctl(ARCH_CET_LEGACY_BITMAP, unsigned long *addr)
> > > > >     Enable the Indirect Branch Tracking legacy code bitmap.
> > > > > 
> > > > >     The parameter 'addr' is a pointer to a user buffer.
> > > > >     On returning to the caller, the kernel fills the following:
> > > > > 
> > > > >     *addr = IBT bitmap base address
> > > > >     *(addr + 1) = IBT bitmap size
> > > > 
> > > > Again, some structure with a size field would be better from
> > > > UAPI/extensibility standpoint.
> > > > 
> > > > One additional point: "size" in the structure from kernel should have
> > > > structure size expected by kernel, and at least providing there "0" from
> > > > user space shouldn't lead to failure (in fact, it is possible to provide
> > > > structure size back to userspace even if buffer is too small, along
> > > > with error).
> > > 
> > > This has been in GLIBC v2.28.  We cannot change it anymore.
> > 
> > In theory, you could, if you change the ARCH_CET_LEGACY_BITMAP
> > constant, so that glibc will not use the different arch_prctl
> > operation.  We could backport the change into the glibc 2.28 dynamic
> > linker, so that existing binaries will start using CET again.  Then
> > only statically linked binaries will be impacted.
> > 
> > It's definitely not ideal, but it's doable if the interface is
> > terminally broken or otherwise unacceptable.  But to me it looks like
> > this threshold isn't reached here.
> 
> I tend to agree.
> 
> But I do think there's a real problem that should be fixed and won't
> affect ABI: the *name* of the prctl is pretty bad.  I read the test
> several times trying to decide if you meant
> ARCH_GET_CET_LEGACY_BITMAP?  But you don't.
> 
> Maybe name it ARCH_CET_CREATE_LEGACY_BITMAP?  And explicitly document
> what it does if legacy bitmap already exists?

I will fix it.

Yu-cheng