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From:   Andy Lutomirski <luto@amacapital.net>
Date:   Thu, 4 Oct 2018 09:12:04 -0700
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Subject: Re: [RFC PATCH v4 6/9] x86/cet/ibt: Add arch_prctl functions for IBT
To:     Florian Weimer <fw@deneb.enyo.de>
Cc:     Yu-cheng Yu <yu-cheng.yu@intel.com>,
        Eugene Syromiatnikov <esyr@redhat.com>,
        X86 ML <x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
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        Balbir Singh <bsingharora@gmail.com>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Florian Weimer <fweimer@redhat.com>,
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On Thu, Oct 4, 2018 at 9:08 AM Florian Weimer <fw@deneb.enyo.de> 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?

--Andy