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

[Andy Lutomirski]

> On Oct 4, 2018, at 8:37 AM, Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote:
>
>> 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.

Sure you can. Just change ARCH_CET_LEGACY_BITMAP to a new number.  You
might need to change all the constants.  And if the ELF note by itself
causes a problem too, you may need to rename it.  And maybe ask glibc
to kindly not enable code that depends on non-upstreamed kernel
features.

There is not, and has never been, any ABI compatibility requirement
that says that, if glibc 2.28 "enables" a feature, that the kernel
will ever enable it in a way that makes glibc 2.28 actually support
it.  All the kernel needs to do is avoid making glibc 2.28 *crash*.