> 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*.
