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
