On Mon, 2020-05-18 at 06:41 -0700, Dave Hansen wrote: > On 5/15/20 7:53 PM, Yu-cheng Yu wrote: > > On Fri, 2020-05-15 at 16:56 -0700, Dave Hansen wrote: > > > What's my recourse as an end user? I want to run my app and turn off > > > CET for that app. How can I do that? > > > > GLIBC_TUNABLES=glibc.tune.hwcaps=-SHSTK,-IBT > > Like I mentioned to H.J., this is something that we need to at least > acknowledge the existence of in the changelog and probably even the > Documentation/. Sure. I will do that. > > > > > > I think you're saying that the CET-enabled binary would do > > > > > arch_setup_elf_property() when it was first exec()'d. Later, it could > > > > > use the new prctl(ARCH_X86_CET_DISABLE) to disable its shadow stack, > > > > > then fork() and the child would not be using CET. Right? > > > > > > > > > > What is ARCH_X86_CET_DISABLE used for, anyway? > > > > > > > > Both the parent and the child can do ARCH_X86_CET_DISABLE, if CET is > > > > not locked. > > > > > > Could you please describe a real-world example of why > > > ARCH_X86_CET_DISABLE exists? What kinds of apps will use it, or *are* > > > using it? Why was it created in the first place? > > > > Currently, ld-linux turns off CET if the binary being loaded does not support > > CET. > > Great! Could this please be immortalized in the documentation for the > prctl()? Yes. > > > > > > > > Does this *code* work? Could you please indicate which JITs have been > > > > > > > enabled to use the code in this series? How much of the new ABI is in use? > > > > > > > > > > > > JIT does not necessarily use all of the ABI. The JIT changes mainly fix stack > > > > > > frames and insert ENDBRs. I do not work on JIT. What I found is LLVM JIT fixes > > > > > > are tested and in the master branch. Sljit fixes are in the release. > > > > > > > > > > Huh, so who is using the new prctl() ABIs? > > > > > > > > Any code can use the ABI, but JIT code CET-enabling part mostly do not use these > > > > new prctl()'s, except, probably to get CET status. > > > > > > Which applications specifically are going to use the new prctl()s which > > > this series adds? How are they going to use them? > > > > > > "Any code can use them" is not a specific enough answer. > > > > We have four arch_ptctl() calls. ARCH_X86_CET_DISABLE and ARCH_X86_CET_LOCK are > > used by ld-linux. ARCH_X86_CET_STATUS are used in many places to determine if > > CET is on. ARCH_X86_CET_ALLOC_SHSTK is used in ucontext related handling, but > > it can be use by any application to switch shadow stacks. > > Could some of this information be added to the documentation, please? > It would also be nice to have some more details about how apps end up > using ARCH_X86_CET_STATUS. Why would they care that CET is on? Yes. Yu-cheng
