> On Aug 27, 2020, at 11:13 AM, Yu, Yu-cheng <yu-cheng.yu@xxxxxxxxx> wrote: > > On 8/27/2020 6:36 AM, Florian Weimer wrote: >> * H. J. Lu: >>>> On Thu, Aug 27, 2020 at 6:19 AM Florian Weimer <fweimer@xxxxxxxxxx> wrote: >>>>> >>>>> * Dave Martin: >>>>> >>>>>> You're right that this has implications: for i386, libc probably pulls >>>>>> more arguments off the stack than are really there in some situations. >>>>>> This isn't a new problem though. There are already generic prctls with >>>>>> fewer than 4 args that are used on x86. >>>>> >>>>> As originally posted, glibc prctl would have to know that it has to pull >>>>> an u64 argument off the argument list for ARCH_X86_CET_DISABLE. But >>>>> then the u64 argument is a problem for arch_prctl as well. >>>>> >>> >>> Argument of ARCH_X86_CET_DISABLE is int and passed in register. >> The commit message and the C source say otherwise, I think (not sure >> about the C source, not a kernel hacker). > > H.J. Lu suggested that we fix x86 arch_prctl() to take four arguments, and then keep MMAP_SHSTK as an arch_prctl(). Because now the map flags and size are all in registers, this also solves problems being pointed out earlier. Without a wrapper, the shadow stack mmap call (from user space) will be: > > syscall(_NR_arch_prctl, ARCH_X86_CET_MMAP_SHSTK, size, MAP_32BIT). I admit I don’t see a show stopping technical reason we can’t add arguments to an existing syscall, but I’m pretty sure it’s unprecedented, and it doesn’t seem like a good idea.
