On Thu, Aug 27, 2020 at 12:38 PM H.J. Lu <hjl.to...@gmail.com> wrote: > > On Thu, Aug 27, 2020 at 11:56 AM Andy Lutomirski <l...@amacapital.net> wrote: > > > > > > > > > On Aug 27, 2020, at 11:13 AM, Yu, Yu-cheng <yu-cheng...@intel.com> 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 <fwei...@redhat.com> > > >>>> 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. > > prctl prototype is: > > extern int prctl (int __option, ...) > > and implemented in kernel as: > > int prctl(int option, unsigned long arg2, unsigned long arg3, > unsigned long arg4, unsigned long arg5); > > Not all prctl operations take all 5 arguments. It also applies > to arch_prctl. It is quite normal for different operations of > arch_prctl to take different numbers of arguments.
If by "quite normal" you mean "does not happen", then I agree. In any event, I will not have anything to do with a patch that changes an existing syscall signature unless Linus personally acks it. So if you want to email him and linux-abi, be my guest.
