On Thu, Aug 27, 2020 at 6:35 PM Andy Lutomirski <l...@kernel.org> wrote: > > 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.
Can you think of ANY issues of passing more arguments to arch_prctl? syscall () provided by glibc always passes 6 arguments to the kernel. Arguments are already in the registers. What kind of problems do you see? -- H.J.
