> On Nov 2, 2017, at 12:33 PM, Thomas Gleixner <t...@linutronix.de> wrote: > >> On Thu, 2 Nov 2017, Andy Lutomirski wrote: >>> On Wed, Nov 1, 2017 at 3:20 PM, Thomas Gleixner <t...@linutronix.de> wrote: >>>> On Wed, 1 Nov 2017, Linus Torvalds wrote: >>>>> On Wed, Nov 1, 2017 at 2:52 PM, Dave Hansen <dave.han...@linux.intel.com> >>>>> wrote: >>>>>> On 11/01/2017 02:28 PM, Thomas Gleixner wrote: >>>>>>> On Wed, 1 Nov 2017, Andy Lutomirski wrote: >>>>>>> The vsyscall page is _PAGE_USER and lives in init_mm via the fixmap. >>>>>> >>>>>> Groan, forgot about that abomination, but still there is no point in >>>>>> having >>>>>> it marked PAGE_USER in the init_mm at all, kaiser or not. >>>>> >>>>> So shouldn't this patch effectively make the vsyscall page unusable? >>>>> Any idea why that didn't show up in any of the x86 selftests? >>>> >>>> I actually think there may be two issues here: >>>> >>>> - vsyscall isn't even used much - if any - any more >>> >>> Only legacy user space uses it. >>> >>>> - the vsyscall emulation works fine without _PAGE_USER, since the >>>> whole point is that we take a fault on it and then emulate. >>>> >>>> We do expose the vsyscall page read-only to user space in the >>>> emulation case, but I'm not convinced that's even required. >>> >>> I don't see a reason why it needs to be mapped at all for emulation. >> >> At least a couple years ago, the maintainers of some userspace tracing >> tools complained very loudly at the early versions of the patches. >> There are programs like pin (semi-open-source IIRC) that parse >> instructions, make an instrumented copy, and run it. This means that >> the vsyscall page needs to contain text that is semantically >> equivalent to what calling it actually does. >> >> So yes, read access needs to work. I should add a selftest for this. >> >> This is needed in emulation mode as well as native mode, so removing >> native mode is totally orthogonal. > > Fair enough. I enabled function tracing with emulate_vsyscall as the filter > on a couple of machines and so far I have no hit at all. Though I found a > VM with a real old user space (~2005) and that actually used it. > > So for the problem at hand, I'd suggest we disable the vsyscall stuff if > CONFIG_KAISER=y and be done with it.
I think that time() on not-so-old glibc uses it. Even more recent versions of Go use it. :( > > Thanks, > > tglx > > > > >
