On Thu, 2 Nov 2017, Andy Lutomirski wrote: > On Wed, Nov 1, 2017 at 3:20 PM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote: > > On Wed, 1 Nov 2017, Linus Torvalds wrote: > >> On Wed, Nov 1, 2017 at 2:52 PM, Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> 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. Thanks, tglx -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
