> On Jan 4, 2018, at 12:57 PM, Hugh Dickins <hughd@xxxxxxxxxx> wrote: > >> On Thu, Jan 4, 2018 at 12:43 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: >> >>>> On Jan 4, 2018, at 12:29 PM, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: >>>> >>>> On Thu, Jan 4, 2018 at 12:16 PM, Thomas Voegtle <tv@xxxxxxxx> wrote: >>>> >>>> Attached a screenshot. >>>> Is that useful? Are there some debug options I can add? >>> >>> Not much of an oops, because the SIGSEGV happens in user space. The >>> only reason you get any kernel stack printout at all is because 'init' >>> dying will make the kernel print that out. >>> >>> The segfault address for init looks like the fixmap area to me (first >>> byte in the last page of the fixmap?). "Error 5" means that it's a >>> user-space read that got a protection fault. So it's not a LDT of GDT >>> update or anything like that, it's a normal access from user space (or >>> a qemu emulation bug, but that sounds unlikely). >>> >>> Is that the vsyscall page? >>> >>> Adding Luto to the participants. I think he noticed one of the >>> vsyscall patches missing earlier in the 4.9 series. Maybe the 4.4 >>> series had something similar.. >>> >> >> That's almost certainly it. > > I'm hopeless on the FIXMAP arithmetic, but I'm pretty sure that > ffffffffff5ff000 is either VSYSCALL page or PVCLOCK page (I think it > was VVAR page when init segfaulted on it in my 3.2). Nah, that's one page below VSYSCALL. Vvar is 0x7fff... I don't have the actual screenshot, I think. > > I'll forward Borislav's suggested 4.4 VSYSCALL patch from the kaiser > backports ml to Thomas, to see if that sorts his crash (forwarding in > the hope that gmail doesn't mess up the patch). > > Seems odd that 4.4 should be broken but 4.9 not broken here, I'd > expect them to be equally known broken with respect to VSYSCALL; but > perhaps it's a matter of userspace trying different fallbacks > according to what kernel supports, and only hitting this on 4.4. I don't think any current userspace is that dumb. But Go was still using vsyscall fairly recently. I may be able to look for real tonight. > > Hugh > >> >> I'll try to find some time today Thomnor tomorrow to add a proper selftest. >> >>> Linus