On 05/15/2014 11:42 PM, Andy Lutomirski wrote: > On May 14, 2014 8:36 PM, "Pavel Emelyanov" <xemul@xxxxxxxxxxxxx> wrote: >> >> On 05/15/2014 02:23 AM, Andy Lutomirski wrote: >>> On Wed, May 14, 2014 at 3:11 PM, Cyrill Gorcunov <gorcunov@xxxxxxxxx> wrote: >>>> On Wed, May 14, 2014 at 02:33:54PM -0700, Andy Lutomirski wrote: >>>>> On Wed, May 14, 2014 at 2:31 PM, Andrew Morton >>>>> <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: >>>>>> On Wed, 14 May 2014 17:11:00 -0400 Sasha Levin <sasha.levin@xxxxxxxxxx> wrote: >>>>>> >>>>>>>> In my linux-next all that code got deleted by Andy's "x86, vdso: >>>>>>>> Reimplement vdso.so preparation in build-time C" anyway. What kernel >>>>>>>> were you looking at? >>>>>>> >>>>>>> Deleted? It appears in today's -next. arch/x86/vdso/vma.c:124 . >>>>>>> >>>>>>> I don't see Andy's patch removing that code either. >>>>>> >>>>>> ah, OK, it got moved from arch/x86/vdso/vdso32-setup.c into >>>>>> arch/x86/vdso/vma.c. >>>>>> >>>>>> Maybe you managed to take a fault against the symbol area between the >>>>>> _install_special_mapping() and the remap_pfn_range() call, but mmap_sem >>>>>> should prevent that. >>>>>> >>>>>> Or the remap_pfn_range() call never happened. Should map_vdso() be >>>>>> running _install_special_mapping() at all if >>>>>> image->sym_vvar_page==NULL? >>>>> >>>>> I'm confused: are we talking about 3.15-rcsomething or linux-next? >>>>> That code changed. >>>>> >>>>> Would this all make more sense if there were just a single vma in >>>>> here? cc: Pavel and Cyrill, who might have to deal with this stuff in >>>>> CRIU >>>> >>>> Well, for criu we've not modified any vdso kernel's code (except >>>> setting VM_SOFTDIRTY for this vdso VMA in _install_special_mapping). >>>> And never experienced problems Sasha points. Looks like indeed in >>>> -next code is pretty different from mainline one. To figure out >>>> why I need to fetch -next branch and get some research. I would >>>> try to do that tomorrow (still hoping someone more experienced >>>> in mm system would beat me on that). >>> >>> I can summarize: >>> >>> On 3.14 and before, the vdso is just a bunch of ELF headers and >>> executable data. When executed by 64-bit binaries, it reads from the >>> fixmap to do its thing. That is, it reads from kernel addresses that >>> don't have vmas. When executed by 32-bit binaries, it doesn't read >>> anything, since there was no 32-bit timing code. >>> >>> On 3.15, the x86_64 vdso is unchanged. The 32-bit vdso is preceded by >>> a separate vma containing two pages worth of time-varying read-only >>> data. The vdso reads those pages using PIC references. >>> >>> On linux-next, all vdsos work the same way. There are two vmas. The >>> first vma is executable text, which can be poked at by ptrace, etc >>> normally. The second vma contains time-varying state, should not >>> allow poking, and is accessed by PIC references. >> >> Is this 2nd vma seen in /proc/pid/maps? And if so, is it marked somehow? > > It is in maps, and it's not marked. I can write a patch to change > that. I imagine it shouldn't be called [vdso], though. That would be great. >> >>> What does CRIU do to restore the vdso? Will 3.15 and/or linux-next >>> need to make some concession for CRIU? >> >> We detect the vdso by "[vdso]" mark in proc at dump time and mark it in >> the images. At restore time we check that vdso symbols layout hasn't changed >> and just remap it in proper location. >> >> If this remains the same in -next, then we're fine :) > > If you just remap the vdso, you'll crash. > > This is the case in 3.15, too, for 32-bit apps, anyway. > > What happens if you try to checkpoint a program that's in the vdso or, > worse, in a signal frame with the vdso on the stack? Nothing good, unfortunately :( And this is one of the things we're investigating. Cyrill can shed more light on it, as he's the one in charge. > --Andy -- 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>