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. > > > 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? --Andy > > Thanks, > Pavel -- 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>