Messages in this thread | | | Date | Sat, 20 Aug 2011 23:38:41 +0200 | From | Richard Weinberger <> | Subject | Re: [RFC] weird crap with vdso on uml/i386 |
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Am 20.08.2011 23:26, schrieb Andrew Lutomirski: > On Sat, Aug 20, 2011 at 4:55 PM, Richard Weinberger<richard@nod.at> wrote: >> Am 20.08.2011 22:14, schrieb Al Viro: >>> >>> On Sat, Aug 20, 2011 at 05:22:23PM +0200, Richard Weinberger wrote: >>> >>>> Hmmm, very strange. >>>> Sadly I cannot reproduce the issue. :( >>>> Everything works fine within UML. >>>> (Of course I've applied your vDSO/i386 patches) >>>> >>>> My test setup: >>>> Host kernel: 2.6.37 and 3.0.1 >>>> Distro: openSUSE 11.4/x86_64 >>>> >>>> UML kernel: 3.1-rc2 >>>> Distro: openSUSE 11.1/i386 >>>> >>>> Does the problem also occur with another host kernel or a different >>>> guest image? >>> >>> Could you check what you get in __kernel_vsyscall()? On iAMD64 box >>> where that sucker contains sysenter-based variant the bug is not >>> present. IOW, it's sensitive to syscall vs. systenter vs. int 0x80 >>> differences. >> >> OK, this explains why I cannot reproduce it. >> My Intel Core2 box is sysenter-based. >> >> (gdb) disass __kernel_vsyscall >> 0xffffe420<__kernel_vsyscall+0>: push %ecx >> 0xffffe421<__kernel_vsyscall+1>: push %edx >> 0xffffe422<__kernel_vsyscall+2>: push %ebp >> 0xffffe423<__kernel_vsyscall+3>: mov %esp,%ebp >> 0xffffe425<__kernel_vsyscall+5>: sysenter >> 0xffffe427<__kernel_vsyscall+7>: nop >> 0xffffe428<__kernel_vsyscall+8>: nop >> 0xffffe429<__kernel_vsyscall+9>: nop >> 0xffffe42a<__kernel_vsyscall+10>: nop >> 0xffffe42b<__kernel_vsyscall+11>: nop >> 0xffffe42c<__kernel_vsyscall+12>: nop >> 0xffffe42d<__kernel_vsyscall+13>: nop >> 0xffffe42e<__kernel_vsyscall+14>: jmp 0xffffe423<__kernel_vsyscall+3> >> 0xffffe430<__kernel_vsyscall+16>: pop %ebp >> 0xffffe431<__kernel_vsyscall+17>: pop %edx >> 0xffffe432<__kernel_vsyscall+18>: pop %ecx >> 0xffffe433<__kernel_vsyscall+19>: ret >> >>> I can throw the trimmed-down fs image your way, BTW (66MB of bzipped ext2 >>> ;-/) >>> if you want to see if that gets reproduced on your box. I'll drop it on >>> anonftp if you are interested. FWIW, the same kernel binary/same image >>> result in >>> * K7 box - no breakage, SYSENTER-based vdso >>> * K8 box - breakage as described, SYSCALL-based vdso32 >>> * P4 box - no breakage, SYSENTER-based vdso32 >>> Hell knows... In theory that would seem to point towards >>> ia32_cstar_target(), >>> so I'm going to RTFS carefully through that animal. >> >> Now I'm testing with a Debian fs from: >> http://fs.devloop.org.uk/filesystems/Debian-Squeeze/ >> >>> The thing is, whatever happens happens when victim gets resumed inside >>> vdso page. I'll try to dump PTRACE_SETREGS and see the values host >>> kernel asked to set and work from there, but the interesting part is >>> bloody hard to singlestep through - the victim is back to user mode and >>> it is already traced by the guest kernel, so it's not as if we could >>> attach host gdb to it and walk through that crap. And guest gdb is not >>> going to be able to set breakpoints in there - vdso page is r/o... >> >> [ CC'ing luto@mit.edu ] >> Andy, do you have an idea? >> You can find Al's original report here: >> http://marc.info/?l=linux-kernel&m=131380315624244&w=2 > > I'm missing a bit of the background. Is the user-on-UML app calling > into a vdso entry provided by UML or into a vdso entry provided by the > host?
UML/i386 reuses the host's vDSO page. IOW it does not have it's own vDSO like UML/x86_64.
Thanks, //richard
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