Dmitry Safonov <0x7f454c46@xxxxxxxxx> writes: > Hello, > > On Wed, 26 Aug 2020 at 15:39, Michael Ellerman <mpe@xxxxxxxxxxxxxx> wrote: >> Christophe Leroy <christophe.leroy@xxxxxxxxxx> writes: > [..] >> > arch_remap() gets replaced by vdso_remap() >> > >> > For arch_unmap(), I'm wondering how/what other architectures do, because >> > powerpc seems to be the only one to erase the vdso context pointer when >> > unmapping the vdso. >> >> Yeah. The original unmap/remap stuff was added for CRIU, which I thought >> people tested on other architectures (more than powerpc even). >> >> Possibly no one really cares about vdso unmap though, vs just moving the >> vdso. >> >> We added a test for vdso unmap recently because it happened to trigger a >> KAUP failure, and someone actually hit it & reported it. > > You right, CRIU cares much more about moving vDSO. > It's done for each restoree and as on most setups vDSO is premapped and > used by the application - it's actively tested. > Speaking about vDSO unmap - that's concerning only for heterogeneous C/R, > i.e when an application is migrated from a system that uses vDSO to the one > which doesn't - it's much rare scenario. > (for arm it's !CONFIG_VDSO, for x86 it's `vdso=0` boot parameter) Ah OK that explains it. The case we hit of VDSO unmapping was some strange "library OS" thing which had explicitly unmapped the VDSO, so also very rare. > Looking at the code, it seems quite easy to provide/maintain .close() for > vm_special_mapping. A bit harder to add a test from CRIU side > (as glibc won't know on restore that it can't use vdso anymore), > but totally not impossible. > >> Running that test on arm64 segfaults: >> >> # ./sigreturn_vdso >> VDSO is at 0xffff8191f000-0xffff8191ffff (4096 bytes) >> Signal delivered OK with VDSO mapped >> VDSO moved to 0xffff8191a000-0xffff8191afff (4096 bytes) >> Signal delivered OK with VDSO moved >> Unmapped VDSO >> Remapped the stack executable >> [ 48.556191] potentially unexpected fatal signal 11. >> [ 48.556752] CPU: 0 PID: 140 Comm: sigreturn_vdso Not tainted 5.9.0-rc2-00057-g2ac69819ba9e #190 >> [ 48.556990] Hardware name: linux,dummy-virt (DT) >> [ 48.557336] pstate: 60001000 (nZCv daif -PAN -UAO BTYPE=--) >> [ 48.557475] pc : 0000ffff8191a7bc >> [ 48.557603] lr : 0000ffff8191a7bc >> [ 48.557697] sp : 0000ffffc13c9e90 >> [ 48.557873] x29: 0000ffffc13cb0e0 x28: 0000000000000000 >> [ 48.558201] x27: 0000000000000000 x26: 0000000000000000 >> [ 48.558337] x25: 0000000000000000 x24: 0000000000000000 >> [ 48.558754] x23: 0000000000000000 x22: 0000000000000000 >> [ 48.558893] x21: 00000000004009b0 x20: 0000000000000000 >> [ 48.559046] x19: 0000000000400ff0 x18: 0000000000000000 >> [ 48.559180] x17: 0000ffff817da300 x16: 0000000000412010 >> [ 48.559312] x15: 0000000000000000 x14: 000000000000001c >> [ 48.559443] x13: 656c626174756365 x12: 7865206b63617473 >> [ 48.559625] x11: 0000000000000003 x10: 0101010101010101 >> [ 48.559828] x9 : 0000ffff818afda8 x8 : 0000000000000081 >> [ 48.559973] x7 : 6174732065687420 x6 : 64657070616d6552 >> [ 48.560115] x5 : 000000000e0388bd x4 : 000000000040135d >> [ 48.560270] x3 : 0000000000000000 x2 : 0000000000000001 >> [ 48.560412] x1 : 0000000000000003 x0 : 00000000004120b8 >> Segmentation fault >> # >> >> So I think we need to keep the unmap hook. Maybe it should be handled by >> the special_mapping stuff generically. > > I'll cook a patch for vm_special_mapping if you don't mind :-) That would be great, thanks! cheers
