Le 26/03/2021 à 20:17, Dmitry Safonov a écrit :
Since commit 511157ab641e ("powerpc/vdso: Move vdso datapage up front")
VVAR page is in front of the VDSO area. In result it breaks CRIU
(Checkpoint Restore In Userspace) [1], where CRIU expects that "[vdso]"
from /proc/../maps points at ELF/vdso image, rather than at VVAR data page.
Laurent made a patch to keep CRIU working (by reading aux vector).
But I think it still makes sence to separate two mappings into different
VMAs. It will also make ppc64 less "special" for userspace and as
a side-bonus will make VVAR page un-writable by debugger (which previously
would COW page and can be unexpected).
I opportunistically Cc stable on it: I understand that usually such
stuff isn't a stable material, but that will allow us in CRIU have
one workaround less that is needed just for one release (v5.11) on
one platform (ppc64), which we otherwise have to maintain.
Why is that a workaround, and why for one release only ? I think the solution proposed by Laurentto
use the aux vector AT_SYSINFO_EHDR should work with any past and future release.
I wouldn't go as far as to say that the commit 511157ab641e is ABI
regression as no other userspace got broken, but I'd really appreciate
if it gets backported to v5.11 after v5.12 is released, so as not
to complicate already non-simple CRIU-vdso code. Thanks!
Cc: Andrei Vagin <avagin@xxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
Cc: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
Cc: Laurent Dufour <ldufour@xxxxxxxxxxxxx>
Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Cc: Paul Mackerras <paulus@xxxxxxxxx>
Cc: linuxppc-dev@xxxxxxxxxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx # v5.11
[1]: https://github.com/checkpoint-restore/criu/issues/1417
Signed-off-by: Dmitry Safonov <dima@xxxxxxxxxx>
Tested-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
I tested it with sifreturn_vdso selftest and it worked, because that selftest doesn't involve VDSO data.
But if I do a mremap() on the VDSO text vma without remapping VVAR to keep the same distance between
the two vmas, gettimeofday() crashes. The reason is that the code obtains the address of the data by
calculating a fix difference from its own address with the below macro, the delta being resolved at
link time:
.macro get_datapage ptr
bcl 20, 31, .+4
999:
mflr \ptr
#if CONFIG_PPC_PAGE_SHIFT > 14
addis \ptr, \ptr, (_vdso_datapage - 999b)@ha
#endif
addi \ptr, \ptr, (_vdso_datapage - 999b)@l
.endm
So the datapage needs to remain at the same distance from the code at all time.
Wondering how the other architectures do to have two independant VMAs and be able to move one
independantly of the other.
Christophe
