Re: [PATCH] powerpc/vdso: Separate vvar vma from vdso

[Laurent Dufour]

Hi Christophe and Dimitry,

Le 27/03/2021 à 18:43, Dmitry Safonov a écrit :
> Hi Christophe,
>
> On 3/27/21 5:19 PM, Christophe Leroy wrote:
> [..]
> > > 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.
>
> Yeah, I guess.
> Previously, (before v5.11/power) all kernels had ELF start at "[vdso]"
> VMA start, now we'll have to carry the offset in the VMA. Probably, not
> the worst thing, but as it will be only for v5.11 release it can break,
> so needs separate testing.
> Kinda life was a bit easier without this additional code.
The assumption that ELF header is at the start of "[vdso]" is perhaps not a good 
one, but using a "[vvar]" section looks more conventional and allows to clearly 
identify the data part. I'd argue for this option.

> > > 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.
>
> Thanks again on helping with testing it, I appreciate it!
>
> > 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 independent VMAs
> > and be able to move one independently of the other.
>
> It's alright as far as I know. If userspace remaps vdso/vvar it should
> be aware of this (CRIU keeps this in mind, also old vdso image is dumped
> to compare on restore with the one that the host has).

I do agree, playing with the VDSO mapping needs the application to be aware of 
the mapping details, and prior to 83d3f0e90c6c "powerpc/mm: tracking vDSO 
remap", remapping the VDSO was not working on PowerPC and nobody complained...

Laurent.