[CRIU] Dealing with VDSO remap

[Cyrill Gorcunov]

On Fri, Mar 06, 2015 at 03:15:56PM +0100, Laurent Dufour wrote:
> Hi,
> 
> I'm porting CRIU to the PopwerPC architecture, and among other issues,
> I'm facing a major one with the VDSO remapping at restart time.
> 
> On PowerPC, as on ARM64, the kernel keeps track of the VDSO base address
> because it is using it to jump back to a sigreturn trampoline at the end
> of a signal processing (see handle_rt_signal64 in
> arch/powerpc/kernel/signal_64.c, and for ARM64, setup_return in
> arch/arm64/kernel/signal.c).
> 
> When remapping the VDSO at restart time, the kernel keep the reference
> to the previous VDSO mapping, the one inheriting from the criu, so
> handling signal after the restart leads to unpredictable results, most
> of the time a SIGSEGV is raised.

Hi Laurent. As far as I remember this indeed a problem on arm64, and
there should be some CONFIG_ option for that (but I have a little in
my memory what exactly workaround was). Thus the real fix is somehow
tune the kernel itself but looks like nothing was implemented in this
area yet :/ So any help would be appreciated.

> I didn't find a smart way to update the kernel reference to the vdso
> mapping once the VDSO is remapped, so no way to work around that today.
> 
> Furthermore, since this is the same picture on ARM 64, I'm wondering how
> it could work on this architecture. Am I missing a major thing here ?
> 
> If not, is there a plan in the CRIU project to to deal with that, other
> than by hacking the kernel to update its reference at restart time ?