Re: [PATCHv2] ARM32: Support mremap() for sigpage/vDSO

[Dmitry Safonov]

+ Adding back to CC x86 guys - as I've removed in ping messages.

On 05/23/2017 11:09 PM, Russell King - ARM Linux wrote:
> On Thu, May 18, 2017 at 02:13:29PM +0300, Dmitry Safonov wrote:
> > On 04/25/2017 08:18 PM, Dmitry Safonov wrote:
> > > On 04/14/2017 04:25 PM, Dmitry Safonov wrote:
> > > > CRIU restores application mappings on the same place where they
> > > > were before Checkpoint. That means, that we need to move vDSO
> > > > and sigpage during restore on exactly the same place where
> > > > they were before C/R.
> > > >
> > > > Make mremap() code update mm->context.{sigpage,vdso} pointers
> > > > during VMA move. Sigpage is used for landing after handling
> > > > a signal - if the pointer is not updated during moving, the
> > > > application might crash on any signal after mremap().
> > > >
> > > > vDSO pointer on ARM32 is used only for setting auxv at this moment,
> > > > update it during mremap() in case of future usage.
> > > >
> > > > Without those updates, current work of CRIU on ARM32 is not reliable.
> > > > Historically, we error Checkpointing if we find vDSO page on ARM32
> > > > and suggest user to disable CONFIG_VDSO.
> > > > But that's not correct - it goes from x86 where signal processing
> > > > is ended in vDSO blob. For arm32 it's sigpage, which is not disabled
> > > > with `CONFIG_VDSO=n'.
> > > >
> > > > Looks like C/R was working by luck - because userspace on ARM32 at
> > > > this moment always sets SA_RESTORER.
> > > >
> > > > Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> > > > Cc: Russell King <linux@xxxxxxxxxxxxxxx>
> > > > Cc: Will Deacon <will.deacon@xxxxxxx>
> > > > Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> > > > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > > > Cc: Cyrill Gorcunov <gorcunov@xxxxxxxxxx>
> > > > Cc: Pavel Emelyanov <xemul@xxxxxxxxxxxxx>
> > > > Cc: Christopher Covington <cov@xxxxxxxxxxxxxx>
> > > > Signed-off-by: Dmitry Safonov <dsafonov@xxxxxxxxxxxxx>
> > > > ---
> > > > v2: (buildbot) Fix (unsinged long) to (void*) cast warning.
> > > >
> > > >   arch/arm/kernel/process.c |  8 ++++++++
> > > >   arch/arm/kernel/vdso.c    | 18 ++++++++++++++++++
> > > >   arch/x86/entry/vdso/vma.c |  3 ---
> > > >   mm/mmap.c                 |  4 ++++
> > > >   4 files changed, 30 insertions(+), 3 deletions(-)
> > >
> > > Ping?
> >
> > Ping?
>
> I'm mostly happy with the ARM bits, but I can't take the patch without
> acks from others because it touches other architectures/generic code.

Fair enough, thanks.

Andy, does moving of this WARN_ON() looks good to you?
I've done it to reuse it over arches and between vdso/sigpage mappings,
reducing code duplication.

Here is the patch, so you don't need to search it in the mailbox:
https://patchwork.kernel.org/patch/9681273/

--
             Dmitry