Re: [PATCH 3/3] prctl,syscall_user_dispatch: add a getter for configuration info

[Gregory Price <gregory.price@memverge.com>]

On Thu, Jan 12, 2023 at 10:15:39AM -0800, Andrei Vagin wrote:
> On Mon, Jan 09, 2023 at 10:33:48AM -0500, Gregory Price wrote:
> > This patch implements simple getter interface for syscall user dispatch
> > configuration info.
> > 
> > To support checkpoint/resume of a syscall user dispatch process,
> > the prctl settings for syscall user dispatch must be fetchable.
> > Presently, these settings are write-only, making it impossible to
> > implement transparent checkpoint (coordination with the software is
> > required).
> > 
> > As Syscall User Dispatch is explicitly not for secure-container
> > development, exposing the configuration state via prctl does not
> > violate the original design intent.
> > 
> > Signed-off-by: Gregory Price <gregory.price@memverge.com>
> > ---
> >  .../admin-guide/syscall-user-dispatch.rst     | 18 +++++++
> >  include/linux/syscall_user_dispatch.h         |  7 +++
> >  include/uapi/linux/prctl.h                    |  3 ++
> >  kernel/entry/syscall_user_dispatch.c          | 14 +++++
> >  kernel/sys.c                                  |  4 ++
> >  .../syscall_user_dispatch/sud_test.c          | 54 +++++++++++++++++++
> >  6 files changed, 100 insertions(+)
> > 
> > diff --git a/Documentation/admin-guide/syscall-user-dispatch.rst b/Documentation/admin-guide/syscall-user-dispatch.rst
> > index 60314953c728..8b2c8b6441b7 100644
> > --- a/Documentation/admin-guide/syscall-user-dispatch.rst
> > +++ b/Documentation/admin-guide/syscall-user-dispatch.rst
> > @@ -45,6 +45,10 @@ only the syscall dispatcher address and the userspace key.
> >  As the ABI of these intercepted syscalls is unknown to Linux, these
> >  syscalls are not instrumentable via ptrace or the syscall tracepoints.
> >  
> > +A getter interface is supplied for the purpose of userland
> > +checkpoint/restore software being able to suspend and restore the
> > +current state of the system.
> > +
> >  Interface
> >  ---------
> >  
> > @@ -73,6 +77,20 @@ thread-wide, without the need to invoke the kernel directly.  selector
> >  can be set to SYSCALL_DISPATCH_FILTER_ALLOW or SYSCALL_DISPATCH_FILTER_BLOCK.
> >  Any other value should terminate the program with a SIGSYS.
> >  
> > +
> > +A thread can fetch the current Syscall User Dispatch configuration with the following prctl:
> > +
> > +  prctl(PR_GET_SYSCALL_USER_DISPATCH, <dispatch_config>))
> > +
> > +<dispatch_config> is a pointer to a ``struct syscall_user_dispatch`` as defined in ``linux/include/linux/syscall_user_dispatch.h``::
> 
> syscall_user_dispatch.h isn't a part of uapi, so I am not sure that it
> is a good idea to use it here.
> 
> For criu, it is much more convinient to have a ptrace interface to get
> this sort of parameters. prctl requires to execute a system call from a
> context of the target process. It is tricky so we want to minimize a
> number of such calls.
> 
> Thanks,
> Andrei


Thank you for the feedback.

I think you're right.  A Ptrace for this seems more in-line with the
SECCOMP filter exporting that CRIU uses too.

I'll look at implementing that instead.