On Thu, Jan 26, 2023 at 01:30:08AM +0100, Oleg Nesterov wrote:

On 01/24, Gregory Price wrote:

quoted

Adds PTRACE_O_SUSPEND_SYSCALL_USER_DISPATCH to ptrace options, and
modify Syscall User Dispatch to suspend interception when enabled.

This is modeled after the SUSPEND_SECCOMP feature, which suspends
SECCOMP interposition.  Without doing this, software like CRIU will
inject system calls into a process and be intercepted by Syscall
User Dispatch, either causing a crash (due to blocked signals) or
the delivery of those signals to a ptracer (not the intended behavior).

Cough... Gregory, I am sorry ;)

but can't we drop this patch to ?

CRIU needs to do PTRACE_SET_SYSCALL_USER_DISPATCH_CONFIG and check
config->mode anyway as we discussed.

Then it can simply set *config->selector = SYSCALL_DISPATCH_FILTER_ALLOW
with the same effect, no?

Oleg.

The selector is optional, but the core idea seems reasonable.

Though I think this complicates the quiesce vs checkpoint phases a bit.

My best understanding of CRIU is there are (at least) two checkpoint
phases: quiesce and checkpoint. The intent of patch 1/2 is to aid the
quiesce phase, not the checkpoint phase.

In both phases the `compel` code is used to inject system calls, so
turning SUD off is required.  That can obviously be achieved via saving
with get_config, and just clearing it entirely with set_config.

I'm NOT sure whether the `compel` code can save settings that the
`cr-check` code then saves to disc, or if `compel` is standalone. I will
go check this and report back.

The only other concern is one of how it's restored, and in what order
compared to SECCOMP - for the absolute insane case of someone running a
SUD task inside a locked down cgroup? Technically possible (TM)!

We may find that the suspend flag is "just easier" but not required.

I do think more-simple-is-more-better, though, so I will investigate.

~Gregory