Re: [RFD] posix-timers: CRIU woes

[Pavel Tikhomirov]

On 11.05.2023 21:42, Thomas Gleixner wrote:
> On Thu, May 11 2023 at 17:52, Pavel Tikhomirov wrote:
> > On 11.05.2023 17:36, Thomas Gleixner wrote:
> > > On Thu, May 11 2023 at 11:17, Pavel Tikhomirov wrote:
> > > > On 10.05.2023 16:16, Andrey Vagin wrote:
> > > > > > So because of that half thought out user space ABI we are now up the
> > > > > > regression creek without a paddle, unless CRIU can accomodate to a
> > > > > > different restore mechanism to lift this restriction from the kernel.
> > > > > If you give us a new API to create timers with specified id-s, we will
> > > > > figure out how to live with it. It isn't good to ask users to update
> > > > > CRIU to work on new kernels, but here are reasons and event improvements
> > > > > for CRIU, so I think it's worth it.
> > > >
> > > > I agree, any API to create timers with specified id-s would work for new
> > > > CRIU versions.
> > >
> > > The real question is whether this will cause any upheaval when a new
> > > kernel meets a non-updated CRIU stack.
> >
> > Creation of posix timer would hang forever in this loop
> > https://github.com/checkpoint-restore/criu/blob/33dd66c6fc93c47213aaa0447a94d97ba1fa56ba/criu/pie/restorer.c#L1185
> > if old criu is run on new kernel (without consecutive id allocation) AFAICS.
>
> Yes, because that "sanity" check
>
>       if ((long)next_id > args->posix_timers[i].spt.it_id)
>
> which tries to establish whether the kernel provides timer IDs in strict
> increasing order does not work for that case.

Yes, this check is not perfect, but it does at least something: It 
detects that posix timer creation missed needed id (if you start from 0 
and increase by 1 each time you can not reach number > N before reaching 
N).

> It "works" to detect the IDR case on older kernels by chance, but not
> under all circumstances. Assume the following case:
>
>        Global IDR has a free slot at index 1
>
>        Restore tries to create a timer for index 2
>
> That will also loop forever, unless some other process creates a timer
> and occupies the free slot at index 1, right?

Yes on old-old kernel, where there were no ids increasing on 
create/delete, CRIU is broken, but CRIU does not support kernels earlier 
than 3.11 (https://criu.org/Check_the_kernel#Basic) so probably we are fine.

git describe --contains 5ed67f05f66c
v3.10-rc1~171^2~9

> So this needs a fix anyway, which should be done so that the new kernel
> case is at least properly detected.
>
> But even then there is still the problem of "it worked before I upgraded
> the kernel".
>
> IOW, we are still up a creek without a paddle, unless you would be
> willing to utilize the existing CRIU bug to distribute the 'deal with
> new kernel' mechanics as a bug bounty :) >
> Fix for the loop termination below.

It fixes the loop for new kernel, I agree.

> Thanks,
>
>          tglx
> ---
>   criu/pie/restorer.c |   24 +++++++++++++-----------
>   1 file changed, 13 insertions(+), 11 deletions(-)
>
> --- a/criu/pie/restorer.c
> +++ b/criu/pie/restorer.c
> @@ -1169,10 +1169,10 @@ static int timerfd_arm(struct task_resto
>   static int create_posix_timers(struct task_restore_args *args)
>   {
>   	int ret, i;
> -	kernel_timer_t next_id;
> +	kernel_timer_t next_id, timer_id;
>   	struct sigevent sev;
>   
> -	for (i = 0; i < args->posix_timers_n; i++) {
> +	for (i = 0, next_id = 0; i < args->posix_timers_n; i++) {
>   		sev.sigev_notify = args->posix_timers[i].spt.it_sigev_notify;
>   		sev.sigev_signo = args->posix_timers[i].spt.si_signo;
>   #ifdef __GLIBC__
> @@ -1183,25 +1183,27 @@ static int create_posix_timers(struct ta
>   		sev.sigev_value.sival_ptr = args->posix_timers[i].spt.sival_ptr;
>   
>   		while (1) {
> -			ret = sys_timer_create(args->posix_timers[i].spt.clock_id, &sev, &next_id);
> +			ret = sys_timer_create(args->posix_timers[i].spt.clock_id, &sev, &timer_id);
>   			if (ret < 0) {
>   				pr_err("Can't create posix timer - %d\n", i);
>   				return ret;
>   			}
>   
> -			if (next_id == args->posix_timers[i].spt.it_id)
> +			if (timer_id != next_id) {
> +				pr_err("Can't create timers, kernel don't give them consequently\n");
> +				return -1;
> +			}
> +
> +			next_id++;
> +
> +			if (timer_id == args->posix_timers[i].spt.it_id)
>   				break;
>   
> -			ret = sys_timer_delete(next_id);
> +			ret = sys_timer_delete(timer_id);
>   			if (ret < 0) {
> -				pr_err("Can't remove temporaty posix timer 0x%x\n", next_id);
> +				pr_err("Can't remove temporaty posix timer 0x%x\n", timer_id);
>   				return ret;
>   			}
> -
> -			if ((long)next_id > args->posix_timers[i].spt.it_id) {
> -				pr_err("Can't create timers, kernel don't give them consequently\n");
> -				return -1;
> -			}
>   		}
>   	}
>   

--
Best regards, Tikhomirov Pavel
Senior Software Developer, Virtuozzo.