Subject: Re: [PATCH net-next 2/2] ipv6: mcast: annotate igmp6 timer expiry race

From: Ido Schimmel <[email-masked]>
Date: Tue, 9 Jun 2026 10:44:47 +0300

On Fri, Jun 05, 2026 at 11:57:59PM +0900, Yuyang Huang wrote:
> /proc/net/igmp6 walks IPv6 multicast memberships under RCU and reads
> mca_work.timer.expires to print the remaining multicast timer. The
> delayed-work timer can be updated concurrently.
> 
> Annotate the intentional lockless procfs snapshot with READ_ONCE().
> 
> Signed-off-by: Yuyang Huang <[email-masked]>
> ---
>  net/ipv6/mcast.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/net/ipv6/mcast.c b/net/ipv6/mcast.c
> index bd3972730aa0..184e57469086 100644
> --- a/net/ipv6/mcast.c
> +++ b/net/ipv6/mcast.c
> @@ -2983,6 +2983,7 @@ static int igmp6_mc_seq_show(struct seq_file *seq, void *v)
>  {
>  	struct ifmcaddr6 *im = (struct ifmcaddr6 *)v;
>  	struct igmp6_mc_iter_state *state = igmp6_mc_seq_private(seq);
> +	unsigned long expires = READ_ONCE(im->mca_work.timer.expires);
>  	unsigned int mca_flags = READ_ONCE(im->mca_flags);

The comment from Sashiko about inverting the order looks valid. In the write
path, the MAF_TIMER_RUNNING flag is always set after modifying the timer:

"
Does unconditionally hoisting the read of timer.expires before mca_flags
create a time-of-check to time-of-use race?

If a newly allocated multicast group has expires initialized to 0, this
sequence could happen in igmp6_mc_seq_show():

CPU1 reads expires as 0:
    expires = READ_ONCE(im->mca_work.timer.expires);

CPU2 concurrently arms the timer and sets the flag:
    mod_delayed_work(...)
    im->mca_flags |= MAF_TIMER_RUNNING;

CPU1 then reads mca_flags:
    mca_flags = READ_ONCE(im->mca_flags);

Because the MAF_TIMER_RUNNING flag is now set, CPU1 evaluates the timer
output as (expires - jiffies), which is (0 - jiffies). Does this
underflow and cause /proc/net/igmp6 to print a massive garbage timer value?
"

>  
>  	seq_printf(seq,
> @@ -2991,7 +2992,7 @@ static int igmp6_mc_seq_show(struct seq_file *seq, void *v)
>  		   &im->mca_addr,
>  		   READ_ONCE(im->mca_users), mca_flags,
>  		   (mca_flags & MAF_TIMER_RUNNING) ?
> -		   jiffies_to_clock_t(im->mca_work.timer.expires - jiffies) : 0);
> +		   jiffies_to_clock_t(expires - jiffies) : 0);
>  	return 0;
>  }
>  
> -- 
> 2.43.0
>