Subject: Re: [PATCH net 5/6] igc: Fix launchtime before start of cycle

From: Leon Romanovsky <[email-masked]>
Date: Tue, 11 Jul 2023 10:09:02 +0300

On Mon, Jul 10, 2023 at 09:35:02AM -0700, Tony Nguyen wrote:
> From: Florian Kauer <[email-masked]>
> 
> It is possible (verified on a running system) that frames are processed
> by igc_tx_launchtime with a txtime before the start of the cycle
> (baset_est).
> 
> However, the result of txtime - baset_est is written into a u32,
> leading to a wrap around to a positive number. The following
> launchtime > 0 check will only branch to executing launchtime = 0
> if launchtime is already 0.
> 
> Fix it by using a s32 before checking launchtime > 0.
> 
> Fixes: db0b124f02ba ("igc: Enhance Qbv scheduling by using first flag bit")
> Signed-off-by: Florian Kauer <[email-masked]>
> Reviewed-by: Kurt Kanzenbach <[email-masked]>
> Tested-by: Naama Meir <[email-masked]>
> Signed-off-by: Tony Nguyen <[email-masked]>
> ---
>  drivers/net/ethernet/intel/igc/igc_main.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/intel/igc/igc_main.c b/drivers/net/ethernet/intel/igc/igc_main.c
> index 5d24930fed8f..4855caa3bae4 100644
> --- a/drivers/net/ethernet/intel/igc/igc_main.c
> +++ b/drivers/net/ethernet/intel/igc/igc_main.c
> @@ -1016,7 +1016,7 @@ static __le32 igc_tx_launchtime(struct igc_ring *ring, ktime_t txtime,
>  	ktime_t base_time = adapter->base_time;
>  	ktime_t now = ktime_get_clocktai();
>  	ktime_t baset_est, end_of_cycle;
> -	u32 launchtime;
> +	s32 launchtime;

The rest of igc_tx_launchtime() function is very questionable,
as ktime_sub_ns() returns ktime_t which is s64.

  1049         launchtime = ktime_sub_ns(txtime, baset_est);
  1050         if (launchtime > 0)
  1051                 div_s64_rem(launchtime, cycle_time, &launchtime);
  1052         else
  1053                 launchtime = 0;
  1054
  1055         return cpu_to_le32(launchtime);

>  	s64 n;
>  
>  	n = div64_s64(ktime_sub_ns(now, base_time), cycle_time);
> -- 
> 2.38.1
> 
>