git: 6a376af0cd21 - main - TCP PRR: Patch div/0 in tcp_prr_partialack

[Michael Tuexen]

> On 26. Jan 2021, at 16:09, Richard Scheffenegger <rscheff at FreeBSD.org> wrote:
> 
> The branch main has been updated by rscheff:
> 
> URL: https://cgit.FreeBSD.org/src/commit/?id=6a376af0cd212be4e16d013d35a0e2eec1dbb8ae
> 
> commit 6a376af0cd212be4e16d013d35a0e2eec1dbb8ae
> Author:     Richard Scheffenegger <rscheff at FreeBSD.org>
> AuthorDate: 2021-01-26 15:06:32 +0000
> Commit:     Richard Scheffenegger <rscheff at FreeBSD.org>
> CommitDate: 2021-01-26 15:06:32 +0000
> 
>    TCP PRR: Patch div/0 in tcp_prr_partialack
> 
>    With clearing of recover_fs in bc7ee8e5bc555, div/0
>    was observed while processing partial_acks.
> 
>    Suspect that rewind of an erraneous RTO may be
>    causing this - with the above change, recover_fs
>    would no longer retained at the last calculated
>    value, and reset. But CC_RTO_ERR can reenable
>    IN_RECOVERY(), without setting this again.
> 
>    Adding a safety net prior to the division in that
>    function, which I missed in D28114.
Any plans to MFC this to stable/13?

This commit intends to address an issue reported by mm@ in
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=252958

Best regards
Michael
> ---
> sys/netinet/tcp_input.c | 8 +++++---
> 1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/sys/netinet/tcp_input.c b/sys/netinet/tcp_input.c
> index 4b8f91ed9d0b..459b78cd444a 100644
> --- a/sys/netinet/tcp_input.c
> +++ b/sys/netinet/tcp_input.c
> @@ -510,7 +510,6 @@ cc_post_recovery(struct tcpcb *tp, struct tcphdr *th)
> 	}
> 	/* XXXLAS: EXIT_RECOVERY ? */
> 	tp->t_bytes_acked = 0;
> -	tp->sackhint.recover_fs = 0;
> }
> 
> /*
> @@ -3948,10 +3947,13 @@ tcp_prr_partialack(struct tcpcb *tp, struct tcphdr *th)
> 	/*
> 	 * Proportional Rate Reduction
> 	 */
> -	if (pipe > tp->snd_ssthresh)
> +	if (pipe > tp->snd_ssthresh) {
> +		if (tp->sackhint.recover_fs == 0)
> +			tp->sackhint.recover_fs =
> +			    max(1, tp->snd_nxt - tp->snd_una);
> 		snd_cnt = (tp->sackhint.prr_delivered * tp->snd_ssthresh /
> 		    tp->sackhint.recover_fs) - tp->sackhint.sack_bytes_rexmit;
> -	else {
> +	} else {
> 		if (V_tcp_do_prr_conservative)
> 			limit = tp->sackhint.prr_delivered -
> 			    tp->sackhint.sack_bytes_rexmit;