From: Claudio Jeker <cjeker@diehard.n-r-g.com>
Subject: Re: let bpf pick the first bpf_if, not the bpf_if with the lowest dlt
To: David Gwynne <david@gwynne.id.au>
Cc: tech@openbsd.org
Date: Thu, 21 Nov 2024 08:03:11 +0100

On Wed, Nov 20, 2024 at 10:18:35AM +1000, David Gwynne wrote:
> this lets bpf_setif pick the first found dlt in the bpf_if list when
> attaching a bpf descriptor to an interface. back in bpf.c r1.52
> claudio changed bpf_setif so it would prefer lower numbered DLTs
> when attaching a bpf descriptor to an interface. before that it would
> pick the last bpf_if that was added because of how the bpf_if list was
> ordered.
> 
> claudio made the change so things would show the "most common" DLT
> by default, and the example given was an atw(4) wifi interface.
> wifi interfaces provide at least two and often more DLTs,
> 
> wifi interfaces attach a DLT_EN10MB interface first, which is what
> people expect when they tcpdump on an interface, so this doesn't break
> the change in 1.52.
> 
> however, i want to attach multiple DLTs to some other interfaces, but
> control which one is the default regardless of the number used for the
> DLT identifier. letting bpf pick the first attached dlt on an interface
> is the simplest fix for this, and doesn't seem to break existing use
> cases.
> 
> ok?

OK claudio@

The behaviour remains the same but the logic is a bit simpler. If there is
a driver that attaches the wrong way it can be fixed easily.
 
> Index: bpf.c
> ===================================================================
> RCS file: /cvs/src/sys/net/bpf.c,v
> diff -u -p -r1.227 bpf.c
> --- bpf.c	19 Nov 2024 23:26:35 -0000	1.227
> +++ bpf.c	19 Nov 2024 23:29:03 -0000
> @@ -1180,22 +1180,19 @@ bpf_setf(struct bpf_d *d, struct bpf_pro
>  int
>  bpf_setif(struct bpf_d *d, struct ifreq *ifr)
>  {
> -	struct bpf_if *bp, *candidate = NULL;
> +	struct bpf_if *bp;
>  	int error = 0;
>  
>  	/*
>  	 * Look through attached interfaces for the named one.
>  	 */
>  	TAILQ_FOREACH(bp, &bpf_iflist, bif_next) {
> -		if (strcmp(bp->bif_name, ifr->ifr_name) != 0)
> -			continue;
> -
> -		if (candidate == NULL || candidate->bif_dlt > bp->bif_dlt)
> -			candidate = bp;
> +		if (strcmp(bp->bif_name, ifr->ifr_name) == 0)
> +			break;
>  	}
>  
>  	/* Not found. */
> -	if (candidate == NULL)
> +	if (bp == NULL)
>  		return (ENXIO);
>  
>  	/*
> @@ -1208,12 +1205,12 @@ bpf_setif(struct bpf_d *d, struct ifreq 
>  		if ((error = bpf_allocbufs(d)))
>  			goto out;
>  	}
> -	if (candidate != d->bd_bif) {
> +	if (bp != d->bd_bif) {
>  		/*
>  		 * Detach if attached to something else.
>  		 */
>  		bpf_detachd(d);
> -		bpf_attachd(d, candidate);
> +		bpf_attachd(d, bp);
>  	}
>  	bpf_resetd(d);
>  out:
> 

-- 
:wq Claudio