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From: Johannes Berg <johannes-cdvu00un1VgdHxzADdlk8Q@public.gmane.org>
To: Guy Harris <guy-FrUbXkNCsVf2fBVCVOL8/A@public.gmane.org>
Cc: radiotap-S783fYmB3Ccdnm+yROfE0A@public.gmane.org
Subject: Re: [RFA] 0-length PSDU reporting
Date: Tue, 14 Aug 2018 10:30:04 +0200
Message-ID: <1534235404.3547.10.camel@sipsolutions.net> (raw)
In-Reply-To: <1534156537.3093.7.camel-cdvu00un1VgdHxzADdlk8Q@public.gmane.org>

On Mon, 2018-08-13 at 12:35 +0200, Johannes Berg wrote:

> Yes, one case for example is when you know the packet metadata because
> you received it, but were unable to capture the data because it was a
> multi-user transmission to a different user, or because it was a multi-
> user transmission with more streams than you have RX chains, e.g. In
> this case I'm not even sure you know the real length of the packet, to
> be able to indicate it in the "Original Packet Length" field.
> 
> Additionally, I'm not sure it's possible to indicate an "Original Length
> Field" all the way through from the kernel while capturing, I believe
> only tcpdump/etc. will set this to long than the data frame from the
> kernel when they explicitly truncate data.

To clarify - I think this field will also be used (and I think that
should be valid) when you have a PPDU that did carry a PSDU, but you
don't know its length, and thus report

  frame length == captured length == radiotap header length.

Like I said, I don't currently see a way to - at least on Linux - report
frame length > captured length. Looking at libpcap's pcap-linux.c, we
see

        caplen = packet_len;
        if (caplen > handle->snapshot)
                caplen = handle->snapshot;

We do get packet_len as recvmsg(..., MSG_TRUNC), but inside the kernel
we cannot represent a message as simply not having the data and getting
truncated even without MSG_TRUNC, so that we can't actually get out of
recvmsg() with a packet_len bigger than the in-kernel buffer.

Guy, does that address your question sufficiently?

johannes

  parent reply index

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-06-18 14:37 [RFC] 0-length PPDU reporting Johannes Berg
     [not found] ` <1529332670.3490.50.camel-cdvu00un1VgdHxzADdlk8Q@public.gmane.org>
2018-06-18 16:05   ` Simon Barber
     [not found]     ` <00986C20-9443-40FC-9A7E-DE636A743951-vp0mx6+5gkqFX2APIN6yfw@public.gmane.org>
2018-06-18 19:20       ` Johannes Berg
     [not found]         ` <1529349637.3092.8.camel-cdvu00un1VgdHxzADdlk8Q@public.gmane.org>
2018-06-19 21:20           ` Guy Harris
     [not found]             ` <3028EC8F-6CFD-4F52-8349-D49B1F37AC22-FrUbXkNCsVf2fBVCVOL8/A@public.gmane.org>
2018-06-19 22:50               ` Guy Harris
     [not found]                 ` <BC6DCA08-4049-4B7A-A7C0-9FC545173FE5-FrUbXkNCsVf2fBVCVOL8/A@public.gmane.org>
2018-06-29 10:49                   ` Johannes Berg
2018-07-02 14:33   ` Johannes Berg
     [not found]     ` <1530542000.3117.4.camel-cdvu00un1VgdHxzADdlk8Q@public.gmane.org>
2018-07-03 12:13       ` Johannes Berg
2018-07-09  8:30   ` [RFA] 0-length PSDU reporting Johannes Berg
     [not found]     ` <1531125042.3298.5.camel-cdvu00un1VgdHxzADdlk8Q@public.gmane.org>
2018-08-09  1:53       ` Guy Harris
     [not found]         ` <AEBBAD8F-A779-4E0A-B40D-659D55DFE847-FrUbXkNCsVf2fBVCVOL8/A@public.gmane.org>
2018-08-13 10:35           ` Johannes Berg
     [not found]             ` <1534156537.3093.7.camel-cdvu00un1VgdHxzADdlk8Q@public.gmane.org>
2018-08-14  8:30               ` Johannes Berg [this message]
2018-09-04 12:10   ` [ADOPTION] " Johannes Berg

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