Date: Fri, 10 Sep 2021 15:35:58 -0400 From: Mark Johnston <markj@freebsd.org> To: Andriy Gapon <avg@freebsd.org> Cc: "net@FreeBSD.org" <net@freebsd.org>, hackers@freebsd.org Subject: Re: recvmsg() "short receive" after FIONREAD Message-ID: <YTuznrhho4qGXqu8@nuc> In-Reply-To: <6f455869-cbdd-ee20-f2f8-f633e22071e9@FreeBSD.org> References: <500a2272-c1b3-3f97-0096-9fe8117c4b95@FreeBSD.org> <6f455869-cbdd-ee20-f2f8-f633e22071e9@FreeBSD.org>
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On Fri, Sep 10, 2021 at 10:15:37PM +0300, Andriy Gapon wrote: > On 10/09/2021 21:51, Andriy Gapon wrote: > > > > > > I observe a problem with the code that can be seen here: > > https://gitlab.freedesktop.org/pulseaudio/pulseaudio/-/blob/master/src/modules/rtp/sap.c#L142 > > > > > > The code uses ioctl(FIONREAD) to check the size of available data in a socket. > > Does / should this work? > > > > Then the code calls recvmsg() on the socket with single vector with iov_len > > equal to the size obtained earlier. > > > > But the return value from recvmsg() is smaller than the iov_len value. > > In my test I see 215 vs expected 263 (so, the difference is 48). > > > > Does this ring a bell to anyone? > > I see this on a month old 14.0-CURRENT arm64. > > > > From a quick look at soreceive_dgram() and some dtrace-ing, it seems that each > time recvmsg() is called soreceive_dgram() gets an mbuf chain where the first > mbuf is MT_SONAME (8), the second one is MT_CONTROL (14) and only the third one > is MT_DATA. > > Could it be that data in the first two mbuf-s (especially the MT_CONTROL one) is > reported by FIONREAD? Or, in other words, accounted in sb_acc? > But then it's not actually returned, of course, in recvmsg() ? Indeed, I suspect that this is the problem. Note that for kevent(EVFILT_READ) we subtract the number of control message bytes from the returned value, see filt_soread(). I wonder if FIONREAD should do the same thing.
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