Date: Sun, 28 Apr 2013 19:04:54 -0400 From: Mark Johnston <markjdb@gmail.com> To: Barry Spinney <spinney@tilera.com> Cc: "freebsd-net@freebsd.org" <freebsd-net@freebsd.org> Subject: Re: TF_NEEDSYN flag never set. Message-ID: <20130428230454.GA31215@gloom> In-Reply-To: <73BC01C897E9F642AC962BF311A45ACB100B0057@USMAExch1.tad.internal.tilera.com> References: <73BC01C897E9F642AC962BF311A45ACB100B0057@USMAExch1.tad.internal.tilera.com>
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On Sun, Apr 28, 2013 at 10:31:48PM +0000, Barry Spinney wrote:
> I am sorry if this is a dumb question, but I was trying to understand the FreeBSD TCP stack,
> and In particular I was trying to understand the use of the TF_NEEDSYN flag. This flag
> is referenced a number of times in tcp_input.c and tcp_output.c, but I don't think that
> it can ever be set.
>
> In particular grepping through the "../src/sys/netinet", one discovers that the only code
> that can set this flag is lines 1018 and 1020 of tcp_input.c. But, it appears to me that
> none of the lines in tcp_input.c from 999 thru 1023 are even reachable! The reason they
> are not reachable is because just ahead of them are the following lines:
>
> if (!syncache_add(&inc, &to, th, &so, m))
> goto drop;
> if (so == NULL) {
> ... // uninteresting lines, but no gotos
> return;
> }
> ... /unreachable code here
>
>
> Studying syncache_add (in file tcp_syncache.c), reveals three return statements.
> One of the returns, returns the value 0, which will cause the "goto drop" to be executed.
> The other two returns, return both the value 1 AND set "*sop = NULL", which should cause
> the following "if (so == NULL)" to execute the subsequent return statement.
>
> Is this intentional? (i.e. dead code awaiting future development?), or a bug?
> Or I am going blind to something obvious?
>
> Thanx Barry Spinney.
>
> (p.s. I doubt it matters which version of code, but to be precise this is from the
> /pub/FreeBSD/development/tarballs named "src_stable_6.tar.gz" dated "4/21/2013 01:15 AM",
> gotten from ftp1.us.freebsd.org<ftp://ftp1.us.freebsd.org>)
That tarball presumably contains sources for the stable branch of
FreeBSD 6, which probably isn't what you're looking for - the last
release from that branch was in 2008.
The relevant code in FreeBSD-CURRENT is different and your observations
don't seem to apply there. Based on a comment added in r156125, you seem
to be correct though. :)
http://svnweb.freebsd.org/base?view=revision&revision=156125
I'd suggest fetching src_current.tar.gz from the FTP same directory and
looking at that instead - you're more likely to get replies to questions
about the current tip of development.
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