svn commit: r278729 - head/sys/sys

[hiren panchasara]

On 03/16/15 at 03:39P, Gleb Smirnoff wrote:
> On Wed, Mar 11, 2015 at 02:36:07PM -0700, hiren panchasara wrote:
> h> On 02/13/15 at 11:19P, Simon J. Gerraty wrote:
> h> > Author: sjg
> h> > Date: Fri Feb 13 23:19:35 2015
> h> > New Revision: 278729
> h> > URL: https://svnweb.freebsd.org/changeset/base/278729
> h> > 
> h> > Log:
> h> >   sbspace: size of bleft, mleft must match sockbuf fields to avoid
> h> >   overflow on amd64
> h> >   
> h> >   Submitted by:	anshukla at juniper.net
> h> >   Obtained from:	Juniper Networks
> h> 
> h> Talking to sjg on -arch to MFC this. If he cannot get around doing that,
> h> I'll do it tomorrow. 
> h> 
> h> Letting people know here to see if there are any objections.
> 
> Would that fix the bug we've been discussing?

Unsure as I am not sure what caused the issue I saw.

For those who do not know the details, we recently saw a userland
process stuck spinning at 100% around sbcut_internal(). Inside
sbflush_internal(), the sb_cc was grown to be about 4G. And before
passing it to sbcut_internal(), we cast it from uint to int which
would make that valud -ve.

Gleb pointed out to me that sbspace() is supposed to check/stop sb_cc
from growing that large.

Now, I am not sure if we'd ever run into this situation again but
current fix is a great catch anyways.

I still have 2 questions around what we saw. It'd be great if someone can
clarify them for my understanding:

1) Even if we get into such a scenario that we were in, following would
help by not looping endlessly.

--- uipc_sockbuf.c.0    2015-03-11 15:49:52.000000000 -0700
+++ uipc_sockbuf.c      2015-03-11 15:51:48.000000000 -0700
@@ -877,6 +877,9 @@
{
         struct mbuf *m, *n, *next, *mfree;

+      if (len < 0)
+               panic("%s: len is %d and it is supposed to be +ve",
+		    __func__, len);
+
         next = (m = sb->sb_mb) ? m->m_nextpkt : 0;
         mfree = NULL

2) We need 1) because we are casting a uint to int which _may_ rander a
value -ve. Is there a way we can avoid the casting?

cheers,
Hiren
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