The VideoLAN Forums

As some will be aware, there have recently been a number of SAP storms which appear to be due to a bug in Minisapserver. I've now managed to reproduce this and identify the problem.

This is a serious one - it does not affect minisapserver with the default config, but if you change your config to the values recommended in minisapserver's own documentation, you'll generate a SAP storm (thousands per second). See "How to kill my Cisco router... and those of my neighbours"


miniSAPserver version 0.3.4
on Ubuntu 7.10, gcc 4.1.3


The default config contains:

# Number of seconds between announces. 5 is default. Internet announces better use 30.
sap_delay=5

If you use this default, all is OK. If you add the "-s" flag to the
command, it prints a "." every time it sends out a sap. And as
expected, we get a "." appearing every 5 seconds.

However, if we make the suggested change and increase the value of
sap_delay to 30 (in fact to any value greater than 15!) we get our
SAP storm - thousands of SAPs (and thousands of dots on the screen)
appearing per second.

The problem appears to be an integer overflow in sapserver.cpp

unsigned n = config.Programs.size() ?: 1;
div_t d = div ((1000000000 / n) * config.GetDelay(), 1000000000);
struct timespec delay;
delay.tv_sec = d.quot;
delay.tv_nsec = d.rem;
(snip)
while(!should_exit)
{
for( unsigned int i = 0; i< announces.size() ; i ++ )
{
(snip - send the SAP here)
nanosleep( &delay, NULL );
}
}

So it appears that when config.GetDelay is >15, 1000000000*config.GetDelay
overflows, the arguments to nanosleep are negative.... it doesn't
sleep.... we get a storm.

A quick workaround is to remove the "div" line, and just have
delay.tv_sec = config.GetDelay();
delay.tv_nsec = 0;

I will leave development of a "proper" solution to someone else.
Bruce.

Update:

The fix for this problem is already in the SVN repo, but hasn't made it though to release yet.

Bearing in mind the potential for damage that the current release can cause, I would suggest that creating a new release would be desireable

I saw the following message in another list, it seems the fix is available, maybe it will help:

A new version of the VideoLAN miniSAPserver has been released, which addresses
some serious issues. We strongly recommend that users of minisap upgrade
ASAP. As usual, minisap can be downloaded from:

ftp://ftp.videolan.org/pub/videolan/min ... er/latest/

-- Changes from version 0.3.4 to version 0.3.5
* Fix announce timer
* Fix raw UDP media type