lib/talloc: Disrupt buffer overflow attacks on Samba by using random magic

lib/talloc: Disrupt buffer overflow attacks on Samba by using random magic

By setting the talloc magic to a random value, we make it much harder to overwrite a talloc_chunk
in a valid way with a simple buffer overflow.

The flags are placed before more senstive variables so they have to be overwritten first.

Inspired by the exploit in: http://blog.csnc.ch/wp-content/uploads/2012/07/sambaexploit_v1.0.pdf

Andrew Bartlett

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Signed-off-by: Adrian Cochrane <adrianc@catalyst.net.nz>
Reviewed-by: Jeremy Allison <jra@samba.org>