Add service_opt() string_decode

To control whether stringish Diameter types are decoded to string or
left as binary. The motivation is the same as in the parent commit: to
avoid large strings being copied when incoming Diameter messages are
passed between processes; or *if* in the case of messages destined for
handle_request and handle_answer callbacks, since these are decoded in
the dedicated processes that the callbacks take place in. It would be
possible to do something about other messages without requiring an
option, but disabling the decode is the most effective.

The value is a boolean(), true being the default for backwards
compatibility. Setting false causes both diameter_caps records and
decoded messages to contain binary() in relevant places that previously
had string(): diameter_app(3) callbacks need to be prepared for the
change.

The Diameter types affected are OctetString and the derived types that
can contain arbitrarily large values: OctetString, UTF8String,
DiameterIdentity, DiameterURI, IPFilterRule, and QoSFilterRule. Time and
Address are unaffected.

The DiameterURI decode has been redone using re(3), which both
simplifies and does away with a vulnerability resulting from the
conversion of arbitrary strings to atom.

The solution continues the use and abuse of the process dictionary for
encode/decode purposes, last seen in commit 0f9cdba.

1 files changed, 24 insertions, 0 deletions

diff --git a/lib/diameter/doc/src/diameter.xml b/lib/diameter/doc/src/diameter.xml
index a5a99f7835..cb397614e5 100644
--- a/lib/diameter/doc/src/diameter.xml
+++ b/lib/diameter/doc/src/diameter.xml
@@ -899,6 +899,30 @@ Options <c>monitor</c> and <c>link</c> are ignored.</p>
 Defaults to the empty list.</p>
 </item>
 
+<marker id="string_decode"/>
+<tag><c>{string_decode, boolean()}</c></tag>
+<item>
+<p>
+Whether or not to decode AVPs of type &dict_OctetString; and its
+derived types &dict_DiameterIdentity;, &dict_DiameterURI;,
+&dict_IPFilterRule;, &dict_QoSFilterRule;, and &dict_UTF8String;.
+If <c>true</c> then AVPs of these types are decoded to string().
+If <c>false</c> then values are retained as binary().</p>
+
+<p>
+Defaults to <c>true</c>.</p>
+
+<warning>
+<p>
+This option should be set to <c>false</c>
+since a sufficiently malicious peer can otherwise cause large amounts
+of memory to be consumed when decoded Diameter messages are passed
+between processes.
+The default value is for backwards compatibility.</p>
+</warning>
+
+</item>
+
 <tag><c>{use_shared_peers, boolean() | [node()] | evaluable()}</c></tag>
 <item>
 <p>