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.

3 files changed, 36 insertions, 2 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>
diff --git a/lib/diameter/doc/src/diameter_dict.xml b/lib/diameter/doc/src/diameter_dict.xml
index 810a146b88..9db9bcffde 100644
--- a/lib/diameter/doc/src/diameter_dict.xml
+++ b/lib/diameter/doc/src/diameter_dict.xml
@@ -529,6 +529,11 @@ answer record and passed to a &app_handle_request;
 callback upon reception of an incoming request.</p>
 
 <p>
+In cases in which there is a choice between list() and binary() types
+for OctetString() and derived types, the representation is determined
+by the value of &mod_string_decode;.</p>
+
+<p>
 <em>Basic AVP Data Formats</em></p>
 
 <marker id="OctetString"/>
@@ -541,7 +546,7 @@ callback upon reception of an incoming request.</p>
 <marker id="Grouped"/>
 
 <pre>
-OctetString() = [0..255]
+OctetString() = string() | binary()
 Integer32()   = -2147483647..2147483647
 Integer64()   = -9223372036854775807..9223372036854775807
 Unsigned32()  = 0..4294967295
@@ -603,7 +608,7 @@ and <c>{{2104,2,26},{9,42,23}}</c> (both inclusive) can be encoded.</p>
 
 <marker id="UTF8String"/>
 <pre>
-UTF8String() = [integer()]
+UTF8String() = [integer()] | binary()
 </pre>
 
 <p>
diff --git a/lib/diameter/doc/src/seealso.ent b/lib/diameter/doc/src/seealso.ent
index 44541afb9b..b0e3a2c712 100644
--- a/lib/diameter/doc/src/seealso.ent
+++ b/lib/diameter/doc/src/seealso.ent
@@ -69,6 +69,8 @@ significant.
 <!ENTITY connect_timer '<seealso marker="#connect_timer">connect_timer</seealso>'>
 <!ENTITY watchdog_timer '<seealso marker="#watchdog_timer">watchdog_timer</seealso>'>
 
+<!ENTITY mod_string_decode '<seealso marker="diameter#service_opt">diameter:service_opt()</seealso> <seealso marker="diameter#string_decode">string_decode</seealso>'>
+
 <!-- diameter_app -->
 
 <!ENTITY app_handle_answer '<seealso marker="diameter_app#Mod:handle_answer-4">handle_answer/4</seealso>'>
@@ -102,6 +104,9 @@ significant.
 
 <!ENTITY dict_Address '<seealso marker="diameter_dict#DATA_TYPES">Address()</seealso>'>
 <!ENTITY dict_DiameterIdentity '<seealso marker="diameter_dict#DATA_TYPES">DiameterIdentity()</seealso>'>
+<!ENTITY dict_DiameterURI '<seealso marker="diameter_dict#DATA_TYPES">DiameterURI()</seealso>'>
+<!ENTITY dict_IPFilterRule '<seealso marker="diameter_dict#DATA_TYPES">IPFilterRule()</seealso>'>
+<!ENTITY dict_QoSFilterRule '<seealso marker="diameter_dict#DATA_TYPES">QoSFilterRule()</seealso>'>
 <!ENTITY dict_Grouped '<seealso marker="diameter_dict#DATA_TYPES">Grouped()</seealso>'>
 <!ENTITY dict_OctetString '<seealso marker="diameter_dict#DATA_TYPES">OctetString()</seealso>'>
 <!ENTITY dict_Time '<seealso marker="diameter_dict#DATA_TYPES">Time()</seealso>'>