Fix BIO_get_new_index() to return an error when it is exhausted.

Fixes #23655

BIO_get_new_index() returns a range of 129..255.

It is set to BIO_TYPE_START (128) initially and is incremented on each
call.
>= 256 is reserved for the class type flags (BIO_TYPE_DESCRIPTOR) so it
should error if it reaches the upper bound.

Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/23732)

2 files changed, 14 insertions, 10 deletions

diff --git a/doc/man3/BIO_find_type.pod b/doc/man3/BIO_find_type.pod
index 99bab8cce8..452c29c1bf 100644
--- a/doc/man3/BIO_find_type.pod
+++ b/doc/man3/BIO_find_type.pod
@@ -14,12 +14,12 @@ BIO_find_type, BIO_next, BIO_method_type - BIO chain traversal
 
 =head1 DESCRIPTION
 
-The BIO_find_type() searches for a BIO of a given type in a chain, starting
-at BIO B<b>. If B<type> is a specific type (such as B<BIO_TYPE_MEM>) then a search
-is made for a BIO of that type. If B<type> is a general type (such as
-B<BIO_TYPE_SOURCE_SINK>) then the next matching BIO of the given general type is
-searched for. BIO_find_type() returns the next matching BIO or NULL if none is
-found.
+The BIO_find_type() searches for a B<BIO> of a given type in a chain, starting
+at B<BIO> I<b>. If I<type> is a specific type (such as B<BIO_TYPE_MEM>) then a
+search is made for a B<BIO> of that type. If I<type> is a general type (such as
+B<BIO_TYPE_SOURCE_SINK>) then the next matching B<BIO> of the given general type is
+searched for. BIO_find_type() returns the next matching B<BIO> or NULL if none is
+found. If I<type> is B<BIO_TYPE_NONE> it will not find a match.
 
 The following general types are defined:
 B<BIO_TYPE_DESCRIPTOR>, B<BIO_TYPE_FILTER>, and B<BIO_TYPE_SOURCE_SINK>.
@@ -38,7 +38,7 @@ BIO_find_type() returns a matching BIO or NULL for no match.
 
 BIO_next() returns the next BIO in a chain.
 
-BIO_method_type() returns the type of the BIO B<b>.
+BIO_method_type() returns the type of the BIO I<b>.
 
 =head1 EXAMPLES
 
diff --git a/doc/man3/BIO_meth_new.pod b/doc/man3/BIO_meth_new.pod
index 5be14b2a28..589c1b18fc 100644
--- a/doc/man3/BIO_meth_new.pod
+++ b/doc/man3/BIO_meth_new.pod
@@ -82,9 +82,13 @@ The B<BIO_METHOD> type is a structure used for the implementation of new BIO
 types. It provides a set of functions used by OpenSSL for the implementation
 of the various BIO capabilities. See the L<bio(7)> page for more information.
 
-BIO_meth_new() creates a new B<BIO_METHOD> structure. It should be given a
-unique integer B<type> and a string that represents its B<name>.
-Use BIO_get_new_index() to get the value for B<type>.
+BIO_meth_new() creates a new B<BIO_METHOD> structure that contains a type
+identifier I<type> and a string that represents its B<name>.
+B<type> can be set to either B<BIO_TYPE_NONE> or via BIO_get_new_index() if
+a unique type is required for searching (See L<BIO_find_type(3)>)
+
+Note that BIO_get_new_index() can only be used 127 times before it returns an
+error.
 
 The set of
 standard OpenSSL provided BIO types is provided in F<< <openssl/bio.h> >>.