mbedtls_asn1_store_named_data: clarify val allocation behavior

Document how mbedtls_asn1_store_named_data allocates val.p in the new
or modified entry.

Change the behavior to be more regular, always setting the new length
to val_len. This does not affect the previous documented behavior
since this aspect was not documented. This does not affect current
usage in Mbed TLS's X.509 module where calls with the same OID always
use the same size for the associated value.

Author: gilles-peskine-arm

include/mbedtls/asn1write.h

@@ -333,9 +333,13 @@ int mbedtls_asn1_write_octet_string( unsigned char **p, unsigned char *start,
  *                  through (will be updated in case of a new entry).
  * \param oid       The OID to look for.
  * \param oid_len   The size of the OID.
- * \param val       The data to store (can be \c NULL if you want to fill
- *                  it by hand).
+ * \param val       The associated data to store. If this is \c NULL,
+ *                  no data is copied to the new or existing buffer.
  * \param val_len   The minimum length of the data buffer needed.
+ *                  If this is 0, do not allocate a buffer for the associated
+ *                  data.
+ *                  If the OID was already present, enlarge, shrink or free
+ *                  the existing buffer to fit \p val_len.
  *
  * \return          A pointer to the new / existing entry on success.
  * \return          \c NULL if if there was a memory allocation error.

library/asn1write.c

@@ -432,18 +432,26 @@ mbedtls_asn1_named_data *mbedtls_asn1_store_named_data(
         memcpy( cur->oid.p, oid, oid_len );
 
         cur->val.len = val_len;
-        cur->val.p = mbedtls_calloc( 1, val_len );
-        if( cur->val.p == NULL )
+        if( val_len != 0 )
         {
-            mbedtls_free( cur->oid.p );
-            mbedtls_free( cur );
-            return( NULL );
+            cur->val.p = mbedtls_calloc( 1, val_len );
+            if( cur->val.p == NULL )
+            {
+                mbedtls_free( cur->oid.p );
+                mbedtls_free( cur );
+                return( NULL );
+            }
         }
 
         cur->next = *head;
         *head = cur;
     }
-    else if( cur->val.len < val_len )
+    else if( val_len == 0 )
+    {
+        mbedtls_free( cur->val.p );
+        cur->val.p = NULL;
+    }
+    else if( cur->val.len != val_len )
     {
         /*
          * Enlarge existing value buffer if needed