From a0fb0eaa6780e60b7c5434dfe0e2ed402e5a4ea4 Mon Sep 17 00:00:00 2001
From: "katerina.kubecova" <katerina.kubecova@nic.cz>
Date: Tue, 19 Sep 2023 11:11:24 +0200
Subject: BGP: Setting and unsetting unknown attributes

All these must be declared as bytestring. Allows operators to delete
unwanted attributes breaking the Internet:

    https://blog.benjojo.co.uk/post/bgp-path-attributes-grave-error-handling
---
 doc/bird.sgml      |  7 +++++++
 filter/data.h      |  1 +
 filter/f-inst.c    |  2 +-
 filter/f-inst.h    | 15 +++++++++++++++
 proto/bgp/config.Y | 11 ++++++++++-
 5 files changed, 34 insertions(+), 2 deletions(-)

diff --git a/doc/bird.sgml b/doc/bird.sgml
index 3be266cb..366797ba 100644
--- a/doc/bird.sgml
+++ b/doc/bird.sgml
@@ -3287,6 +3287,13 @@ some of them (marked with `<tt/O/') are optional.
 	name="local Role"> is configured it set automatically.
 </descrip>
 
+<p>For attributes unknown by BIRD, the user can assign a name (on top level)
+to an attribute by its number. This defined name can be used then to both set
+(by a bytestring literal, transitive) or unset the given attribute even though
+BIRD knows nothing about it:
+
+<tt><label id="bgp-attribute-custom">attribute bgp <m/number/ bytestring <m/name/;</tt>
+
 <sect1>Example
 <label id="bgp-exam">
 
diff --git a/filter/data.h b/filter/data.h
index 90e779c8..3430455a 100644
--- a/filter/data.h
+++ b/filter/data.h
@@ -99,6 +99,7 @@ struct f_dynamic_attr {
   u8 bit;		/* For bitfield accessors */
   enum f_type f_type;	/* Filter type */
   uint ea_code;		/* EA code */
+  uint flags;
 };
 
 enum f_sa_code {
diff --git a/filter/f-inst.c b/filter/f-inst.c
index 510c431f..a7bec81e 100644
--- a/filter/f-inst.c
+++ b/filter/f-inst.c
@@ -857,7 +857,7 @@
       l->flags = EALF_SORTED;
       l->count = 1;
       l->attrs[0].id = da.ea_code;
-      l->attrs[0].flags = 0;
+      l->attrs[0].flags = da.flags;
       l->attrs[0].type = da.type;
       l->attrs[0].originated = 1;
       l->attrs[0].fresh = 1;
diff --git a/filter/f-inst.h b/filter/f-inst.h
index 2bde6378..955cfbdc 100644
--- a/filter/f-inst.h
+++ b/filter/f-inst.h
@@ -118,6 +118,21 @@ static inline struct f_dynamic_attr f_new_dynamic_attr_bit(u8 bit, enum f_type f
 static inline struct f_static_attr f_new_static_attr(int f_type, int code, int readonly)
 { return (struct f_static_attr) { .f_type = f_type, .sa_code = code, .readonly = readonly }; }
 
+static inline int f_type_attr(int f_type) {
+  switch (f_type) {
+    case T_INT:		return EAF_TYPE_INT;
+    case T_IP:		return EAF_TYPE_IP_ADDRESS;
+    case T_QUAD:	return EAF_TYPE_ROUTER_ID;
+    case T_PATH:	return EAF_TYPE_AS_PATH;
+    case T_CLIST:	return EAF_TYPE_INT_SET;
+    case T_ECLIST:	return EAF_TYPE_EC_SET;
+    case T_LCLIST:	return EAF_TYPE_LC_SET;
+    case T_BYTESTRING:	return EAF_TYPE_OPAQUE;
+    default:
+      cf_error("Custom route attribute of unsupported type");
+  }
+}
+
 /* Hook for call bt_assert() function in configuration */
 extern void (*bt_assert_hook)(int result, const struct f_line_item *assert);
 
diff --git a/proto/bgp/config.Y b/proto/bgp/config.Y
index a1f1f5ac..d9ff24d8 100644
--- a/proto/bgp/config.Y
+++ b/proto/bgp/config.Y
@@ -362,7 +362,16 @@ dynamic_attr: BGP_LARGE_COMMUNITY
 dynamic_attr: BGP_OTC
 	{ $$ = f_new_dynamic_attr(EAF_TYPE_INT, T_INT, EA_CODE(PROTOCOL_BGP, BA_ONLY_TO_CUSTOMER)); } ;
 
-
+custom_attr: ATTRIBUTE BGP NUM type symbol ';' {
+  if($3 > 255 || $3 < 1)
+    cf_error("Invalid attribute number. (Given %i, must be 1-255.)", $3);
+  if($4 != T_BYTESTRING)
+    cf_error("Attribute type must be bytestring, not %s.", f_type_name($4));
+  struct f_dynamic_attr* a = (struct f_dynamic_attr*) malloc(sizeof(struct f_dynamic_attr));
+  *a = f_new_dynamic_attr(f_type_attr($4), T_BYTESTRING, EA_CODE(PROTOCOL_BGP, $3));
+  a->flags = BAF_TRANSITIVE | BAF_OPTIONAL;
+  cf_define_symbol(new_config, $5, SYM_ATTRIBUTE, attribute, a);
+};
 
 CF_ENUM(T_ENUM_BGP_ORIGIN, ORIGIN_, IGP, EGP, INCOMPLETE)
 
-- 
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