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bgpd: Avoid cluster list attribute truncation #22081

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Merged
donaldsharp merged 1 commit into from
Jun 2, 2026
+30 −19
Merged

bgpd: Avoid cluster list attribute truncation #22081

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49 changes: 30 additions & 19 deletions bgpd/bgp_attr.c
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Original file line number Diff line number Diff line change
Expand Up @@ -5712,27 +5712,38 @@ bgp_size_t bgp_packet_attribute(struct bgp *bgp, struct peer *peer, struct strea
stream_put_in_addr(s, &from->remote_id);

/* Cluster list. */
stream_putc(s, BGP_ATTR_FLAG_OPTIONAL);
stream_putc(s, BGP_ATTR_CLUSTER_LIST);

if (cluster) {
stream_putc(s, cluster->length + 4);
/* If this peer configuration's parent BGP has
* cluster_id. */
if (CHECK_FLAG(bgp->config, BGP_CONFIG_CLUSTER_ID))
stream_put_in_addr(s, &bgp->cluster_id);
else
stream_put_in_addr(s, &bgp->router_id);
stream_put(s, cluster->list, cluster->length);
/* RFC 4271 4.3 says:
* The fourth high-order bit (bit 3) of the Attribute Flags octet
* is the Extended Length bit. It defines whether the Attribute
* Length is one octet (if set to 0) or two octets (if set to 1).
*
* If the Extended Length bit of the Attribute Flags octet is set
* to 0, the third octet of the Path Attribute contains the length
* of the attribute data in octets.
*
* If the Extended Length bit of the Attribute Flags octet is set
* to 1, the third and fourth octets of the path attribute contain
* the length of the attribute data in octets.
*/
if (cluster && cluster->length + 4 > 255) {
stream_putc(s, BGP_ATTR_FLAG_OPTIONAL | BGP_ATTR_FLAG_EXTLEN);
stream_putc(s, BGP_ATTR_CLUSTER_LIST);
stream_putw(s, cluster->length + 4);
} else {
stream_putc(s, 4);
/* If this peer configuration's parent BGP has
* cluster_id. */
if (CHECK_FLAG(bgp->config, BGP_CONFIG_CLUSTER_ID))
stream_put_in_addr(s, &bgp->cluster_id);
else
stream_put_in_addr(s, &bgp->router_id);
stream_putc(s, BGP_ATTR_FLAG_OPTIONAL);
stream_putc(s, BGP_ATTR_CLUSTER_LIST);
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@donaldsharp donaldsharp May 28, 2026

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Why not move these two lines outside the if/else and remove lines 5730 and 5731

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@ton31337 ton31337 May 28, 2026

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That's valid, but IMO, this doesn't do anything with the relevant fix. We should then fix this together with BGP_ATTR_LARGE_COMMUNITIES, BGP_ATTR_COMMUNITIES, etc. (the same pattern).

stream_putc(s, cluster ? cluster->length + 4 : 4);
}

/* If this peer configuration's parent BGP has
* cluster_id. */
if (CHECK_FLAG(bgp->config, BGP_CONFIG_CLUSTER_ID))
stream_put_in_addr(s, &bgp->cluster_id);
else
stream_put_in_addr(s, &bgp->router_id);

if (cluster)
stream_put(s, cluster->list, cluster->length);
}

/* Extended IPv6/Communities attributes. */
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