bgpd: check and set extra num_labels

The handling of MPLS labels in BGP faces an issue due to the way labels
are stored in memory. They are stored in bgp_path_info but not in
bgp_adj_in and bgp_adj_out structures. As a consequence, some
configuration changes result in losing labels or even a bgpd crash. For
example, when retrieving routes from the Adj-RIB-in table
("soft-reconfiguration inbound" enabled), labels are missing.

bgp_path_info stores the MPLS labels, as shown below:

> struct bgp_path_info {
>   struct bgp_path_info_extra *extra;
>   [...]
> struct bgp_path_info_extra {
>	mpls_label_t label[BGP_MAX_LABELS];
>	uint32_t num_labels;
>   [...]

To solve those issues, a solution would be to set label data to the
bgp_adj_in and bgp_adj_out structures in addition to the
bgp_path_info_extra structure. The idea is to reference a common label
pointer in all these three structures. And to store the data in a hash
list in order to save memory.

However, an issue in the code prevents us from setting clean data
without a rework. The extra->num_labels field, which is intended to
indicate the number of labels in extra->label[], is not reliably checked
or set. The code often incorrectly assumes that if the extra pointer is
present, then a label must also be present, leading to direct access to
extra->label[] without verifying extra->num_labels. This assumption
usually works because extra->label[0] is set to MPLS_INVALID_LABEL when
a new bgp_path_info_extra is created, but it is technically incorrect.

Cleanup the label code by setting num_labels each time values are set in
extra->label[] and checking extra->num_labels before accessing the
labels.

Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>

0 files changed, 0 insertions, 0 deletions