ospf6d: clear local ifp per ECMP path rebuild

[florath]

ospf6_intra_prefix_route_ecmp_path() can reuse a stale ifp value while rebuilding the effective nexthop set for an existing intra-prefix route.

What changes

• reset ifp to NULL at the start of each old_route->paths iteration

Why
The current loop only assigns ifp for self-originated Intra-Prefix-LSAs. Without an explicit reset, a previous successful interface lookup can leak into a later unrelated path and cause FRR to install a wrong on-link nexthop.

Impact

• fixes incorrect direct-connect nexthop reuse

Fixes #21036

[greptile-apps]

Greptile Summary

This PR fixes a stale ifp pointer bug in ospf6_intra_prefix_route_ecmp_path() (ospf6d/ospf6_intra.c). When rebuilding the effective nexthop set for an existing intra-prefix route, the loop over old_route->paths only assigns ifp for self-originated Intra-Prefix-LSAs (where intra_prefix_lsa->ref_adv_router == oa->ospf6->router_id). Without an explicit reset, a non-NULL ifp from one iteration would silently carry into the next, causing the direct-connect nexthop path (ospf6_route_add_nexthop) to be taken instead of the correct ospf6_route_merge_nexthops path for unrelated paths.

Key changes:

• A single ifp = NULL; statement is inserted at the top of the for (ALL_LIST_ELEMENTS_RO(old_route->paths, anode, o_path)) loop body (line 1535), ensuring the pointer is fresh for every path entry.
• The fix is minimal, targeted, and has no side-effects on the surrounding logic.
• The ifp variable is declared as struct interface *ifp = NULL; at function scope (line 1362), so the declaration already sets it correctly for the first iteration — but only an explicit per-iteration reset prevents leakage on subsequent iterations.

Confidence Score: 5/5

• This PR is safe to merge — it is a single-line, clearly correct bug fix with no risk of regression.
• The change is minimal (one line added), the root cause is well understood and described, the fix directly addresses the issue (stale ifp pointer in a per-path loop), and the surrounding logic is unchanged. No new branches, no new data structures, no API changes.
• No files require special attention.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A["ospf6_intra_prefix_route_ecmp_path()"] --> B["Iterate old_route->paths"]
    B --> C["ifp = NULL  (NEW FIX)"]
    C --> D["Lookup ls_entry in spf_table"]
    D --> E{ls_entry found?}
    E -- No --> F["continue to next path"]
    E -- Yes --> G["Lookup lsa in lsdb"]
    G --> H{lsa found?}
    H -- No --> F
    H -- Yes --> I["intra_prefix_lsa = lsa_after_header"]
    I --> J{Self-originated LSA?}
    J -- Yes --> K["ifp = if_lookup_prefix()"]
    J -- No --> L["ifp stays NULL"]
    K --> M{ifp != NULL?}
    L --> M
    M -- Yes --> N["ospf6_route_add_nexthop<br/>direct-connect via ifp"]
    M -- No --> O["ospf6_route_merge_nexthops<br/>copy from ls_entry"]
    N --> B
    O --> B
    B --> P["Update RIB/FIB"]

Loading

_{Last reviewed commit: 8d4b765}

[donaldsharp]

@Mergifyio backport stable/10.6

[mergify]

backport stable/10.6

✅ Backports have been created

Details

• #21039 ospf6d: clear local ifp per ECMP path rebuild (backport #21037) has been created for branch stable/10.6