zebra: Fix SRv6 explicit SID allocation to use the provided locator

cscarpitta · cscarpitta · commit a9044d2a62f4 · 2025-10-22T09:54:12.000+02:00
Currently, when staticd allocates an explicit SID using the CLI:

  segment-routing
   srv6
    static-sids
     sid fcbb:bbbb:1:fe00::/64 locator MAIN behavior uDT46 vrf Vrf10

staticd requests the SRv6 SID Manager to allocate SID fcbb:bbbb:1:fe00::
from locator MAIN. However, the SID Manager incorrectly ignores the
provided locator MAIN and instead performs a lookup to find the first
locator that matches the SID prefix, then allocates the SID from that
locator regardless of which locator was requested.

This behavior can lead to SIDs being allocated from unintended locators
when multiple locators have overlapping prefixes, violating the user's
explicit locator specification.

This commit fixes the issue by adding a constraint in zebra_srv6_sid_decompose()
that enforces the SID Manager to allocate the SID only from the provided
locator when one is specified. The fix adds an early check in the locator
lookup loop to skip locators that don't match the caller-provided locator,
ensuring explicit SID allocation respects the user's locator choice.

Signed-off-by: Carmine Scarpitta &lt;cscarpit@cisco.com&gt;
diff --git a/zebra/zebra_srv6.c b/zebra/zebra_srv6.c
@@ -1322,10 +1322,14 @@ static bool zebra_srv6_sid_decompose(struct in6_addr *sid_value,
 	tmp_prefix.prefix = *sid_value;
 
 	/*
-	 * Lookup the parent locator of the SID and return the locator and
-	 * the function of the SID.
+	 * If the parent locator of the SID is provided, allocate the SID from that locator;
+	 * otherwise lookup the parent locator. Return the locator and SID function.
 	 */
 	for (ALL_LIST_ELEMENTS_RO(srv6->locators, node, l)) {
+		/* Skip locators that don't match the provided locator */
+		if (*locator != NULL && *locator != l)
+			continue;
+
 		/*
 		 * Check if the locator prefix includes the temporary prefix
 		 * representing the SID.
@@ -1801,7 +1805,7 @@ static int get_srv6_sid_explicit(struct zebra_srv6_sid **sid, struct srv6_sid_ct
 {
 	struct zebra_srv6_sid_ctx *zctx = NULL;
 	uint32_t sid_func = 0, sid_func_wide = 0;
-	struct srv6_locator *loc = NULL;
+	struct srv6_locator *loc = locator;
 	struct zebra_srv6_sid_block *block = NULL;
 	char buf[256];
 
