The function lowpan_iphc_mcast_ctx_addr_compress() contains two offset
errors that break context-based multicast address compression
(LOWPAN_IPHC_DAM_00).

When compressing the multicast address, the compressed format expects
exactly 6 bytes:
  - Bytes 0-1: Flags, scope, and reserved bits (from s6_addr[1..2])
  - Bytes 2-5: The 4-byte Group ID (from s6_addr[12..15])

Currently, the memcpy() operations use incorrect offsets:
1. The destination offset for the Group ID is &data[1] instead of
   &data[2]. This overwrites the previously copied scope byte.
2. The source offset for the Group ID is &ipaddr->s6_addr[11] instead
   of &ipaddr->s6_addr[12].

This mismatch results in a corrupted compressed address being
transmitted. Consequently, the receiving side fails to reconstruct the
original IPv6 address via lowpan_uncompress_multicast_ctx_daddr() since
it expects the Group ID to start at data[2].

Fix the logic by correcting both the destination and source offsets
so that the 6-byte compressed representation is assembled correctly.

Signed-off-by: Quan Sun <2022090917019@std.uestc.edu.cn>
---
 net/6lowpan/iphc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/6lowpan/iphc.c b/net/6lowpan/iphc.c
index e116d308a8df6..29ae68ca3ec15 100644
--- a/net/6lowpan/iphc.c
+++ b/net/6lowpan/iphc.c
@@ -1091,7 +1091,7 @@ static u8 lowpan_iphc_mcast_ctx_addr_compress(u8 **hc_ptr,
 	/* flags/scope, reserved (RIID) */
 	memcpy(data, &ipaddr->s6_addr[1], 2);
 	/* group ID */
-	memcpy(&data[1], &ipaddr->s6_addr[11], 4);
+	memcpy(&data[2], &ipaddr->s6_addr[12], 4);
 	lowpan_push_hc_data(hc_ptr, data, 6);
 
 	return LOWPAN_IPHC_DAM_00;

base-commit: 95084f1883a760e0d4290698346759d58e2b944a
-- 
2.43.0