diff --git a/tools/testing/selftests/mm/pkey-arm64.h b/tools/testing/selftests/mm/pkey-arm64.h
index 8e9685e03c44..c5a78a2f211d 100644
--- a/tools/testing/selftests/mm/pkey-arm64.h
+++ b/tools/testing/selftests/mm/pkey-arm64.h
@@ -130,9 +130,10 @@ static inline u64 get_pkey_bits(u64 reg, int pkey)
 static inline void aarch64_write_signal_pkey(ucontext_t *uctxt, u64 pkey)
 {
 	struct _aarch64_ctx *ctx = GET_UC_RESV_HEAD(uctxt);
+	size_t resv_size = GET_UCP_RESV_SIZE(uctxt);
 	struct poe_context *poe_ctx =
 		(struct poe_context *) get_header(ctx, POE_MAGIC,
-						sizeof(uctxt->uc_mcontext), NULL);
+						  resv_size, NULL);
 	if (poe_ctx)
 		poe_ctx->por_el0 = pkey;
 }