KeyGen: Remove size annotation from gen_matrix_entry_x4()

gen_matrix_entry_x4() expects a slice of 4 polynomials to be filled.
In the caller gen_matrix(), this slice is constructed as a pointer
into an array of MLKEM_K polyvecs. Importantly, however, the slice of
4 poly's passed to gen_matrix_entry_x4() can cross the boundary
between two polyvec instances. Some compilers notice that from
the signature of gen_matrix_entry_x4(), and fail.

The behaviour of the code is intentional and safe here. To suppress
the compiler warning, this commit changes the signature of
gen_matrix_entry_x4() to use a generic poly* pointer, rather than
a pointer to a poly[4].

Signed-off-by: Hanno Becker <[email protected]>

mlkem/indcpa.c

@@ -149,7 +149,7 @@ static void unpack_ciphertext(polyvec *b, poly *v,
 // sampling on the output of a XOF.
 // clang-format off
 STATIC_TESTABLE
-void gen_matrix_entry_x4(poly vec[4], uint8_t *seed[4])
+void gen_matrix_entry_x4(poly *vec, uint8_t *seed[4])
   REQUIRES(IS_FRESH(vec, sizeof(poly) * 4))
   REQUIRES(IS_FRESH(seed, sizeof(uint8_t*) * 4))
   REQUIRES(IS_FRESH(seed[0], MLKEM_SYMBYTES + 2))
@@ -299,6 +299,8 @@ void gen_matrix(polyvec *a, const uint8_t seed[MLKEM_SYMBYTES],
       }
     }
 
+    // This call writes across polyvec boundaries for K=2 and K=3.
+    // This is intentional and safe.
     gen_matrix_entry_x4(&a[0].vec[0] + i, seedxy);
   }