bpf: Permit NULL checking pointer with non-zero fixed offset

Pointer increment on seeing PTR_MAYBE_NULL is already protected against,
hence make an exception for PTR_TO_BTF_ID | MEM_ALLOC while still
keeping the warning for other unintended cases that might creep in.

bpf_list_pop_{front,_back} helpers planned to be introduced in next
commit will return a MEM_ALLOC register with incremented offset pointing
to bpf_list_node field. The user is supposed to then obtain the pointer
to the entry using container_of after NULL checking it. The current
restrictions trigger a warning when doing the NULL checking. Revisiting
the reason, it is meant as an assertion which seems to actually work and
catch the bad case.

Hence, under no other circumstances can reg->off be non-zero for a
register that has the PTR_MAYBE_NULL type flag set.
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20221118015614.2013203-16-memxor@gmail.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/verifier.c

@@ -10791,16 +10791,19 @@ static void mark_ptr_or_null_reg(struct bpf_func_state *state,
 {
 	if (type_may_be_null(reg->type) && reg->id == id &&
 	    !WARN_ON_ONCE(!reg->id)) {
-		if (WARN_ON_ONCE(reg->smin_value || reg->smax_value ||
-				 !tnum_equals_const(reg->var_off, 0) ||
-				 reg->off)) {
-			/* Old offset (both fixed and variable parts) should
-			 * have been known-zero, because we don't allow pointer
-			 * arithmetic on pointers that might be NULL. If we
-			 * see this happening, don't convert the register.
-			 */
+		/* Old offset (both fixed and variable parts) should have been
+		 * known-zero, because we don't allow pointer arithmetic on
+		 * pointers that might be NULL. If we see this happening, don't
+		 * convert the register.
+		 *
+		 * But in some cases, some helpers that return local kptrs
+		 * advance offset for the returned pointer. In those cases, it
+		 * is fine to expect to see reg->off.
+		 */
+		if (WARN_ON_ONCE(reg->smin_value || reg->smax_value || !tnum_equals_const(reg->var_off, 0)))
+			return;
+		if (reg->type != (PTR_TO_BTF_ID | MEM_ALLOC | PTR_MAYBE_NULL) && WARN_ON_ONCE(reg->off))
 			return;
-		}
 		if (is_null) {
 			reg->type = SCALAR_VALUE;
 			/* We don't need id and ref_obj_id from this point