[PATCH bpf-next v4 21/24] bpf: Permit NULL checking pointer with non-zero fixed offset

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

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

bpf_list_del{,tail} helpers return a local kptr 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>
---
 kernel/bpf/verifier.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 1e72e559ea6b..58e58678382a 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -10622,15 +10622,20 @@ 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)) {
+		if (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.
+			 *
+			 * 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.
 			 */
-			return;
+			if (WARN_ON_ONCE(reg->type != (PTR_TO_BTF_ID | MEM_TYPE_LOCAL | PTR_MAYBE_NULL)))
+				return;
+			if (WARN_ON_ONCE(reg->smin_value || reg->smax_value || !tnum_equals_const(reg->var_off, 0)))
+				return;
 		}
 		if (is_null) {
 			reg->type = SCALAR_VALUE;
-- 
2.38.1