bpf: Fix check_func_arg_reg_off bug for graph root/node

The commit being fixed introduced a hunk into check_func_arg_reg_off
that bypasses reg->off == 0 enforcement when offset points to a graph
node or root. This might possibly be done for treating bpf_rbtree_remove
and others as KF_RELEASE and then later check correct reg->off in helper
argument checks.

But this is not the case, those helpers are already not KF_RELEASE and
permit non-zero reg->off and verify it later to match the subobject in
BTF type.

However, this logic leads to bpf_obj_drop permitting free of register
arguments with non-zero offset when they point to a graph root or node
within them, which is not ok.

For instance:

struct foo {
int i;
int j;
struct bpf_rb_node node;
};

struct foo *f = bpf_obj_new(typeof(*f));
if (!f) ...
bpf_obj_drop(f); // OK
bpf_obj_drop(&f->i); // still ok from verifier PoV
bpf_obj_drop(&f->node); // Not OK, but permitted right now

Fix this by dropping the whole part of code altogether.

Fixes: 6a3cd3318ff6 ("bpf: Migrate release_on_unlock logic to non-owning ref semantics")
Signed-off-by: Kumar Kartikeya Dwivedi <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

kernel/bpf/verifier.c

···
 		if (arg_type_is_dynptr(arg_type) && type == PTR_TO_STACK)
 			return 0;
 
-		if ((type_is_ptr_alloc_obj(type) || type_is_non_owning_ref(type)) && reg->off) {
-			if (reg_find_field_offset(reg, reg->off, BPF_GRAPH_NODE_OR_ROOT))
-				return __check_ptr_off_reg(env, reg, regno, true);
-
-			verbose(env, "R%d must have zero offset when passed to release func\n",
-				regno);
-			verbose(env, "No graph node or root found at R%d type:%s off:%d\n", regno,
-				btf_type_name(reg->btf, reg->btf_id), reg->off);
-			return -EINVAL;
-		}
-
 		/* Doing check_ptr_off_reg check for the offset will catch this
 		 * because fixed_off_ok is false, but checking here allows us
 		 * to give the user a better error message.