bpf: verify callbacks as if they are called unknown number of times

Prior to this patch callbacks were handled as regular function calls,
execution of callback body was modeled exactly once.
This patch updates callbacks handling logic as follows:
- introduces a function push_callback_call() that schedules callback
  body verification in env->head stack;
- updates prepare_func_exit() to reschedule callback body verification
  upon BPF_EXIT;
- as calls to bpf_*_iter_next(), calls to callback invoking functions
  are marked as checkpoints;
- is_state_visited() is updated to stop callback based iteration when
  some identical parent state is found.

Paths with callback function invoked zero times are now verified first,
which leads to necessity to modify some selftests:
- the following negative tests required adding release/unlock/drop
  calls to avoid previously masked unrelated error reports:
  - cb_refs.c:underflow_prog
  - exceptions_fail.c:reject_rbtree_add_throw
  - exceptions_fail.c:reject_with_cp_reference
- the following precision tracking selftests needed change in expected
  log trace:
  - verifier_subprog_precision.c:callback_result_precise
    (note: r0 precision is no longer propagated inside callback and
           I think this is a correct behavior)
  - verifier_subprog_precision.c:parent_callee_saved_reg_precise_with_callback
  - verifier_subprog_precision.c:parent_stack_slot_precise_with_callback

Reported-by: Andrew Werner <[email protected]>
Closes: https://lore.kernel.org/bpf/CA+vRuzPChFNXmouzGG+wsy=6eMcfr1mFG0F3g7rbg-sedGKW3w@mail.gmail.com/
Acked-by: Andrii Nakryiko <[email protected]>
Signed-off-by: Eduard Zingerman <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

Author: eddyz87

include/linux/bpf_verifier.h

@@ -400,6 +400,7 @@ struct bpf_verifier_state {
 	struct bpf_idx_pair *jmp_history;
 	u32 jmp_history_cnt;
 	u32 dfs_depth;
+	u32 callback_unroll_depth;
 };
 
 #define bpf_get_spilled_reg(slot, frame, mask)				\
@@ -511,6 +512,10 @@ struct bpf_insn_aux_data {
 	 * this instruction, regardless of any heuristics
 	 */
 	bool force_checkpoint;
+	/* true if instruction is a call to a helper function that
+	 * accepts callback function as a parameter.
+	 */
+	bool calls_callback;
 };
 
 #define MAX_USED_MAPS 64 /* max number of maps accessed by one eBPF program */