Commit 4b5ce570 authored by Andrii Nakryiko's avatar Andrii Nakryiko Committed by Alexei Starovoitov
Browse files

bpf: ensure state checkpointing at iter_next() call sites 



State equivalence check and checkpointing performed in is_state_visited()
employs certain heuristics to try to save memory by avoiding state checkpoints
if not enough jumps and instructions happened since last checkpoint. This leads
to unpredictability of whether a particular instruction will be checkpointed
and how regularly. While normally this is not causing much problems (except
inconveniences for predictable verifier tests, which we overcome with
BPF_F_TEST_STATE_FREQ flag), turns out it's not the case for open-coded
iterators.

Checking and saving state checkpoints at iter_next() call is crucial for fast
convergence of open-coded iterator loop logic, so we need to force it. If we
don't do that, is_state_visited() might skip saving a checkpoint, causing
unnecessarily long sequence of not checkpointed instructions and jumps, leading
to exhaustion of jump history buffer, and potentially other undesired outcomes.
It is expected that with correct open-coded iterators convergence will happen
quickly, so we don't run a risk of exhausting memory.

This patch adds, in addition to prune and jump instruction marks, also a
"forced checkpoint" mark, and makes sure that any iter_next() call instruction
is marked as such.

Signed-off-by: default avatarAndrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20230310060149.625887-1-andrii@kernel.org


Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
parent 1456ddcc

include/linux/bpf_verifier.h

+5 −1
Original line number Diff line number Diff line
@@ -477,8 +477,12 @@ struct bpf_insn_aux_data { 


	/* below fields are initialized once */

	unsigned int orig_idx; /* original instruction index */

	bool prune_point;

	bool jmp_point;

	bool prune_point;

	/* ensure we check state equivalence and save state checkpoint and

	 * this instruction, regardless of any heuristics

	 */

	bool force_checkpoint;

};



#define MAX_USED_MAPS 64 /* max number of maps accessed by one eBPF program */

kernel/bpf/verifier.c

+28 −3
Original line number Diff line number Diff line
@@ -13865,6 +13865,17 @@ static bool is_prune_point(struct bpf_verifier_env *env, int insn_idx) 
	return env->insn_aux_data[insn_idx].prune_point;

}









static void mark_force_checkpoint(struct bpf_verifier_env *env, int idx)









{









	env->insn_aux_data[idx].force_checkpoint = true;









}

















static bool is_force_checkpoint(struct bpf_verifier_env *env, int insn_idx)









{









	return env->insn_aux_data[insn_idx].force_checkpoint;









}

























enum {









	DONE_EXPLORING = 0,









	KEEP_EXPLORING = 1,










@@ -13984,8 +13995,21 @@ static int visit_insn(int t, struct bpf_verifier_env *env)







			struct bpf_kfunc_call_arg_meta meta;

















			ret = fetch_kfunc_meta(env, insn, &meta, NULL);









			if (ret == 0 && is_iter_next_kfunc(&meta))









			if (ret == 0 && is_iter_next_kfunc(&meta)) {









				mark_prune_point(env, t);









				/* Checking and saving state checkpoints at iter_next() call









				 * is crucial for fast convergence of open-coded iterator loop









				 * logic, so we need to force it. If we don't do that,









				 * is_state_visited() might skip saving a checkpoint, causing









				 * unnecessarily long sequence of not checkpointed









				 * instructions and jumps, leading to exhaustion of jump









				 * history buffer, and potentially other undesired outcomes.









				 * It is expected that with correct open-coded iterators









				 * convergence will happen quickly, so we don't run a risk of









				 * exhausting memory.









				 */









				mark_force_checkpoint(env, t);









			}









		}









		return visit_func_call_insn(t, insns, env, insn->src_reg == BPF_PSEUDO_CALL);


















@@ -15172,7 +15196,8 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)







	struct bpf_verifier_state_list *sl, **pprev;









	struct bpf_verifier_state *cur = env->cur_state, *new;









	int i, j, err, states_cnt = 0;









	bool add_new_state = env->test_state_freq ? true : false;









	bool force_new_state = env->test_state_freq || is_force_checkpoint(env, insn_idx);









	bool add_new_state = force_new_state;

















	/* bpf progs typically have pruning point every 4 instructions









	 * http://vger.kernel.org/bpfconf2019.html#session-1










@@ -15269,7 +15294,7 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)







			 * at the end of the loop are likely to be useful in pruning.









			 */









skip_inf_loop_check:









			if (!env->test_state_freq &&









			if (!force_new_state &&









			    env->jmps_processed - env->prev_jmps_processed < 20 &&









			    env->insn_processed - env->prev_insn_processed < 100)









				add_new_state = false;