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: Andrii Nakryiko <andrii@xxxxxxxxxx>
---
include/linux/bpf_verifier.h | 6 +++++-
kernel/bpf/verifier.c | 31 ++++++++++++++++++++++++++++---
2 files changed, 33 insertions(+), 4 deletions(-)
diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 0c052bc79940..81d525d057c7 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -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 */
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 45a082284464..13fd4c893f3b 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -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;
--
2.34.1
