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bpf: ensure state checkpointing at iter_next() call sites

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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@kernel.org>
Link: https://lore.kernel.org/r/20230310060149.625887-1-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Andrii Nakryiko 2023-03-09 22:01:49 -08:00 committed by Alexei Starovoitov
parent 1456ddcce5
commit 4b5ce570db
2 changed files with 33 additions and 4 deletions
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6
include/linux/bpf_verifier.h
View File

@ -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 */

31
kernel/bpf/verifier.c
View File

@ -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;