It turns out that .branches > 0 in is_state_visited() is not a
sufficient condition to identify if two verifier states form a loop
when iterators convergence is computed. This commit adds logic to
distinguish situations like below:
(I) initial (II) initial
| |
V V
.---------> hdr ..
| | |
| V V
| .------... .------..
| | | | |
| V V V V
| ... ... .-> hdr ..
| | | | | |
| V V | V V
| succ <- cur | succ <- cur
| | | |
| V | V
| ... | ...
| | | |
'----' '----'
For both (I) and (II) successor 'succ' of the current state 'cur' was
previously explored and has branches count at 0. However, loop entry
'hdr' corresponding to 'succ' might be a part of current DFS path.
If that is the case 'succ' and 'cur' are members of the same loop
and have to be compared exactly.
Co-developed-by: Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx>
Co-developed-by: Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx>
Reviewed-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
Signed-off-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
---
include/linux/bpf_verifier.h | 15 +++
kernel/bpf/verifier.c | 207 ++++++++++++++++++++++++++++++++++-
2 files changed, 218 insertions(+), 4 deletions(-)
diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 38b788228594..24213a99cc79 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -373,10 +373,25 @@ struct bpf_verifier_state {
struct bpf_active_lock active_lock;
bool speculative;
bool active_rcu_lock;
+ /* If this state was ever pointed-to by other state's loop_entry field
+ * this flag would be set to true. Used to avoid freeing such states
+ * while they are still in use.
+ */
+ bool used_as_loop_entry;
/* first and last insn idx of this verifier state */
u32 first_insn_idx;
u32 last_insn_idx;
+ /* If this state is a part of states loop this field points to some
+ * parent of this state such that:
+ * - it is also a member of the same states loop;
+ * - DFS states traversal starting from initial state visits loop_entry
+ * state before this state.
+ * Used to compute topmost loop entry for state loops.
+ * State loops might appear because of open coded iterators logic.
+ * See get_loop_entry() for more information.
+ */
+ struct bpf_verifier_state *loop_entry;
/* jmp history recorded from first to last.
* backtracking is using it to go from last to first.
* For most states jmp_history_cnt is [0-3].
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 6573e8c77329..f23fbfe82c59 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1803,6 +1803,7 @@ static int copy_verifier_state(struct bpf_verifier_state *dst_state,
dst_state->first_insn_idx = src->first_insn_idx;
dst_state->last_insn_idx = src->last_insn_idx;
dst_state->dfs_depth = src->dfs_depth;
+ dst_state->used_as_loop_entry = src->used_as_loop_entry;
for (i = 0; i <= src->curframe; i++) {
dst = dst_state->frame[i];
if (!dst) {
@@ -1845,11 +1846,176 @@ static bool same_callsites(struct bpf_verifier_state *a, struct bpf_verifier_sta
return true;
}
+/* Open coded iterators allow back-edges in the state graph in order to
+ * check unbounded loops that iterators.
+ *
+ * In is_state_visited() it is necessary to know if explored states are
+ * part of some loops in order to decide whether non-exact states
+ * comparison could be used:
+ * - non-exact states comparison establishes sub-state relation and uses
+ * read and precision marks to do so, these marks are propagated from
+ * children states and thus are not guaranteed to be final in a loop;
+ * - exact states comparison just checks if current and explored states
+ * are identical (and thus form a back-edge).
+ *
+ * Paper "A New Algorithm for Identifying Loops in Decompilation"
+ * by Tao Wei, Jian Mao, Wei Zou and Yu Chen [1] presents a convenient
+ * algorithm for loop structure detection and gives an overview of
+ * relevant terminology. It also has helpful illustrations.
+ *
+ * [1] https://api.semanticscholar.org/CorpusID:15784067
+ *
+ * We use a similar algorithm but because loop nested structure is
+ * irrelevant for verifier ours is significantly simpler and resembles
+ * strongly connected components algorithm from Sedgewick's textbook.
+ *
+ * Define topmost loop entry as a first node of the loop traversed in a
+ * depth first search starting from initial state. The goal of the loop
+ * tracking algorithm is to associate topmost loop entries with states
+ * derived from these entries.
+ *
+ * For each step in the DFS states traversal algorithm needs to identify
+ * the following situations:
+ *
+ * initial initial initial
+ * | | |
+ * V V V
+ * ... ... .---------> hdr
+ * | | | |
+ * V V | V
+ * cur .-> succ | .------...
+ * | | | | | |
+ * V | V | V V
+ * succ '-- cur | ... ...
+ * | | |
+ * | V V
+ * | succ <- cur
+ * | |
+ * | V
+ * | ...
+ * | |
+ * '----'
+ *
+ * (A) successor state of cur (B) successor state of cur or it's entry
+ * not yet traversed are in current DFS path, thus cur and succ
+ * are members of the same outermost loop
+ *
+ * initial initial
+ * | |
+ * V V
+ * ... ...
+ * | |
+ * V V
+ * .------... .------...
+ * | | | |
+ * V V V V
+ * .-> hdr ... ... ...
+ * | | | | |
+ * | V V V V
+ * | succ <- cur succ <- cur
+ * | | |
+ * | V V
+ * | ... ...
+ * | | |
+ * '----' exit
+ *
+ * (C) successor state of cur is a part of some loop but this loop
+ * does not include cur or successor state is not in a loop at all.
+ *
+ * Algorithm could be described as the following python code:
+ *
+ * traversed = set() # Set of traversed nodes
+ * entries = {} # Mapping from node to loop entry
+ * depths = {} # Depth level assigned to graph node
+ * path = set() # Current DFS path
+ *
+ * # Find outermost loop entry known for n
+ * def get_loop_entry(n):
+ * h = entries.get(n, None)
+ * while h in entries and entries[h] != h:
+ * h = entries[h]
+ * return h
+ *
+ * # Update n's loop entry if h's outermost entry comes
+ * # before n's outermost entry in current DFS path.
+ * def update_loop_entry(n, h):
+ * n1 = get_loop_entry(n) or n
+ * h1 = get_loop_entry(h) or h
+ * if h1 in path and depths[h1] <= depths[n1]:
+ * entries[n] = h1
+ *
+ * def dfs(n, depth):
+ * traversed.add(n)
+ * path.add(n)
+ * depths[n] = depth
+ * for succ in G.successors(n):
+ * if succ not in traversed:
+ * # Case A: explore succ and update cur's loop entry
+ * # only if succ's entry is in current DFS path.
+ * dfs(succ, depth + 1)
+ * h = get_loop_entry(succ)
+ * update_loop_entry(n, h)
+ * else:
+ * # Case B or C depending on `h1 in path` check in update_loop_entry().
+ * update_loop_entry(n, succ)
+ * path.remove(n)
+ *
+ * To adapt this algorithm for use with verifier:
+ * - use st->branch == 0 as a signal that DFS of succ had been finished
+ * and cur's loop entry has to be updated (case A), handle this in
+ * update_branch_counts();
+ * - use st->branch > 0 as a signal that st is in the current DFS path;
+ * - handle cases B and C in is_state_visited();
+ * - update topmost loop entry for intermediate states in get_loop_entry().
+ */
+static struct bpf_verifier_state *get_loop_entry(struct bpf_verifier_state *st)
+{
+ struct bpf_verifier_state *topmost = st->loop_entry, *old;
+
+ while (topmost && topmost->loop_entry && topmost != topmost->loop_entry)
+ topmost = topmost->loop_entry;
+ /* Update loop entries for intermediate states to avoid this
+ * traversal in future get_loop_entry() calls.
+ */
+ while (st && st->loop_entry != topmost) {
+ old = st->loop_entry;
+ st->loop_entry = topmost;
+ st = old;
+ }
+ return topmost;
+}
+
+static void update_loop_entry(struct bpf_verifier_state *cur, struct bpf_verifier_state *hdr)
+{
+ struct bpf_verifier_state *cur1, *hdr1;
+
+ cur1 = get_loop_entry(cur) ?: cur;
+ hdr1 = get_loop_entry(hdr) ?: hdr;
+ /* The head1->branches check decides between cases B and C in
+ * comment for get_loop_entry(). If hdr1->branches == 0 then
+ * head's topmost loop entry is not in current DFS path,
+ * hence 'cur' and 'hdr' are not in the same loop and there is
+ * no need to update cur->loop_entry.
+ */
+ if (hdr1->branches && hdr1->dfs_depth <= cur1->dfs_depth) {
+ cur->loop_entry = hdr;
+ hdr->used_as_loop_entry = true;
+ }
+}
+
static void update_branch_counts(struct bpf_verifier_env *env, struct bpf_verifier_state *st)
{
while (st) {
u32 br = --st->branches;
+ /* br == 0 signals that DFS exploration for 'st' is finished,
+ * thus it is necessary to update parent's loop entry if it
+ * turned out that st is a part of some loop.
+ * This is a part of 'case A' in get_loop_entry() comment.
+ */
+ if (br == 0 && st->parent && st->loop_entry)
+ update_loop_entry(st->parent, st->loop_entry);
+
/* WARN_ON(br > 1) technically makes sense here,
* but see comment in push_stack(), hence:
*/
@@ -16650,10 +16816,11 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
{
struct bpf_verifier_state_list *new_sl;
struct bpf_verifier_state_list *sl, **pprev;
- struct bpf_verifier_state *cur = env->cur_state, *new;
+ struct bpf_verifier_state *cur = env->cur_state, *new, *loop_entry;
int i, j, n, err, states_cnt = 0;
bool force_new_state = env->test_state_freq || is_force_checkpoint(env, insn_idx);
bool add_new_state = force_new_state;
+ bool force_exact;
/* bpf progs typically have pruning point every 4 instructions
* http://vger.kernel.org/bpfconf2019.html#session-1
@@ -16748,8 +16915,10 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
*/
spi = __get_spi(iter_reg->off + iter_reg->var_off.value);
iter_state = &func(env, iter_reg)->stack[spi].spilled_ptr;
- if (iter_state->iter.state == BPF_ITER_STATE_ACTIVE)
+ if (iter_state->iter.state == BPF_ITER_STATE_ACTIVE) {
+ update_loop_entry(cur, &sl->state);
goto hit;
+ }
}
goto skip_inf_loop_check;
}
@@ -16780,7 +16949,36 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
add_new_state = false;
goto miss;
}
- if (states_equal(env, &sl->state, cur, false)) {
+ /* If sl->state is a part of a loop and this loop's entry is a part of
+ * current verification path then states have to be compared exactly.
+ * 'force_exact' is needed to catch the following case:
+ *
+ * initial Here state 'succ' was processed first,
+ * | it was eventually tracked to produce a
+ * V state identical to 'hdr'.
+ * .---------> hdr All branches from 'succ' had been explored
+ * | | and thus 'succ' has its .branches == 0.
+ * | V
+ * | .------... Suppose states 'cur' and 'succ' correspond
+ * | | | to the same instruction + callsites.
+ * | V V In such case it is necessary to check
+ * | ... ... if 'succ' and 'cur' are states_equal().
+ * | | | If 'succ' and 'cur' are a part of the
+ * | V V same loop exact flag has to be set.
+ * | succ <- cur To check if that is the case, verify
+ * | | if loop entry of 'succ' is in current
+ * | V DFS path.
+ * | ...
+ * | |
+ * '----'
+ *
+ * Additional details are in the comment before get_loop_entry().
+ */
+ loop_entry = get_loop_entry(&sl->state);
+ force_exact = loop_entry && loop_entry->branches > 0;
+ if (states_equal(env, &sl->state, cur, force_exact)) {
+ if (force_exact)
+ update_loop_entry(cur, loop_entry);
hit:
sl->hit_cnt++;
/* reached equivalent register/stack state,
@@ -16829,7 +17027,8 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
* speed up verification
*/
*pprev = sl->next;
- if (sl->state.frame[0]->regs[0].live & REG_LIVE_DONE) {
+ if (sl->state.frame[0]->regs[0].live & REG_LIVE_DONE &&
+ !sl->state.used_as_loop_entry) {
u32 br = sl->state.branches;
WARN_ONCE(br,
--
2.42.0
