[PATCH bpf-next v3 05/13] bpf: 4-byte precise clean_verifier_state

Eduard Zingerman
2026-04-10T09:29:35Z

Migrate clean_verifier_state() and its liveness queries from 8-byte
SPI granularity to 4-byte half-slot granularity.

In __clean_func_state(), each SPI is cleaned in two independent
halves:
- half_spi 2*i (lo): slot_type[0..3]
- half_spi 2*i+1 (hi): slot_type[4..7]

Slot types STACK_DYNPTR, STACK_ITER and STACK_IRQ_FLAG are never
cleaned, as their slot type markers are required by
destroy_if_dynptr_stack_slot(), is_iter_reg_valid_uninit() and
is_irq_flag_reg_valid_uninit() for correctness.

When only the hi half is dead, spilled_ptr metadata is destroyed and
the lo half's STACK_SPILL bytes are downgraded to STACK_MISC or
STACK_ZERO. When only the lo half is dead, spilled_ptr is preserved
because the hi half may still need it for state comparison.

Signed-off-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
---
kernel/bpf/liveness.c | 14 ++++----
kernel/bpf/verifier.c | 94 ++++++++++++++++++++++++++++++++++++++++++---------
2 files changed, 85 insertions(+), 23 deletions(-)

diff --git a/kernel/bpf/liveness.c b/kernel/bpf/liveness.c
index 9e36ea5f2eec..e2697cd9ae27 100644
--- a/kernel/bpf/liveness.c
+++ b/kernel/bpf/liveness.c
@@ -732,13 +732,12 @@ int bpf_update_live_stack(struct bpf_verifier_env *env)
return 0;
}

-static bool is_live_before(struct func_instance *instance, u32 insn_idx, u32 frameno, u32 spi)
+static bool is_live_before(struct func_instance *instance, u32 insn_idx, u32 frameno, u32 half_spi)
{
struct per_frame_masks *masks;

masks = get_frame_masks(instance, frameno, insn_idx);
- return masks && (spis_test_bit(masks->live_before, spi * 2) ||
- spis_test_bit(masks->live_before, spi * 2 + 1));
+ return masks && spis_test_bit(masks->live_before, half_spi);
}

int bpf_live_stack_query_init(struct bpf_verifier_env *env, struct bpf_verifier_state *st)
@@ -759,7 +758,7 @@ int bpf_live_stack_query_init(struct bpf_verifier_env *env, struct bpf_verifier_
return 0;
}

-bool bpf_stack_slot_alive(struct bpf_verifier_env *env, u32 frameno, u32 spi)
+bool bpf_stack_slot_alive(struct bpf_verifier_env *env, u32 frameno, u32 half_spi)
{
/*
* Slot is alive if it is read before q->st->insn_idx in current func instance,
@@ -773,15 +772,16 @@ bool bpf_stack_slot_alive(struct bpf_verifier_env *env, u32 frameno, u32 spi)
bool alive;

curframe_instance = q->instances[q->curframe];
- if (is_live_before(curframe_instance, q->insn_idx, frameno, spi))
+ alive = is_live_before(curframe_instance, q->insn_idx, frameno, half_spi);
+ if (alive)
return true;

for (i = frameno; i < q->curframe; i++) {
callsite = curframe_instance->callchain.callsites[i];
instance = q->instances[i];
alive = bpf_calls_callback(env, callsite)
- ? is_live_before(instance, callsite, frameno, spi)
- : is_live_before(instance, callsite + 1, frameno, spi);
+ ? is_live_before(instance, callsite, frameno, half_spi)
+ : is_live_before(instance, callsite + 1, frameno, half_spi);
if (alive)
return true;
}
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index ee3945a919e3..0731e99aa541 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -20059,11 +20059,10 @@ static bool check_scalar_ids(u32 old_id, u32 cur_id, struct bpf_idmap *idmap)
return check_ids(old_id, cur_id, idmap);
}

-static void clean_func_state(struct bpf_verifier_env *env,
- struct bpf_func_state *st,
- u32 ip)
+static void __clean_func_state(struct bpf_verifier_env *env,
+ struct bpf_func_state *st,
+ u16 live_regs, int frame)
{
- u16 live_regs = env->insn_aux_data[ip].live_regs_before;
int i, j;

for (i = 0; i < BPF_REG_FP; i++) {
@@ -20075,26 +20074,83 @@ static void clean_func_state(struct bpf_verifier_env *env,
__mark_reg_not_init(env, &st->regs[i]);
}

+ /*
+ * Clean dead 4-byte halves within each SPI independently.
+ * half_spi 2*i → lower half: slot_type[0..3] (closer to FP)
+ * half_spi 2*i+1 → upper half: slot_type[4..7] (farther from FP)
+ */
for (i = 0; i < st->allocated_stack / BPF_REG_SIZE; i++) {
- if (!bpf_stack_slot_alive(env, st->frameno, i)) {
- __mark_reg_not_init(env, &st->stack[i].spilled_ptr);
- for (j = 0; j < BPF_REG_SIZE; j++)
+ bool lo_live = bpf_stack_slot_alive(env, frame, i * 2);
+ bool hi_live = bpf_stack_slot_alive(env, frame, i * 2 + 1);
+
+ if (!hi_live || !lo_live) {
+ int start = !lo_live ? 0 : BPF_REG_SIZE / 2;
+ int end = !hi_live ? BPF_REG_SIZE : BPF_REG_SIZE / 2;
+ u8 stype = st->stack[i].slot_type[7];
+
+ /*
+ * Don't clear special slots.
+ * destroy_if_dynptr_stack_slot() needs STACK_DYNPTR to
+ * detect overwrites and invalidate associated data slices.
+ * is_iter_reg_valid_uninit() and is_irq_flag_reg_valid_uninit()
+ * check for their respective slot types to detect double-create.
+ */
+ if (stype == STACK_DYNPTR || stype == STACK_ITER ||
+ stype == STACK_IRQ_FLAG)
+ continue;
+
+ /*
+ * Only destroy spilled_ptr when hi half is dead.
+ * If hi half is still live with STACK_SPILL, the
+ * spilled_ptr metadata is needed for correct state
+ * comparison in stacksafe().
+ * is_spilled_reg() is using slot_type[7], but
+ * is_spilled_scalar_after() check either slot_type[0] or [4]
+ */
+ if (!hi_live) {
+ struct bpf_reg_state *spill = &st->stack[i].spilled_ptr;
+
+ if (lo_live && stype == STACK_SPILL) {
+ u8 val = STACK_MISC;
+
+ /*
+ * 8 byte spill of scalar 0 where half slot is dead
+ * should become STACK_ZERO in lo 4 bytes.
+ */
+ if (register_is_null(spill))
+ val = STACK_ZERO;
+ for (j = 0; j < 4; j++) {
+ u8 *t = &st->stack[i].slot_type[j];
+
+ if (*t == STACK_SPILL)
+ *t = val;
+ }
+ }
+ __mark_reg_not_init(env, spill);
+ }
+ for (j = start; j < end; j++)
st->stack[i].slot_type[j] = STACK_INVALID;
}
}
}

-static void clean_verifier_state(struct bpf_verifier_env *env,
+static int clean_verifier_state(struct bpf_verifier_env *env,
struct bpf_verifier_state *st)
{
- int i, ip;
+ int i, err;

- bpf_live_stack_query_init(env, st);
- st->cleaned = true;
+ if (env->cur_state != st)
+ st->cleaned = true;
+ err = bpf_live_stack_query_init(env, st);
+ if (err)
+ return err;
for (i = 0; i <= st->curframe; i++) {
- ip = frame_insn_idx(st, i);
- clean_func_state(env, st->frame[i], ip);
+ u32 ip = frame_insn_idx(st, i);
+ u16 live_regs = env->insn_aux_data[ip].live_regs_before;
+
+ __clean_func_state(env, st->frame[i], live_regs, i);
}
+ return 0;
}

/* the parentage chains form a tree.
@@ -20193,11 +20249,12 @@ static void clear_singular_ids(struct bpf_verifier_env *env,
}));
}

-static void clean_live_states(struct bpf_verifier_env *env, int insn,
+static int clean_live_states(struct bpf_verifier_env *env, int insn,
struct bpf_verifier_state *cur)
{
struct bpf_verifier_state_list *sl;
struct list_head *pos, *head;
+ int err;

head = explored_state(env, insn);
list_for_each(pos, head) {
@@ -20212,8 +20269,11 @@ static void clean_live_states(struct bpf_verifier_env *env, int insn,
continue;
if (incomplete_read_marks(env, &sl->state))
continue;
- clean_verifier_state(env, &sl->state);
+ err = clean_verifier_state(env, &sl->state);
+ if (err)
+ return err;
}
+ return 0;
}

static bool regs_exact(const struct bpf_reg_state *rold,
@@ -20916,7 +20976,9 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
env->insn_processed - env->prev_insn_processed >= 8)
add_new_state = true;

- clean_live_states(env, insn_idx, cur);
+ err = clean_live_states(env, insn_idx, cur);
+ if (err)
+ return err;

loop = false;
head = explored_state(env, insn_idx);

--
2.53.0