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Merge branch 'bpf-verifier-improve-state-pruning-for-scalar-registers'
Puranjay Mohan says: ==================== bpf: Improve state pruning for scalar registers V2: https://lore.kernel.org/all/20260203022229.1630849-1-puranjay@kernel.org/ Changes in V3: - Fix spelling mistakes in commit logs (AI) - Fix an incorrect comment in the selftest added in patch 5 (AI) - Improve the title of patch 5 V1: https://lore.kernel.org/all/20260202104414.3103323-1-puranjay@kernel.org/ Changes in V2: - Collected acked by Eduard - Removed some unnecessary comments - Added a selftest for id=0 equivalence in Patch 5 This series improves BPF verifier state pruning by relaxing scalar ID equivalence requirements. Scalar register IDs are used to track relationships between registers for bounds propagation. However, once an ID becomes "singular" (only one register/stack slot carries it), it can no longer participate in bounds propagation and becomes stale. These stale IDs can prevent pruning of otherwise equivalent states. The series addresses this in four patches: Patch 1: Assign IDs on stack fills to ensure stack slots have IDs before being read into registers, preparing for the singular ID clearing in patch 2. Patch 2: Clear IDs that appear only once before caching, as they cannot contribute to bounds propagation. Patch 3: Relax maybe_widen_reg() to only compare value-tracking fields (bounds, tnum, var_off) rather than also requiring ID matches. Two scalars with identical value constraints but different IDs represent the same abstract value and don't need widening. Patch 4: Relax scalar ID equivalence in state comparison by treating rold->id == 0 as "independent". If the old state didn't rely on ID relationships for a register, any linking in the current state only adds constraints and is safe to accept for pruning. Patch 5: Add a selftest to show the exact case being handled by Patch 4 I ran veristat on BPF programs from sched_ext, meta's internal programs, and on selftest programs, showing programs with insn diff > 5%: Scx Progs File Program States (A) States (B) States (DIFF) Insns (A) Insns (B) Insns (DIFF) ------------------ ------------------- ---------- ---------- ------------- --------- --------- --------------- scx_rusty.bpf.o rusty_set_cpumask 320 230 -90 (-28.12%) 4478 3259 -1219 (-27.22%) scx_bpfland.bpf.o bpfland_select_cpu 55 49 -6 (-10.91%) 691 618 -73 (-10.56%) scx_beerland.bpf.o beerland_select_cpu 27 25 -2 (-7.41%) 320 295 -25 (-7.81%) scx_p2dq.bpf.o p2dq_init 265 250 -15 (-5.66%) 3423 3233 -190 (-5.55%) scx_layered.bpf.o layered_enqueue 1461 1386 -75 (-5.13%) 14541 13792 -749 (-5.15%) FB Progs File Program States (A) States (B) States (DIFF) Insns (A) Insns (B) Insns (DIFF) ------------ ------------------- ---------- ---------- -------------- --------- --------- --------------- bpf007.bpf.o bpfj_free 1726 1342 -384 (-22.25%) 25671 19096 -6575 (-25.61%) bpf041.bpf.o armr_net_block_init 22373 20411 -1962 (-8.77%) 651697 602873 -48824 (-7.49%) bpf227.bpf.o layered_quiescent 28 26 -2 (-7.14%) 365 340 -25 (-6.85%) bpf248.bpf.o p2dq_init 263 248 -15 (-5.70%) 3370 3159 -211 (-6.26%) bpf254.bpf.o p2dq_init 263 248 -15 (-5.70%) 3388 3177 -211 (-6.23%) bpf241.bpf.o p2dq_init 264 249 -15 (-5.68%) 3428 3240 -188 (-5.48%) bpf230.bpf.o p2dq_init 287 271 -16 (-5.57%) 3666 3431 -235 (-6.41%) bpf251.bpf.o lavd_cpu_offline 321 316 -5 (-1.56%) 6221 5891 -330 (-5.30%) bpf251.bpf.o lavd_cpu_online 321 316 -5 (-1.56%) 6219 5889 -330 (-5.31%) Selftest Progs File Program States (A) States (B) States (DIFF) Insns (A) Insns (B) Insns (DIFF) ---------------------------------- ----------------- ---------- ---------- ------------- --------- --------- --------------- verifier_iterating_callbacks.bpf.o test2 4 2 -2 (-50.00%) 29 18 -11 (-37.93%) verifier_iterating_callbacks.bpf.o test3 4 2 -2 (-50.00%) 31 19 -12 (-38.71%) strobemeta_bpf_loop.bpf.o on_event 318 221 -97 (-30.50%) 3938 2755 -1183 (-30.04%) bpf_qdisc_fq.bpf.o bpf_fq_dequeue 133 105 -28 (-21.05%) 1686 1385 -301 (-17.85%) iters.bpf.o delayed_read_mark 6 5 -1 (-16.67%) 60 46 -14 (-23.33%) arena_strsearch.bpf.o arena_strsearch 107 106 -1 (-0.93%) 1394 1258 -136 (-9.76%) ==================== Link: https://patch.msgid.link/20260203165102.2302462-1-puranjay@kernel.org Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Alexei Starovoitov
commit
f941479a70
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@ -697,8 +697,11 @@ struct bpf_idmap {
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};
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struct bpf_idset {
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u32 count;
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u32 ids[BPF_ID_MAP_SIZE];
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u32 num_ids;
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struct {
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u32 id;
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u32 cnt;
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} entries[BPF_ID_MAP_SIZE];
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};
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/* see verifier.c:compute_scc_callchain() */
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@ -5518,6 +5518,12 @@ static int check_stack_read_fixed_off(struct bpf_verifier_env *env,
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*/
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s32 subreg_def = state->regs[dst_regno].subreg_def;
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if (env->bpf_capable && size == 4 && spill_size == 4 &&
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get_reg_width(reg) <= 32)
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/* Ensure stack slot has an ID to build a relation
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* with the destination register on fill.
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*/
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assign_scalar_id_before_mov(env, reg);
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copy_register_state(&state->regs[dst_regno], reg);
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state->regs[dst_regno].subreg_def = subreg_def;
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@ -5563,6 +5569,11 @@ static int check_stack_read_fixed_off(struct bpf_verifier_env *env,
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}
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} else if (dst_regno >= 0) {
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/* restore register state from stack */
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if (env->bpf_capable)
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/* Ensure stack slot has an ID to build a relation
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* with the destination register on fill.
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*/
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assign_scalar_id_before_mov(env, reg);
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copy_register_state(&state->regs[dst_regno], reg);
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/* mark reg as written since spilled pointer state likely
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* has its liveness marks cleared by is_state_visited()
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@ -8984,15 +8995,24 @@ static bool regs_exact(const struct bpf_reg_state *rold,
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const struct bpf_reg_state *rcur,
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struct bpf_idmap *idmap);
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/*
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* Check if scalar registers are exact for the purpose of not widening.
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* More lenient than regs_exact()
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*/
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static bool scalars_exact_for_widen(const struct bpf_reg_state *rold,
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const struct bpf_reg_state *rcur)
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{
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return !memcmp(rold, rcur, offsetof(struct bpf_reg_state, id));
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}
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static void maybe_widen_reg(struct bpf_verifier_env *env,
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struct bpf_reg_state *rold, struct bpf_reg_state *rcur,
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struct bpf_idmap *idmap)
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struct bpf_reg_state *rold, struct bpf_reg_state *rcur)
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{
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if (rold->type != SCALAR_VALUE)
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return;
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if (rold->type != rcur->type)
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return;
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if (rold->precise || rcur->precise || regs_exact(rold, rcur, idmap))
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if (rold->precise || rcur->precise || scalars_exact_for_widen(rold, rcur))
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return;
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__mark_reg_unknown(env, rcur);
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}
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@ -9004,7 +9024,6 @@ static int widen_imprecise_scalars(struct bpf_verifier_env *env,
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struct bpf_func_state *fold, *fcur;
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int i, fr, num_slots;
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reset_idmap_scratch(env);
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for (fr = old->curframe; fr >= 0; fr--) {
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fold = old->frame[fr];
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fcur = cur->frame[fr];
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@ -9012,8 +9031,7 @@ static int widen_imprecise_scalars(struct bpf_verifier_env *env,
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for (i = 0; i < MAX_BPF_REG; i++)
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maybe_widen_reg(env,
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&fold->regs[i],
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&fcur->regs[i],
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&env->idmap_scratch);
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&fcur->regs[i]);
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num_slots = min(fold->allocated_stack / BPF_REG_SIZE,
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fcur->allocated_stack / BPF_REG_SIZE);
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@ -9024,8 +9042,7 @@ static int widen_imprecise_scalars(struct bpf_verifier_env *env,
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maybe_widen_reg(env,
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&fold->stack[i].spilled_ptr,
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&fcur->stack[i].spilled_ptr,
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&env->idmap_scratch);
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&fcur->stack[i].spilled_ptr);
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}
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}
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return 0;
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@ -19370,13 +19387,29 @@ static bool check_ids(u32 old_id, u32 cur_id, struct bpf_idmap *idmap)
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return false;
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}
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/* Similar to check_ids(), but allocate a unique temporary ID
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* for 'old_id' or 'cur_id' of zero.
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* This makes pairs like '0 vs unique ID', 'unique ID vs 0' valid.
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/*
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* Compare scalar register IDs for state equivalence.
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*
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* When old_id == 0, the old register is independent - not linked to any
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* other register. Any linking in the current state only adds constraints,
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* making it more restrictive. Since the old state didn't rely on any ID
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* relationships for this register, it's always safe to accept cur regardless
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* of its ID. Hence, return true immediately.
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*
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* When old_id != 0 but cur_id == 0, we need to ensure that different
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* independent registers in cur don't incorrectly satisfy the ID matching
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* requirements of linked registers in old.
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*
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* Example: if old has r6.id=X and r7.id=X (linked), but cur has r6.id=0
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* and r7.id=0 (both independent), without temp IDs both would map old_id=X
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* to cur_id=0 and pass. With temp IDs: r6 maps X->temp1, r7 tries to map
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* X->temp2, but X is already mapped to temp1, so the check fails correctly.
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*/
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static bool check_scalar_ids(u32 old_id, u32 cur_id, struct bpf_idmap *idmap)
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{
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old_id = old_id ? old_id : ++idmap->tmp_id_gen;
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if (!old_id)
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return true;
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cur_id = cur_id ? cur_id : ++idmap->tmp_id_gen;
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return check_ids(old_id, cur_id, idmap);
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@ -19450,6 +19483,72 @@ static void clean_verifier_state(struct bpf_verifier_env *env,
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* doesn't meant that the states are DONE. The verifier has to compare
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* the callsites
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*/
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/* Find id in idset and increment its count, or add new entry */
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static void idset_cnt_inc(struct bpf_idset *idset, u32 id)
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{
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u32 i;
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for (i = 0; i < idset->num_ids; i++) {
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if (idset->entries[i].id == id) {
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idset->entries[i].cnt++;
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return;
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}
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}
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/* New id */
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if (idset->num_ids < BPF_ID_MAP_SIZE) {
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idset->entries[idset->num_ids].id = id;
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idset->entries[idset->num_ids].cnt = 1;
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idset->num_ids++;
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}
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}
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/* Find id in idset and return its count, or 0 if not found */
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static u32 idset_cnt_get(struct bpf_idset *idset, u32 id)
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{
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u32 i;
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for (i = 0; i < idset->num_ids; i++) {
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if (idset->entries[i].id == id)
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return idset->entries[i].cnt;
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}
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return 0;
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}
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/*
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* Clear singular scalar ids in a state.
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* A register with a non-zero id is called singular if no other register shares
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* the same base id. Such registers can be treated as independent (id=0).
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*/
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static void clear_singular_ids(struct bpf_verifier_env *env,
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struct bpf_verifier_state *st)
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{
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struct bpf_idset *idset = &env->idset_scratch;
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struct bpf_func_state *func;
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struct bpf_reg_state *reg;
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idset->num_ids = 0;
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bpf_for_each_reg_in_vstate(st, func, reg, ({
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if (reg->type != SCALAR_VALUE)
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continue;
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if (!reg->id)
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continue;
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idset_cnt_inc(idset, reg->id & ~BPF_ADD_CONST);
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}));
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bpf_for_each_reg_in_vstate(st, func, reg, ({
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if (reg->type != SCALAR_VALUE)
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continue;
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if (!reg->id)
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continue;
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if (idset_cnt_get(idset, reg->id & ~BPF_ADD_CONST) == 1) {
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reg->id = 0;
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reg->off = 0;
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}
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}));
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}
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static void clean_live_states(struct bpf_verifier_env *env, int insn,
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struct bpf_verifier_state *cur)
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{
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@ -19535,11 +19634,21 @@ static bool regsafe(struct bpf_verifier_env *env, struct bpf_reg_state *rold,
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}
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if (!rold->precise && exact == NOT_EXACT)
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return true;
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if ((rold->id & BPF_ADD_CONST) != (rcur->id & BPF_ADD_CONST))
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return false;
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if ((rold->id & BPF_ADD_CONST) && (rold->off != rcur->off))
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return false;
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/* Why check_ids() for scalar registers?
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/*
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* Linked register tracking uses rold->id to detect relationships.
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* When rold->id == 0, the register is independent and any linking
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* in rcur only adds constraints. When rold->id != 0, we must verify
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* id mapping and (for BPF_ADD_CONST) offset consistency.
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*
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* +------------------+-----------+------------------+---------------+
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* | | rold->id | rold + ADD_CONST | rold->id == 0 |
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* |------------------+-----------+------------------+---------------|
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* | rcur->id | range,ids | false | range |
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* | rcur + ADD_CONST | false | range,ids,off | range |
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* | rcur->id == 0 | range,ids | false | range |
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* +------------------+-----------+------------------+---------------+
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*
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* Why check_ids() for scalar registers?
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*
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* Consider the following BPF code:
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* 1: r6 = ... unbound scalar, ID=a ...
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@ -19563,9 +19672,22 @@ static bool regsafe(struct bpf_verifier_env *env, struct bpf_reg_state *rold,
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* ---
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* Also verify that new value satisfies old value range knowledge.
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*/
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return range_within(rold, rcur) &&
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tnum_in(rold->var_off, rcur->var_off) &&
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check_scalar_ids(rold->id, rcur->id, idmap);
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/* ADD_CONST mismatch: different linking semantics */
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if ((rold->id & BPF_ADD_CONST) && !(rcur->id & BPF_ADD_CONST))
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return false;
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if (rold->id && !(rold->id & BPF_ADD_CONST) && (rcur->id & BPF_ADD_CONST))
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return false;
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/* Both have offset linkage: offsets must match */
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if ((rold->id & BPF_ADD_CONST) && rold->off != rcur->off)
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return false;
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if (!check_scalar_ids(rold->id, rcur->id, idmap))
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return false;
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return range_within(rold, rcur) && tnum_in(rold->var_off, rcur->var_off);
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case PTR_TO_MAP_KEY:
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case PTR_TO_MAP_VALUE:
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case PTR_TO_MEM:
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@ -20448,6 +20570,8 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
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if (env->bpf_capable)
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mark_all_scalars_imprecise(env, cur);
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clear_singular_ids(env, cur);
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/* add new state to the head of linked list */
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new = &new_sl->state;
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err = copy_verifier_state(new, cur);
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@ -715,6 +715,51 @@ __naked void ignore_unique_scalar_ids_old(void)
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: __clobber_all);
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}
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/* Check that two registers with 0 scalar IDs in a verified state can be mapped
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* to the same scalar ID in current state.
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*/
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SEC("socket")
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__success __log_level(2)
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/* The states should be equivalent on reaching insn 12.
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*/
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__msg("12: safe")
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__msg("processed 17 insns")
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__flag(BPF_F_TEST_STATE_FREQ)
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__naked void two_nil_old_ids_one_cur_id(void)
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{
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asm volatile (
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/* Give unique scalar IDs to r{6,7} */
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"call %[bpf_ktime_get_ns];"
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"r0 &= 0xff;"
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"r6 = r0;"
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"r6 *= 1;"
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"call %[bpf_ktime_get_ns];"
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"r0 &= 0xff;"
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"r7 = r0;"
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"r7 *= 1;"
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"r0 = 0;"
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/* Maybe make r{6,7} IDs identical */
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"if r6 > r7 goto l0_%=;"
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"goto l1_%=;"
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"l0_%=:"
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"r6 = r7;"
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"l1_%=:"
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/* Mark r{6,7} precise.
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* Get here in two states:
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* - first: r6{.id=0}, r7{.id=0} (cached state)
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* - second: r6{.id=A}, r7{.id=A}
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* Verifier considers such states equivalent.
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* Thus "exit;" would be verified only once.
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*/
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"r2 = r10;"
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"r2 += r6;"
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"r2 += r7;"
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"exit;"
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:
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: __imm(bpf_ktime_get_ns)
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: __clobber_all);
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}
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/* Check that two different scalar IDs in a verified state can't be
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* mapped to the same scalar ID in current state.
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*/
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@ -723,9 +768,9 @@ __success __log_level(2)
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/* The exit instruction should be reachable from two states,
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* use two matches and "processed .. insns" to ensure this.
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*/
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__msg("13: (95) exit")
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__msg("13: (95) exit")
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__msg("processed 18 insns")
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__msg("15: (95) exit")
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__msg("15: (95) exit")
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__msg("processed 20 insns")
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__flag(BPF_F_TEST_STATE_FREQ)
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__naked void two_old_ids_one_cur_id(void)
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{
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@ -734,9 +779,11 @@ __naked void two_old_ids_one_cur_id(void)
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"call %[bpf_ktime_get_ns];"
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"r0 &= 0xff;"
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"r6 = r0;"
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"r8 = r0;"
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"call %[bpf_ktime_get_ns];"
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"r0 &= 0xff;"
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"r7 = r0;"
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"r9 = r0;"
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"r0 = 0;"
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/* Maybe make r{6,7} IDs identical */
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"if r6 > r7 goto l0_%=;"
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