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Subject[PATCH net-next] bpf/verifier: track liveness for pruning
Date
State of a register doesn't matter if it wasn't read in reaching an exit;
a write screens off all reads downstream of it from all explored_states
upstream of it.
This allows us to prune many more branches; here are some processed insn
counts for some Cilium programs:
Program before after
bpf_lb_opt_-DLB_L3.o 6515 3361
bpf_lb_opt_-DLB_L4.o 8976 5176
bpf_lb_opt_-DUNKNOWN.o 2960 1137
bpf_lxc_opt_-DDROP_ALL.o 95412 48537
bpf_lxc_opt_-DUNKNOWN.o 141706 79048
bpf_netdev.o 24251 17995
bpf_overlay.o 10999 9385

The runtime is also improved; here are 'time' results in ms:
Program before after
bpf_lb_opt_-DLB_L3.o 24 6
bpf_lb_opt_-DLB_L4.o 26 11
bpf_lb_opt_-DUNKNOWN.o 11 2
bpf_lxc_opt_-DDROP_ALL.o 1288 152
bpf_lxc_opt_-DUNKNOWN.o 1768 257
bpf_netdev.o 62 31
bpf_overlay.o 15 13

Signed-off-by: Edward Cree <ecree@solarflare.com>
---
include/linux/bpf_verifier.h | 11 ++-
kernel/bpf/verifier.c | 181 +++++++++++++++++++++++++++++++++----------
2 files changed, 151 insertions(+), 41 deletions(-)

diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index c61c3033..91d07ef 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -21,6 +21,12 @@
*/
#define BPF_MAX_VAR_SIZ INT_MAX

+enum bpf_reg_liveness {
+ REG_LIVE_NONE = 0, /* reg hasn't been read or written this branch */
+ REG_LIVE_READ, /* reg was read, so we're sensitive to initial value */
+ REG_LIVE_WRITTEN, /* reg was written first, screening off later reads */
+};
+
struct bpf_reg_state {
enum bpf_reg_type type;
union {
@@ -40,7 +46,7 @@ struct bpf_reg_state {
* came from, when one is tested for != NULL.
*/
u32 id;
- /* These five fields must be last. See states_equal() */
+ /* Ordering of fields matters. See states_equal() */
/* For scalar types (SCALAR_VALUE), this represents our knowledge of
* the actual value.
* For pointer types, this represents the variable part of the offset
@@ -57,6 +63,8 @@ struct bpf_reg_state {
s64 smax_value; /* maximum possible (s64)value */
u64 umin_value; /* minimum possible (u64)value */
u64 umax_value; /* maximum possible (u64)value */
+ /* This field must be last, for states_equal() reasons. */
+ enum bpf_reg_liveness live;
};

enum bpf_stack_slot_type {
@@ -74,6 +82,7 @@ struct bpf_verifier_state {
struct bpf_reg_state regs[MAX_BPF_REG];
u8 stack_slot_type[MAX_BPF_STACK];
struct bpf_reg_state spilled_regs[MAX_BPF_STACK / BPF_REG_SIZE];
+ struct bpf_verifier_state *parent;
};

/* linked list of verifier states used to prune search */
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index ecc590e..dcfe6ab 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -629,8 +629,10 @@ static void init_reg_state(struct bpf_reg_state *regs)
{
int i;

- for (i = 0; i < MAX_BPF_REG; i++)
+ for (i = 0; i < MAX_BPF_REG; i++) {
mark_reg_not_init(regs, i);
+ regs[i].live = REG_LIVE_NONE;
+ }

/* frame pointer */
regs[BPF_REG_FP].type = PTR_TO_STACK;
@@ -647,9 +649,26 @@ enum reg_arg_type {
DST_OP_NO_MARK /* same as above, check only, don't mark */
};

-static int check_reg_arg(struct bpf_reg_state *regs, u32 regno,
+static void mark_reg_read(const struct bpf_verifier_state *state, u32 regno)
+{
+ struct bpf_verifier_state *parent = state->parent;
+
+ while (parent) {
+ /* if read wasn't screened by an earlier write ... */
+ if (state->regs[regno].live & REG_LIVE_WRITTEN)
+ break;
+ /* ... then we depend on parent's value */
+ parent->regs[regno].live |= REG_LIVE_READ;
+ state = parent;
+ parent = state->parent;
+ }
+}
+
+static int check_reg_arg(struct bpf_verifier_env *env, u32 regno,
enum reg_arg_type t)
{
+ struct bpf_reg_state *regs = env->cur_state.regs;
+
if (regno >= MAX_BPF_REG) {
verbose("R%d is invalid\n", regno);
return -EINVAL;
@@ -661,12 +680,14 @@ static int check_reg_arg(struct bpf_reg_state *regs, u32 regno,
verbose("R%d !read_ok\n", regno);
return -EACCES;
}
+ mark_reg_read(&env->cur_state, regno);
} else {
/* check whether register used as dest operand can be written to */
if (regno == BPF_REG_FP) {
verbose("frame pointer is read only\n");
return -EACCES;
}
+ regs[regno].live |= REG_LIVE_WRITTEN;
if (t == DST_OP)
mark_reg_unknown(regs, regno);
}
@@ -695,7 +716,7 @@ static bool is_spillable_regtype(enum bpf_reg_type type)
static int check_stack_write(struct bpf_verifier_state *state, int off,
int size, int value_regno)
{
- int i;
+ int i, spi = (MAX_BPF_STACK + off) / BPF_REG_SIZE;
/* caller checked that off % size == 0 and -MAX_BPF_STACK <= off < 0,
* so it's aligned access and [off, off + size) are within stack limits
*/
@@ -710,15 +731,14 @@ static int check_stack_write(struct bpf_verifier_state *state, int off,
}

/* save register state */
- state->spilled_regs[(MAX_BPF_STACK + off) / BPF_REG_SIZE] =
- state->regs[value_regno];
+ state->spilled_regs[spi] = state->regs[value_regno];
+ state->spilled_regs[spi].live |= REG_LIVE_WRITTEN;

for (i = 0; i < BPF_REG_SIZE; i++)
state->stack_slot_type[MAX_BPF_STACK + off + i] = STACK_SPILL;
} else {
/* regular write of data into stack */
- state->spilled_regs[(MAX_BPF_STACK + off) / BPF_REG_SIZE] =
- (struct bpf_reg_state) {};
+ state->spilled_regs[spi] = (struct bpf_reg_state) {};

for (i = 0; i < size; i++)
state->stack_slot_type[MAX_BPF_STACK + off + i] = STACK_MISC;
@@ -726,11 +746,26 @@ static int check_stack_write(struct bpf_verifier_state *state, int off,
return 0;
}

+static void mark_stack_slot_read(const struct bpf_verifier_state *state, int slot)
+{
+ struct bpf_verifier_state *parent = state->parent;
+
+ while (parent) {
+ /* if read wasn't screened by an earlier write ... */
+ if (state->spilled_regs[slot].live & REG_LIVE_WRITTEN)
+ break;
+ /* ... then we depend on parent's value */
+ parent->spilled_regs[slot].live |= REG_LIVE_READ;
+ state = parent;
+ parent = state->parent;
+ }
+}
+
static int check_stack_read(struct bpf_verifier_state *state, int off, int size,
int value_regno)
{
u8 *slot_type;
- int i;
+ int i, spi;

slot_type = &state->stack_slot_type[MAX_BPF_STACK + off];

@@ -746,10 +781,13 @@ static int check_stack_read(struct bpf_verifier_state *state, int off, int size,
}
}

- if (value_regno >= 0)
+ spi = (MAX_BPF_STACK + off) / BPF_REG_SIZE;
+
+ if (value_regno >= 0) {
/* restore register state from stack */
- state->regs[value_regno] =
- state->spilled_regs[(MAX_BPF_STACK + off) / BPF_REG_SIZE];
+ state->regs[value_regno] = state->spilled_regs[spi];
+ mark_stack_slot_read(state, spi);
+ }
return 0;
} else {
for (i = 0; i < size; i++) {
@@ -1167,7 +1205,6 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn

static int check_xadd(struct bpf_verifier_env *env, int insn_idx, struct bpf_insn *insn)
{
- struct bpf_reg_state *regs = env->cur_state.regs;
int err;

if ((BPF_SIZE(insn->code) != BPF_W && BPF_SIZE(insn->code) != BPF_DW) ||
@@ -1177,12 +1214,12 @@ static int check_xadd(struct bpf_verifier_env *env, int insn_idx, struct bpf_ins
}

/* check src1 operand */
- err = check_reg_arg(regs, insn->src_reg, SRC_OP);
+ err = check_reg_arg(env, insn->src_reg, SRC_OP);
if (err)
return err;

/* check src2 operand */
- err = check_reg_arg(regs, insn->dst_reg, SRC_OP);
+ err = check_reg_arg(env, insn->dst_reg, SRC_OP);
if (err)
return err;

@@ -1297,10 +1334,9 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 regno,
if (arg_type == ARG_DONTCARE)
return 0;

- if (type == NOT_INIT) {
- verbose("R%d !read_ok\n", regno);
- return -EACCES;
- }
+ err = check_reg_arg(env, regno, SRC_OP);
+ if (err)
+ return err;

if (arg_type == ARG_ANYTHING) {
if (is_pointer_value(env, regno)) {
@@ -1639,10 +1675,13 @@ static int check_call(struct bpf_verifier_env *env, int func_id, int insn_idx)
}

/* reset caller saved regs */
- for (i = 0; i < CALLER_SAVED_REGS; i++)
+ for (i = 0; i < CALLER_SAVED_REGS; i++) {
mark_reg_not_init(regs, caller_saved[i]);
+ check_reg_arg(env, i, DST_OP_NO_MARK);
+ }

/* update return register */
+ check_reg_arg(env, BPF_REG_0, DST_OP_NO_MARK);
if (fn->ret_type == RET_INTEGER) {
/* sets type to SCALAR_VALUE */
mark_reg_unknown(regs, BPF_REG_0);
@@ -2250,7 +2289,7 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn)
}

/* check src operand */
- err = check_reg_arg(regs, insn->dst_reg, SRC_OP);
+ err = check_reg_arg(env, insn->dst_reg, SRC_OP);
if (err)
return err;

@@ -2261,7 +2300,7 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn)
}

/* check dest operand */
- err = check_reg_arg(regs, insn->dst_reg, DST_OP);
+ err = check_reg_arg(env, insn->dst_reg, DST_OP);
if (err)
return err;

@@ -2274,7 +2313,7 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn)
}

/* check src operand */
- err = check_reg_arg(regs, insn->src_reg, SRC_OP);
+ err = check_reg_arg(env, insn->src_reg, SRC_OP);
if (err)
return err;
} else {
@@ -2285,7 +2324,7 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn)
}

/* check dest operand */
- err = check_reg_arg(regs, insn->dst_reg, DST_OP);
+ err = check_reg_arg(env, insn->dst_reg, DST_OP);
if (err)
return err;

@@ -2328,7 +2367,7 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn)
return -EINVAL;
}
/* check src1 operand */
- err = check_reg_arg(regs, insn->src_reg, SRC_OP);
+ err = check_reg_arg(env, insn->src_reg, SRC_OP);
if (err)
return err;
} else {
@@ -2339,7 +2378,7 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn)
}

/* check src2 operand */
- err = check_reg_arg(regs, insn->dst_reg, SRC_OP);
+ err = check_reg_arg(env, insn->dst_reg, SRC_OP);
if (err)
return err;

@@ -2360,7 +2399,7 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn)
}

/* check dest operand */
- err = check_reg_arg(regs, insn->dst_reg, DST_OP_NO_MARK);
+ err = check_reg_arg(env, insn->dst_reg, DST_OP_NO_MARK);
if (err)
return err;

@@ -2717,7 +2756,7 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
}

/* check src1 operand */
- err = check_reg_arg(regs, insn->src_reg, SRC_OP);
+ err = check_reg_arg(env, insn->src_reg, SRC_OP);
if (err)
return err;

@@ -2734,7 +2773,7 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
}

/* check src2 operand */
- err = check_reg_arg(regs, insn->dst_reg, SRC_OP);
+ err = check_reg_arg(env, insn->dst_reg, SRC_OP);
if (err)
return err;

@@ -2851,7 +2890,7 @@ static int check_ld_imm(struct bpf_verifier_env *env, struct bpf_insn *insn)
return -EINVAL;
}

- err = check_reg_arg(regs, insn->dst_reg, DST_OP);
+ err = check_reg_arg(env, insn->dst_reg, DST_OP);
if (err)
return err;

@@ -2917,7 +2956,7 @@ static int check_ld_abs(struct bpf_verifier_env *env, struct bpf_insn *insn)
}

/* check whether implicit source operand (register R6) is readable */
- err = check_reg_arg(regs, BPF_REG_6, SRC_OP);
+ err = check_reg_arg(env, BPF_REG_6, SRC_OP);
if (err)
return err;

@@ -2928,7 +2967,7 @@ static int check_ld_abs(struct bpf_verifier_env *env, struct bpf_insn *insn)

if (mode == BPF_IND) {
/* check explicit source operand */
- err = check_reg_arg(regs, insn->src_reg, SRC_OP);
+ err = check_reg_arg(env, insn->src_reg, SRC_OP);
if (err)
return err;
}
@@ -3194,7 +3233,11 @@ static bool regsafe(struct bpf_reg_state *rold,
struct bpf_reg_state *rcur,
bool varlen_map_access, struct idpair *idmap)
{
- if (memcmp(rold, rcur, sizeof(*rold)) == 0)
+ if (!(rold->live & REG_LIVE_READ))
+ /* explored state didn't use this */
+ return true;
+
+ if (memcmp(rold, rcur, offsetof(struct bpf_reg_state, live)) == 0)
return true;

if (rold->type == NOT_INIT)
@@ -3372,10 +3415,56 @@ static bool states_equal(struct bpf_verifier_env *env,
return ret;
}

+static bool do_propagate_liveness(const struct bpf_verifier_state *state,
+ struct bpf_verifier_state *parent)
+{
+ bool touched = false; /* any changes made? */
+ int i;
+
+ if (!parent)
+ return touched;
+ /* Propagate read liveness of registers... */
+ BUILD_BUG_ON(BPF_REG_FP + 1 != MAX_BPF_REG);
+ /* We don't need to worry about FP liveness because it's read-only */
+ for (i = 0; i < BPF_REG_FP; i++) {
+ if (parent->regs[i].live & REG_LIVE_READ)
+ continue;
+ if (state->regs[i].live == REG_LIVE_READ) {
+ parent->regs[i].live |= REG_LIVE_READ;
+ touched = true;
+ }
+ }
+ /* ... and stack slots */
+ for (i = 0; i < MAX_BPF_STACK / BPF_REG_SIZE; i++) {
+ if (parent->stack_slot_type[i * BPF_REG_SIZE] != STACK_SPILL)
+ continue;
+ if (state->stack_slot_type[i * BPF_REG_SIZE] != STACK_SPILL)
+ continue;
+ if (parent->spilled_regs[i].live & REG_LIVE_READ)
+ continue;
+ if (state->spilled_regs[i].live == REG_LIVE_READ) {
+ parent->regs[i].live |= REG_LIVE_READ;
+ touched = true;
+ }
+ }
+ return touched;
+}
+
+static void propagate_liveness(const struct bpf_verifier_state *state,
+ struct bpf_verifier_state *parent)
+{
+ while (do_propagate_liveness(state, parent)) {
+ /* Something changed, so we need to feed those changes onward */
+ state = parent;
+ parent = state->parent;
+ }
+}
+
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;
+ int i;

sl = env->explored_states[insn_idx];
if (!sl)
@@ -3385,11 +3474,14 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
return 0;

while (sl != STATE_LIST_MARK) {
- if (states_equal(env, &sl->state, &env->cur_state))
+ if (states_equal(env, &sl->state, &env->cur_state)) {
/* reached equivalent register/stack state,
- * prune the search
+ * prune the search.
+ * Registers read by the continuation are read by us.
*/
+ propagate_liveness(&sl->state, &env->cur_state);
return 1;
+ }
sl = sl->next;
}

@@ -3407,6 +3499,14 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
memcpy(&new_sl->state, &env->cur_state, sizeof(env->cur_state));
new_sl->next = env->explored_states[insn_idx];
env->explored_states[insn_idx] = new_sl;
+ /* connect new state to parentage chain */
+ env->cur_state.parent = &new_sl->state;
+ /* clear liveness marks in current state */
+ for (i = 0; i < BPF_REG_FP; i++)
+ env->cur_state.regs[i].live = REG_LIVE_NONE;
+ for (i = 0; i < MAX_BPF_STACK / BPF_REG_SIZE; i++)
+ if (env->cur_state.stack_slot_type[i * BPF_REG_SIZE] == STACK_SPILL)
+ env->cur_state.spilled_regs[i].live = REG_LIVE_NONE;
return 0;
}

@@ -3430,6 +3530,7 @@ static int do_check(struct bpf_verifier_env *env)
bool do_print_state = false;

init_reg_state(regs);
+ state->parent = NULL;
insn_idx = 0;
env->varlen_map_value_access = false;
for (;;) {
@@ -3500,11 +3601,11 @@ static int do_check(struct bpf_verifier_env *env)
/* check for reserved fields is already done */

/* check src operand */
- err = check_reg_arg(regs, insn->src_reg, SRC_OP);
+ err = check_reg_arg(env, insn->src_reg, SRC_OP);
if (err)
return err;

- err = check_reg_arg(regs, insn->dst_reg, DST_OP_NO_MARK);
+ err = check_reg_arg(env, insn->dst_reg, DST_OP_NO_MARK);
if (err)
return err;

@@ -3554,11 +3655,11 @@ static int do_check(struct bpf_verifier_env *env)
}

/* check src1 operand */
- err = check_reg_arg(regs, insn->src_reg, SRC_OP);
+ err = check_reg_arg(env, insn->src_reg, SRC_OP);
if (err)
return err;
/* check src2 operand */
- err = check_reg_arg(regs, insn->dst_reg, SRC_OP);
+ err = check_reg_arg(env, insn->dst_reg, SRC_OP);
if (err)
return err;

@@ -3589,7 +3690,7 @@ static int do_check(struct bpf_verifier_env *env)
return -EINVAL;
}
/* check src operand */
- err = check_reg_arg(regs, insn->dst_reg, SRC_OP);
+ err = check_reg_arg(env, insn->dst_reg, SRC_OP);
if (err)
return err;

@@ -3643,7 +3744,7 @@ static int do_check(struct bpf_verifier_env *env)
* of bpf_exit, which means that program wrote
* something into it earlier
*/
- err = check_reg_arg(regs, BPF_REG_0, SRC_OP);
+ err = check_reg_arg(env, BPF_REG_0, SRC_OP);
if (err)
return err;

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