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bpf: add self-check logic to liveness analysis

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Introduce REG_LIVE_DONE to check the liveness propagation
and prepare the states for merging.
See algorithm description in clean_live_states().

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
This commit is contained in:
Alexei Starovoitov 2018-12-13 11:42:34 -08:00 committed by Daniel Borkmann
parent 19e2dbb7dd
commit 9242b5f561
2 changed files with 108 additions and 1 deletions
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1
include/linux/bpf_verifier.h
View File

@ -38,6 +38,7 @@ 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 */
REG_LIVE_DONE = 4, /* liveness won't be updating this register anymore */
};
struct bpf_reg_state {

108
kernel/bpf/verifier.c
View File

@ -397,12 +397,14 @@ static char slot_type_char[] = {
static void print_liveness(struct bpf_verifier_env *env,
enum bpf_reg_liveness live)
{
if (live & (REG_LIVE_READ | REG_LIVE_WRITTEN))
if (live & (REG_LIVE_READ | REG_LIVE_WRITTEN | REG_LIVE_DONE))
verbose(env, "_");
if (live & REG_LIVE_READ)
verbose(env, "r");
if (live & REG_LIVE_WRITTEN)
verbose(env, "w");
if (live & REG_LIVE_DONE)
verbose(env, "D");
}
static struct bpf_func_state *func(struct bpf_verifier_env *env,
@ -1132,6 +1134,12 @@ static int mark_reg_read(struct bpf_verifier_env *env,
/* if read wasn't screened by an earlier write ... */
if (writes && state->live & REG_LIVE_WRITTEN)
break;
if (parent->live & REG_LIVE_DONE) {
verbose(env, "verifier BUG type %s var_off %lld off %d\n",
reg_type_str[parent->type],
parent->var_off.value, parent->off);
return -EFAULT;
}
/* ... then we depend on parent's value */
parent->live |= REG_LIVE_READ;
state = parent;
@ -5078,6 +5086,102 @@ static bool check_ids(u32 old_id, u32 cur_id, struct idpair *idmap)
return false;
}
static void clean_func_state(struct bpf_verifier_env *env,
struct bpf_func_state *st)
{
enum bpf_reg_liveness live;
int i, j;
for (i = 0; i < BPF_REG_FP; i++) {
live = st->regs[i].live;
/* liveness must not touch this register anymore */
st->regs[i].live |= REG_LIVE_DONE;
if (!(live & REG_LIVE_READ))
/* since the register is unused, clear its state
* to make further comparison simpler
*/
__mark_reg_not_init(&st->regs[i]);
}
for (i = 0; i < st->allocated_stack / BPF_REG_SIZE; i++) {
live = st->stack[i].spilled_ptr.live;
/* liveness must not touch this stack slot anymore */
st->stack[i].spilled_ptr.live |= REG_LIVE_DONE;
if (!(live & REG_LIVE_READ)) {
__mark_reg_not_init(&st->stack[i].spilled_ptr);
for (j = 0; j < BPF_REG_SIZE; j++)
st->stack[i].slot_type[j] = STACK_INVALID;
}
}
}
static void clean_verifier_state(struct bpf_verifier_env *env,
struct bpf_verifier_state *st)
{
int i;
if (st->frame[0]->regs[0].live & REG_LIVE_DONE)
/* all regs in this state in all frames were already marked */
return;
for (i = 0; i <= st->curframe; i++)
clean_func_state(env, st->frame[i]);
}
/* the parentage chains form a tree.
* the verifier states are added to state lists at given insn and
* pushed into state stack for future exploration.
* when the verifier reaches bpf_exit insn some of the verifer states
* stored in the state lists have their final liveness state already,
* but a lot of states will get revised from liveness point of view when
* the verifier explores other branches.
* Example:
* 1: r0 = 1
* 2: if r1 == 100 goto pc+1
* 3: r0 = 2
* 4: exit
* when the verifier reaches exit insn the register r0 in the state list of
* insn 2 will be seen as !REG_LIVE_READ. Then the verifier pops the other_branch
* of insn 2 and goes exploring further. At the insn 4 it will walk the
* parentage chain from insn 4 into insn 2 and will mark r0 as REG_LIVE_READ.
*
* Since the verifier pushes the branch states as it sees them while exploring
* the program the condition of walking the branch instruction for the second
* time means that all states below this branch were already explored and
* their final liveness markes are already propagated.
* Hence when the verifier completes the search of state list in is_state_visited()
* we can call this clean_live_states() function to mark all liveness states
* as REG_LIVE_DONE to indicate that 'parent' pointers of 'struct bpf_reg_state'
* will not be used.
* This function also clears the registers and stack for states that !READ
* to simplify state merging.
*
* Important note here that walking the same branch instruction in the callee
* doesn't meant that the states are DONE. The verifier has to compare
* the callsites
*/
static void clean_live_states(struct bpf_verifier_env *env, int insn,
struct bpf_verifier_state *cur)
{
struct bpf_verifier_state_list *sl;
int i;
sl = env->explored_states[insn];
if (!sl)
return;
while (sl != STATE_LIST_MARK) {
if (sl->state.curframe != cur->curframe)
goto next;
for (i = 0; i <= cur->curframe; i++)
if (sl->state.frame[i]->callsite != cur->frame[i]->callsite)
goto next;
clean_verifier_state(env, &sl->state);
next:
sl = sl->next;
}
}
/* Returns true if (rold safe implies rcur safe) */
static bool regsafe(struct bpf_reg_state *rold, struct bpf_reg_state *rcur,
struct idpair *idmap)
@ -5396,6 +5500,8 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
*/
return 0;
clean_live_states(env, insn_idx, cur);
while (sl != STATE_LIST_MARK) {
if (states_equal(env, &sl->state, cur)) {
/* reached equivalent register/stack state,