bpf/verifier: document liveness analysis

The liveness tracking algorithm is quite subtle; add comments to explain it.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Edward Cree 2017-08-23 15:11:21 +01:00 committed by David S. Miller
parent 1b688a19a9
commit 8e9cd9ce90
2 changed files with 40 additions and 1 deletions

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@ -21,6 +21,19 @@
*/
#define BPF_MAX_VAR_SIZ INT_MAX
/* Liveness marks, used for registers and spilled-regs (in stack slots).
* Read marks propagate upwards until they find a write mark; they record that
* "one of this state's descendants read this reg" (and therefore the reg is
* relevant for states_equal() checks).
* Write marks collect downwards and do not propagate; they record that "the
* straight-line code that reached this state (from its parent) wrote this reg"
* (and therefore that reads propagated from this state or its descendants
* should not propagate to its parent).
* A state with a write mark can receive read marks; it just won't propagate
* them to its parent, since the write mark is a property, not of the state,
* but of the link between it and its parent. See mark_reg_read() and
* mark_stack_slot_read() in kernel/bpf/verifier.c.
*/
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 */

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@ -3417,6 +3417,12 @@ out_free:
return ret;
}
/* A write screens off any subsequent reads; but write marks come from the
* straight-line code between a state and its parent. When we arrive at a
* jump target (in the first iteration of the propagate_liveness() loop),
* we didn't arrive by the straight-line code, so read marks in state must
* propagate to parent regardless of state's write marks.
*/
static bool do_propagate_liveness(const struct bpf_verifier_state *state,
struct bpf_verifier_state *parent)
{
@ -3457,6 +3463,15 @@ static bool do_propagate_liveness(const struct bpf_verifier_state *state,
return touched;
}
/* "parent" is "a state from which we reach the current state", but initially
* it is not the state->parent (i.e. "the state whose straight-line code leads
* to the current state"), instead it is the state that happened to arrive at
* a (prunable) equivalent of the current state. See comment above
* do_propagate_liveness() for consequences of this.
* This function is just a more efficient way of calling mark_reg_read() or
* mark_stack_slot_read() on each reg in "parent" that is read in "state",
* though it requires that parent != state->parent in the call arguments.
*/
static void propagate_liveness(const struct bpf_verifier_state *state,
struct bpf_verifier_state *parent)
{
@ -3485,6 +3500,12 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
/* reached equivalent register/stack state,
* prune the search.
* Registers read by the continuation are read by us.
* If we have any write marks in env->cur_state, they
* will prevent corresponding reads in the continuation
* from reaching our parent (an explored_state). Our
* own state will get the read marks recorded, but
* they'll be immediately forgotten as we're pruning
* this state and will pop a new one.
*/
propagate_liveness(&sl->state, &env->cur_state);
return 1;
@ -3508,7 +3529,12 @@ static int is_state_visited(struct bpf_verifier_env *env, int 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 */
/* clear write marks in current state: the writes we did are not writes
* our child did, so they don't screen off its reads from us.
* (There are no read marks in current state, because reads always mark
* their parent and current state never has children yet. Only
* explored_states can get read marks.)
*/
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++)