Re: [PATCH net-next 4/4] bpf/verifier: document liveness analysis

On 8/22/17 8:55 AM, Edward Cree wrote:

On 22/08/17 16:42, Alexei Starovoitov wrote:

quoted

On 8/22/17 6:27 AM, Edward Cree wrote:

quoted

 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", so
+ * long as parent != state->parent.
+ */

i'm confused with 'so long as parent != state->parent' which implies
looping and multiple iterations, whereas 'parent != state->parent'
condition is true only for the first iteration of
'while (do_propagate_liveness(state, parent))' loop.
right ?

I phrased it badly.  I mean that, the statement "this function is just a
 way to mark_reg_read() all the things" is true only "so long as" (i.e.
 under the condition) parent != state->parent.

got it.

How about
/* 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.
 */

Thanks. It's more clear to me. Ack