Merge branch 'bpf-bounded-loops'

Alexei Starovoitov says:

====================
v2->v3: fixed issues in backtracking pointed out by Andrii.
The next step is to add a lot more tests for backtracking.

v1->v2: addressed Andrii's feedback.

this patch set introduces verifier support for bounded loops and
adds several other improvements.
Ideally they would be introduced one at a time,
but to support bounded loop the verifier needs to 'step back'
in the patch 1. That patch introduces tracking of spill/fill
of constants through the stack. Though it's a useful feature
it hurts cilium tests.
Patch 3 introduces another feature by extending is_branch_taken
logic to 'if rX op rY' conditions. This feature is also
necessary to support bounded loops.
Then patch 4 adds support for the loops while adding
key heuristics with jmp_processed.
Introduction of parentage chain of verifier states in patch 4
allows patch 9 to add backtracking of precise scalar registers
which finally resolves degradation from patch 1.

The end result is much faster verifier for existing programs
and new support for loops.
See patch 8 for many kinds of loops that are now validated.
Patch 9 is the most tricky one and could be rewritten with
a different algorithm in the future.
====================

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

include/linux/bpf_verifier.h

@@ -139,6 +139,8 @@ struct bpf_reg_state {
 	 */
 	s32 subreg_def;
 	enum bpf_reg_liveness live;
+	/* if (!precise && SCALAR_VALUE) min/max/tnum don't affect safety */
+	bool precise;
 };
 
 enum bpf_stack_slot_type {
@@ -190,14 +192,77 @@ struct bpf_func_state {
 	struct bpf_stack_state *stack;
 };
 
+struct bpf_idx_pair {
+	u32 prev_idx;
+	u32 idx;
+};
+
 #define MAX_CALL_FRAMES 8
 struct bpf_verifier_state {
 	/* call stack tracking */
 	struct bpf_func_state *frame[MAX_CALL_FRAMES];
+	struct bpf_verifier_state *parent;
+	/*
+	 * 'branches' field is the number of branches left to explore:
+	 * 0 - all possible paths from this state reached bpf_exit or
+	 * were safely pruned
+	 * 1 - at least one path is being explored.
+	 * This state hasn't reached bpf_exit
+	 * 2 - at least two paths are being explored.
+	 * This state is an immediate parent of two children.
+	 * One is fallthrough branch with branches==1 and another
+	 * state is pushed into stack (to be explored later) also with
+	 * branches==1. The parent of this state has branches==1.
+	 * The verifier state tree connected via 'parent' pointer looks like:
+	 * 1
+	 * 1
+	 * 2 -> 1 (first 'if' pushed into stack)
+	 * 1
+	 * 2 -> 1 (second 'if' pushed into stack)
+	 * 1
+	 * 1
+	 * 1 bpf_exit.
+	 *
+	 * Once do_check() reaches bpf_exit, it calls update_branch_counts()
+	 * and the verifier state tree will look:
+	 * 1
+	 * 1
+	 * 2 -> 1 (first 'if' pushed into stack)
+	 * 1
+	 * 1 -> 1 (second 'if' pushed into stack)
+	 * 0
+	 * 0
+	 * 0 bpf_exit.
+	 * After pop_stack() the do_check() will resume at second 'if'.
+	 *
+	 * If is_state_visited() sees a state with branches > 0 it means
+	 * there is a loop. If such state is exactly equal to the current state
+	 * it's an infinite loop. Note states_equal() checks for states
+	 * equvalency, so two states being 'states_equal' does not mean
+	 * infinite loop. The exact comparison is provided by
+	 * states_maybe_looping() function. It's a stronger pre-check and
+	 * much faster than states_equal().
+	 *
+	 * This algorithm may not find all possible infinite loops or
+	 * loop iteration count may be too high.
+	 * In such cases BPF_COMPLEXITY_LIMIT_INSNS limit kicks in.
+	 */
+	u32 branches;
 	u32 insn_idx;
 	u32 curframe;
 	u32 active_spin_lock;
 	bool speculative;
+
+	/* first and last insn idx of this verifier state */
+	u32 first_insn_idx;
+	u32 last_insn_idx;
+	/* jmp history recorded from first to last.
+	 * backtracking is using it to go from last to first.
+	 * For most states jmp_history_cnt is [0-3].
+	 * For loops can go up to ~40.
+	 */
+	struct bpf_idx_pair *jmp_history;
+	u32 jmp_history_cnt;
 };
 
 #define bpf_get_spilled_reg(slot, frame)				\
@@ -312,7 +377,9 @@ struct bpf_verifier_env {
 	} cfg;
 	u32 subprog_cnt;
 	/* number of instructions analyzed by the verifier */
-	u32 insn_processed;
+	u32 prev_insn_processed, insn_processed;
+	/* number of jmps, calls, exits analyzed so far */
+	u32 prev_jmps_processed, jmps_processed;
 	/* total verification time */
 	u64 verification_time;
 	/* maximum number of verifier states kept in 'branching' instructions */