bpf: use common instruction history across all states

Instead of allocating and copying instruction history each time we
enqueue child verifier state, switch to a model where we use one common
dynamically sized array of instruction history entries across all states.

The key observation for proving this is correct is that instruction
history is only relevant while state is active, which means it either is
a current state (and thus we are actively modifying instruction history
and no other state can interfere with us) or we are checkpointed state
with some children still active (either enqueued or being current).

In the latter case our portion of instruction history is finalized and
won't change or grow, so as long as we keep it immutable until the state
is finalized, we are good.

Now, when state is finalized and is put into state hash for potentially
future pruning lookups, instruction history is not used anymore. This is
because instruction history is only used by precision marking logic, and
we never modify precision markings for finalized states.

So, instead of each state having its own small instruction history, we
keep a global dynamically-sized instruction history, where each state in
current DFS path from root to active state remembers its portion of
instruction history. Current state can append to this history, but
cannot modify any of its parent histories.

Async callback state enqueueing, while logically detached from parent
state, still is part of verification backtracking tree, so has to follow
the same schema as normal state checkpoints.

Because the insn_hist array can be grown through realloc, states don't
keep pointers, they instead maintain two indices, [start, end), into
global instruction history array. End is exclusive index, so
`start == end` means there is no relevant instruction history.

This eliminates a lot of allocations and minimizes overall memory usage.

For instance, running a worst-case test from [0] (but without the
heuristics-based fix [1]), it took 12.5 minutes until we get -ENOMEM.
With the changes in this patch the whole test succeeds in 10 minutes
(very slow, so heuristics from [1] is important, of course).

To further validate correctness, veristat-based comparison was performed for
Meta production BPF objects and BPF selftests objects. In both cases there
were no differences *at all* in terms of verdict or instruction and state
counts, providing a good confidence in the change.

Having this low-memory-overhead solution of keeping dynamic
per-instruction history cheaply opens up some new possibilities, like
keeping extra information for literally every single validated
instruction. This will be used for simplifying precision backpropagation
logic in follow up patches.

  [0] https://lore.kernel.org/bpf/20241029172641.1042523-2-eddyz87@gmail.com/
  [1] https://lore.kernel.org/bpf/20241029172641.1042523-1-eddyz87@gmail.com/

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20241115001303.277272-1-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 11 insertions, 8 deletions

diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 6b7c91629176..f4290c179bee 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -334,7 +334,7 @@ struct bpf_func_state {
 
 #define MAX_CALL_FRAMES 8
 
-/* instruction history flags, used in bpf_jmp_history_entry.flags field */
+/* instruction history flags, used in bpf_insn_hist_entry.flags field */
 enum {
 	/* instruction references stack slot through PTR_TO_STACK register;
 	 * we also store stack's frame number in lower 3 bits (MAX_CALL_FRAMES is 8)
@@ -352,7 +352,7 @@ enum {
 static_assert(INSN_F_FRAMENO_MASK + 1 >= MAX_CALL_FRAMES);
 static_assert(INSN_F_SPI_MASK + 1 >= MAX_BPF_STACK / 8);
 
-struct bpf_jmp_history_entry {
+struct bpf_insn_hist_entry {
 	u32 idx;
 	/* insn idx can't be bigger than 1 million */
 	u32 prev_idx : 22;
@@ -442,13 +442,14 @@ struct bpf_verifier_state {
 	 * See get_loop_entry() for more information.
 	 */
 	struct bpf_verifier_state *loop_entry;
-	/* 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].
+	/* Sub-range of env->insn_hist[] corresponding to this state's
+	 * instruction history.
+	 * Backtracking is using it to go from last to first.
+	 * For most states instruction history is short, 0-3 instructions.
 	 * For loops can go up to ~40.
 	 */
-	struct bpf_jmp_history_entry *jmp_history;
-	u32 jmp_history_cnt;
+	u32 insn_hist_start;
+	u32 insn_hist_end;
 	u32 dfs_depth;
 	u32 callback_unroll_depth;
 	u32 may_goto_depth;
@@ -738,7 +739,9 @@ struct bpf_verifier_env {
 		int cur_stack;
 	} cfg;
 	struct backtrack_state bt;
-	struct bpf_jmp_history_entry *cur_hist_ent;
+	struct bpf_insn_hist_entry *insn_hist;
+	struct bpf_insn_hist_entry *cur_hist_ent;
+	u32 insn_hist_cap;
 	u32 pass_cnt; /* number of times do_check() was called */
 	u32 subprog_cnt;
 	/* number of instructions analyzed by the verifier */