bpf: encapsulate precision backtracking bookkeeping

Add struct backtrack_state and straightforward API around it to keep
track of register and stack masks used and maintained during precision
backtracking process. Having this logic separately allow to keep
high-level backtracking algorithm cleaner, but also it sets us up to
cleanly keep track of register and stack masks per frame, allowing (with
some further logic adjustments) to perform precision backpropagation
across multiple frames (i.e., subprog calls).

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20230505043317.3629845-4-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: anakryiko

include/linux/bpf_verifier.h

@@ -238,6 +238,10 @@ enum bpf_stack_slot_type {
 
 #define BPF_REG_SIZE 8	/* size of eBPF register in bytes */
 
+#define BPF_REGMASK_ARGS ((1 << BPF_REG_1) | (1 << BPF_REG_2) | \
+			  (1 << BPF_REG_3) | (1 << BPF_REG_4) | \
+			  (1 << BPF_REG_5))
+
 #define BPF_DYNPTR_SIZE		sizeof(struct bpf_dynptr_kern)
 #define BPF_DYNPTR_NR_SLOTS		(BPF_DYNPTR_SIZE / BPF_REG_SIZE)
 
@@ -541,6 +545,15 @@ struct bpf_subprog_info {
 	bool is_async_cb;
 };
 
+struct bpf_verifier_env;
+
+struct backtrack_state {
+	struct bpf_verifier_env *env;
+	u32 frame;
+	u32 reg_masks[MAX_CALL_FRAMES];
+	u64 stack_masks[MAX_CALL_FRAMES];
+};
+
 /* single container for all structs
  * one verifier_env per bpf_check() call
  */
@@ -578,6 +591,7 @@ struct bpf_verifier_env {
 		int *insn_stack;
 		int cur_stack;
 	} cfg;
+	struct backtrack_state bt;
 	u32 pass_cnt; /* number of times do_check() was called */
 	u32 subprog_cnt;
 	/* number of instructions analyzed by the verifier */