bpf: improve precision backtrack logging

Add helper to format register and stack masks in more human-readable
format. Adjust logging a bit during backtrack propagation and especially
during forcing precision fallback logic to make it clearer what's going
on (with log_level=2, of course), and also start reporting affected
frame depth. This is in preparation for having more than one active
frame later when precision propagation between subprog calls is added.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20230505043317.3629845-5-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 65 insertions, 7 deletions

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 9b2e571250e1..5412c8c8511d 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -605,9 +605,9 @@ static const char *reg_type_str(struct bpf_verifier_env *env,
 		 type & PTR_TRUSTED ? "trusted_" : ""
 	);
 
-	snprintf(env->type_str_buf, TYPE_STR_BUF_LEN, "%s%s%s",
+	snprintf(env->tmp_str_buf, TMP_STR_BUF_LEN, "%s%s%s",
 		 prefix, str[base_type(type)], postfix);
-	return env->type_str_buf;
+	return env->tmp_str_buf;
 }
 
 static char slot_type_char[] = {
@@ -3308,6 +3308,45 @@ static inline bool bt_is_slot_set(struct backtrack_state *bt, u32 slot)
 	return bt->stack_masks[bt->frame] & (1ull << slot);
 }
 
+/* format registers bitmask, e.g., "r0,r2,r4" for 0x15 mask */
+static void fmt_reg_mask(char *buf, ssize_t buf_sz, u32 reg_mask)
+{
+	DECLARE_BITMAP(mask, 64);
+	bool first = true;
+	int i, n;
+
+	buf[0] = '\0';
+
+	bitmap_from_u64(mask, reg_mask);
+	for_each_set_bit(i, mask, 32) {
+		n = snprintf(buf, buf_sz, "%sr%d", first ? "" : ",", i);
+		first = false;
+		buf += n;
+		buf_sz -= n;
+		if (buf_sz < 0)
+			break;
+	}
+}
+/* format stack slots bitmask, e.g., "-8,-24,-40" for 0x15 mask */
+static void fmt_stack_mask(char *buf, ssize_t buf_sz, u64 stack_mask)
+{
+	DECLARE_BITMAP(mask, 64);
+	bool first = true;
+	int i, n;
+
+	buf[0] = '\0';
+
+	bitmap_from_u64(mask, stack_mask);
+	for_each_set_bit(i, mask, 64) {
+		n = snprintf(buf, buf_sz, "%s%d", first ? "" : ",", -(i + 1) * 8);
+		first = false;
+		buf += n;
+		buf_sz -= n;
+		if (buf_sz < 0)
+			break;
+	}
+}
+
 /* For given verifier state backtrack_insn() is called from the last insn to
  * the first insn. Its purpose is to compute a bitmask of registers and
  * stack slots that needs precision in the parent verifier state.
@@ -3331,7 +3370,11 @@ static int backtrack_insn(struct bpf_verifier_env *env, int idx,
 	if (insn->code == 0)
 		return 0;
 	if (env->log.level & BPF_LOG_LEVEL2) {
-		verbose(env, "regs=%x stack=%llx before ", bt_reg_mask(bt), bt_stack_mask(bt));
+		fmt_reg_mask(env->tmp_str_buf, TMP_STR_BUF_LEN, bt_reg_mask(bt));
+		verbose(env, "mark_precise: frame%d: regs=%s ",
+			bt->frame, env->tmp_str_buf);
+		fmt_stack_mask(env->tmp_str_buf, TMP_STR_BUF_LEN, bt_stack_mask(bt));
+		verbose(env, "stack=%s before ", env->tmp_str_buf);
 		verbose(env, "%d: ", idx);
 		print_bpf_insn(&cbs, insn, env->allow_ptr_leaks);
 	}
@@ -3531,6 +3574,11 @@ static void mark_all_scalars_precise(struct bpf_verifier_env *env,
 	struct bpf_reg_state *reg;
 	int i, j;
 
+	if (env->log.level & BPF_LOG_LEVEL2) {
+		verbose(env, "mark_precise: frame%d: falling back to forcing all scalars precise\n",
+			st->curframe);
+	}
+
 	/* big hammer: mark all scalars precise in this path.
 	 * pop_stack may still get !precise scalars.
 	 * We also skip current state and go straight to first parent state,
@@ -3542,17 +3590,25 @@ static void mark_all_scalars_precise(struct bpf_verifier_env *env,
 			func = st->frame[i];
 			for (j = 0; j < BPF_REG_FP; j++) {
 				reg = &func->regs[j];
-				if (reg->type != SCALAR_VALUE)
+				if (reg->type != SCALAR_VALUE || reg->precise)
 					continue;
 				reg->precise = true;
+				if (env->log.level & BPF_LOG_LEVEL2) {
+					verbose(env, "force_precise: frame%d: forcing r%d to be precise\n",
+						i, j);
+				}
 			}
 			for (j = 0; j < func->allocated_stack / BPF_REG_SIZE; j++) {
 				if (!is_spilled_reg(&func->stack[j]))
 					continue;
 				reg = &func->stack[j].spilled_ptr;
-				if (reg->type != SCALAR_VALUE)
+				if (reg->type != SCALAR_VALUE || reg->precise)
 					continue;
 				reg->precise = true;
+				if (env->log.level & BPF_LOG_LEVEL2) {
+					verbose(env, "force_precise: frame%d: forcing fp%d to be precise\n",
+						i, -(j + 1) * 8);
+				}
 			}
 		}
 	}
@@ -3716,8 +3772,10 @@ static int __mark_chain_precision(struct bpf_verifier_env *env, int frame, int r
 		DECLARE_BITMAP(mask, 64);
 		u32 history = st->jmp_history_cnt;
 
-		if (env->log.level & BPF_LOG_LEVEL2)
-			verbose(env, "last_idx %d first_idx %d\n", last_idx, first_idx);
+		if (env->log.level & BPF_LOG_LEVEL2) {
+			verbose(env, "mark_precise: frame%d: last_idx %d first_idx %d\n",
+				bt->frame, last_idx, first_idx);
+		}
 
 		if (last_idx < 0) {
 			/* we are at the entry into subprog, which