diff mbox series

[v4,bpf-next,1/2] bpf: Relax precision marking in open coded iters and may_goto loop.

Message ID 20240601034211.63962-1-alexei.starovoitov@gmail.com (mailing list archive)
State Changes Requested
Delegated to: BPF
Headers show
Series [v4,bpf-next,1/2] bpf: Relax precision marking in open coded iters and may_goto loop. | expand

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Commit Message

Alexei Starovoitov June 1, 2024, 3:42 a.m. UTC 
From: Alexei Starovoitov <ast@kernel.org>

v1->v2->v3:
- Algorithm changed completely.
v3->v4:
- Fixed widening for Rx < Ry case and added more tests

Motivation for the patch
------------------------
Open coded iterators and may_goto is a great mechanism to implement loops,
but counted loops are problematic. For example:
  for (i = 0; i < 100 && can_loop; i++)
is verified as a bounded loop, since i < 100 condition forces the verifier to
mark 'i' as precise and loop states at different iterations are not equivalent.
That removes the benefit of open coded iterators and may_goto.
The workaround is to do:
  int zero = 0; /* global or volatile variable */
  for (i = zero; i < 100 && can_loop; i++)
to hide from the verifier the value of 'i'.
It's unnatural and so far users didn't learn such odd programming pattern.

This patch aims to improve the verifier to support
  for (i = 0; i < 100000 && can_loop; i++)
as open coded iter loop (when 'i' doesn't need to be precise).

Algorithm
---------
First of all:
   if (is_may_goto_insn_at(env, insn_idx)) {
+          update_loop_entry(cur, &sl->state);
           if (states_equal(env, &sl->state, cur, RANGE_WITHIN)) {
-                  update_loop_entry(cur, &sl->state);

It changes the definition of the verifier states loop.
Previously, we considered a state loop to be such a sequence of states
Si -> ... -> Sj -> ... -> Sk that states_equal(Si, Sk, RANGE_WITHIN)
is true.

With this change Si -> ... -> Sj -> ... Sk is a loop if call sites and
instruction pointers for Si and Sk match.

Whether or not Si and Sk are in the loop influences two things:
(a) if exact comparison is needed for states cache;
(b) if widening transformation could be applied to some scalars.

All pairs (Si, Sk) marked as a loop using old definition would be
marked as such using new definition (in a addition to some new pairs).

Hence it is safe to apply (a) and (b) in strictly more cases.

Note that update_loop_entry() relies on the following properties:
- every state in the current DFS path (except current)
  has branches > 0;
- states not in the DFS path are either:
  - in explored_states, are fully explored and have branches == 0;
  - in env->stack, are not yet explored and have branches == 0
    (and also not reachable from is_state_visited()).

With that the get_loop_entry() can be used to gate is_branch_taken() logic.
When the verifier sees 'r1 > 1000' inside the loop and it can predict it
instead of marking r1 as precise it widens both branches, so r1 becomes
[0, 1000] in fallthrough and [1001, UMAX] in other_branch.

Consider the loop:
    bpf_for_each(...) {
       if (r1 > 1000)
          break;

       arr[r1] = ..;
    }
At arr[r1] access the r1 is bounded and the loop can quickly converge.

Unfortunately compilers (both GCC and LLVM) often optimize loop exit
condition to equality, so
 for (i = 0; i < 100; i++) arr[i] = 1
becomes
 for (i = 0; i != 100; i++) arr[1] = 1

Hence treat != and == conditions specially in the verifier.
Widen only not-predicted branch and keep predict branch as is. Example:
  r1 = 0
  goto L1
L2:
  arr[r1] = 1
  r1++
L1:
  if r1 != 100 goto L2
  fallthrough: r1=100 after widening
  other_branch: r1 stays as-is (0, 1, 2, ..)

Also recognize the case where both LHS and RHS are constant and equal to each
other. In this case don't widen at all and take the predicted path.
This key heuristic allows the verifier detect loop end condition.
Such 'for (i = 0; i != 100; i++)' is validated just like bounded loop.

With that the users can use 'for (i = 0; ...' pattern everywhere
and many i = zero workarounds can be removed.

One tests has drastic improvement. The rest are noise.

File                  Insns (A)  Insns (B)  Insns     (DIFF)  States (A)  States (B)  States    (DIFF)
--------------------  ---------  ---------  ----------------  ----------  ----------  ----------------
iters_task_vma.bpf.o      22043        132  -21911 (-99.40%)        1006          10    -996 (-99.01%)

Few extra tests are added to iters_task_vma.bpf.o that demonstrate quick
convergence though they iterate over 100k elements.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
---
 include/linux/bpf_verifier.h |   2 +
 kernel/bpf/verifier.c        | 168 +++++++++++++++++++++++++++++------
 2 files changed, 141 insertions(+), 29 deletions(-)
diff mbox series

Patch

diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 50aa87f8d77f..4d9c1a863014 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -750,6 +750,8 @@  struct bpf_verifier_env {
 	 * e.g., in reg_type_str() to generate reg_type string
 	 */
 	char tmp_str_buf[TMP_STR_BUF_LEN];
+	/* temp variables that are too big to keep on stack */
+	struct bpf_reg_state saved_src_reg, saved_dst_reg;
 };
 
 static inline struct bpf_func_info_aux *subprog_aux(struct bpf_verifier_env *env, int subprog)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 77da1f438bec..9bd23ab44eb9 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2321,6 +2321,14 @@  static void __mark_reg_unknown(const struct bpf_verifier_env *env,
 	reg->precise = !env->bpf_capable;
 }
 
+static void widen_reg(struct bpf_reg_state *reg)
+{
+	u32 id = reg->id;
+
+	__mark_reg_unknown_imprecise(reg);
+	reg->id = id;
+}
+
 static void mark_reg_unknown(struct bpf_verifier_env *env,
 			     struct bpf_reg_state *regs, u32 regno)
 {
@@ -15104,10 +15112,11 @@  static int check_cond_jmp_op(struct bpf_verifier_env *env,
 	struct bpf_verifier_state *other_branch;
 	struct bpf_reg_state *regs = this_branch->frame[this_branch->curframe]->regs;
 	struct bpf_reg_state *dst_reg, *other_branch_regs, *src_reg = NULL;
-	struct bpf_reg_state *eq_branch_regs;
+	struct bpf_reg_state *eq_branch_regs, *other_dst_reg, *other_src_reg = NULL;
 	struct bpf_reg_state fake_reg = {};
 	u8 opcode = BPF_OP(insn->code);
-	bool is_jmp32;
+	bool is_jmp32, ignore_pred;
+	bool has_src_reg = false;
 	int pred = -1;
 	int err;
 
@@ -15159,6 +15168,7 @@  static int check_cond_jmp_op(struct bpf_verifier_env *env,
 		if (err)
 			return err;
 
+		has_src_reg = true;
 		src_reg = &regs[insn->src_reg];
 		if (!(reg_is_pkt_pointer_any(dst_reg) && reg_is_pkt_pointer_any(src_reg)) &&
 		    is_pointer_value(env, insn->src_reg)) {
@@ -15177,8 +15187,42 @@  static int check_cond_jmp_op(struct bpf_verifier_env *env,
 	}
 
 	is_jmp32 = BPF_CLASS(insn->code) == BPF_JMP32;
+	if (dst_reg->type != SCALAR_VALUE || src_reg->type != SCALAR_VALUE)
+		ignore_pred = false;
+	/*
+	 * Compilers often optimize loop exit condition to equality, so
+	 *      for (i = 0; i < 100; i++) arr[i] = 1
+	 * becomes
+	 *      for (i = 0; i != 100; i++) arr[1] = 1
+	 * Hence treat != and == conditions specially in the verifier.
+	 * Widen only not-predicted branch and keep predict branch as is. Example:
+	 *    r1 = 0
+	 *    goto L1
+	 * L2:
+	 *    arr[r1] = 1
+	 *    r1++
+	 * L1:
+	 *    if r1 != 100 goto L2
+	 *    fallthrough: r1=100 after widening
+	 *    other_branch: r1 stays as-is (0, 1, 2, ..)
+	 *
+	 *  Also recognize the case where both LHS and RHS are constant and
+	 *  equal to each other. In this case don't widen at all and take the
+	 *  predicted path. This key heuristic allows the verifier detect loop
+	 *  end condition and 'for (i = 0; i != 100; i++)' is validated just
+	 *  like bounded loop.
+	 */
+	else if (is_reg_const(dst_reg, is_jmp32) && is_reg_const(src_reg, is_jmp32) &&
+	    reg_const_value(dst_reg, is_jmp32) == reg_const_value(src_reg, is_jmp32))
+		ignore_pred = false;
+	else
+		ignore_pred = (get_loop_entry(this_branch) ||
+			       this_branch->may_goto_depth) &&
+				/* Gate widen_reg() logic */
+				env->bpf_capable;
+
 	pred = is_branch_taken(dst_reg, src_reg, opcode, is_jmp32);
-	if (pred >= 0) {
+	if (pred >= 0 && !ignore_pred) {
 		/* If we get here with a dst_reg pointer type it is because
 		 * above is_branch_taken() special cased the 0 comparison.
 		 */
@@ -15191,6 +15235,22 @@  static int check_cond_jmp_op(struct bpf_verifier_env *env,
 			return err;
 	}
 
+	if (pred < 0 || ignore_pred) {
+		other_branch = push_stack(env, *insn_idx + insn->off + 1, *insn_idx,
+					  false);
+		if (!other_branch)
+			return -EFAULT;
+		other_branch_regs = other_branch->frame[other_branch->curframe]->regs;
+		other_dst_reg = &other_branch_regs[insn->dst_reg];
+		if (has_src_reg)
+			other_src_reg = &other_branch_regs[insn->src_reg];
+	}
+
+	if (pred >= 0 && ignore_pred && has_src_reg) {
+		env->saved_dst_reg = *dst_reg;
+		env->saved_src_reg = *src_reg;
+	}
+
 	if (pred == 1) {
 		/* Only follow the goto, ignore fall-through. If needed, push
 		 * the fall-through branch for simulation under speculative
@@ -15202,8 +15262,33 @@  static int check_cond_jmp_op(struct bpf_verifier_env *env,
 			return -EFAULT;
 		if (env->log.level & BPF_LOG_LEVEL)
 			print_insn_state(env, this_branch->frame[this_branch->curframe]);
-		*insn_idx += insn->off;
-		return 0;
+		if (ignore_pred) {
+			/* dst and src regs are scalars. Widen them */
+			widen_reg(dst_reg);
+			if (has_src_reg)
+				widen_reg(src_reg);
+			/*
+			 * Widen other branch only if not comparing for equlity.
+			 * Example:
+			 *   r1 = 1
+			 *   if (r1 < 100)
+			 * will produce
+			 *   [0, 99] and [100, UMAX] after widening and reg_set_min_max().
+			 *
+			 *   r1 = 1
+			 *   if (r1 == 100)
+			 * will produce
+			 *    [1] and [100] after widening in other_branch and reg_set_min_max().
+			 */
+			if (opcode != BPF_JEQ && opcode != BPF_JNE) {
+				widen_reg(other_dst_reg);
+				if (has_src_reg)
+					widen_reg(other_src_reg);
+			}
+		} else {
+			*insn_idx += insn->off;
+			return 0;
+		}
 	} else if (pred == 0) {
 		/* Only follow the fall-through branch, since that's where the
 		 * program will go. If needed, push the goto branch for
@@ -15216,23 +15301,50 @@  static int check_cond_jmp_op(struct bpf_verifier_env *env,
 			return -EFAULT;
 		if (env->log.level & BPF_LOG_LEVEL)
 			print_insn_state(env, this_branch->frame[this_branch->curframe]);
-		return 0;
+		if (ignore_pred) {
+			if (opcode != BPF_JEQ && opcode != BPF_JNE) {
+				widen_reg(dst_reg);
+				if (has_src_reg)
+					widen_reg(src_reg);
+			}
+			widen_reg(other_dst_reg);
+			if (has_src_reg)
+				widen_reg(other_src_reg);
+		} else {
+			return 0;
+		}
 	}
 
-	other_branch = push_stack(env, *insn_idx + insn->off + 1, *insn_idx,
-				  false);
-	if (!other_branch)
-		return -EFAULT;
-	other_branch_regs = other_branch->frame[other_branch->curframe]->regs;
-
 	if (BPF_SRC(insn->code) == BPF_X) {
-		err = reg_set_min_max(env,
-				      &other_branch_regs[insn->dst_reg],
-				      &other_branch_regs[insn->src_reg],
-				      dst_reg, src_reg, opcode, is_jmp32);
+		if (pred >= 0 && ignore_pred) {
+			/*
+			 * In case of Rx < Ry both registers
+			 * were widened to unknown scalar, hence
+			 * call reg_set_min_max() twice to learn bounds
+			 * from values Rx and Ry had before widening.
+			 * Ex:
+			 * r1 = 3
+			 * r2 = 5
+			 * if r1 > r2 goto ...
+			 * fallthrough:
+			 * r1 = [0, 5], r2 = [3, UMAX]
+			 */
+			err = reg_set_min_max(env,
+					      other_dst_reg, &env->saved_src_reg,
+					      dst_reg, &env->saved_src_reg, opcode, is_jmp32);
+			if (err)
+				return err;
+			err = reg_set_min_max(env,
+					      &env->saved_dst_reg, other_src_reg,
+					      &env->saved_dst_reg, src_reg, opcode, is_jmp32);
+		} else {
+			err = reg_set_min_max(env,
+					      other_dst_reg, other_src_reg,
+					      dst_reg, src_reg, opcode, is_jmp32);
+		}
 	} else /* BPF_SRC(insn->code) == BPF_K */ {
 		err = reg_set_min_max(env,
-				      &other_branch_regs[insn->dst_reg],
+				      other_dst_reg,
 				      src_reg /* fake one */,
 				      dst_reg, src_reg /* same fake one */,
 				      opcode, is_jmp32);
@@ -15240,16 +15352,16 @@  static int check_cond_jmp_op(struct bpf_verifier_env *env,
 	if (err)
 		return err;
 
-	if (BPF_SRC(insn->code) == BPF_X &&
+	if (has_src_reg &&
 	    src_reg->type == SCALAR_VALUE && src_reg->id &&
-	    !WARN_ON_ONCE(src_reg->id != other_branch_regs[insn->src_reg].id)) {
+	    !WARN_ON_ONCE(src_reg->id != other_src_reg->id)) {
 		find_equal_scalars(this_branch, src_reg);
-		find_equal_scalars(other_branch, &other_branch_regs[insn->src_reg]);
+		find_equal_scalars(other_branch, other_src_reg);
 	}
 	if (dst_reg->type == SCALAR_VALUE && dst_reg->id &&
-	    !WARN_ON_ONCE(dst_reg->id != other_branch_regs[insn->dst_reg].id)) {
+	    !WARN_ON_ONCE(dst_reg->id != other_dst_reg->id)) {
 		find_equal_scalars(this_branch, dst_reg);
-		find_equal_scalars(other_branch, &other_branch_regs[insn->dst_reg]);
+		find_equal_scalars(other_branch, other_dst_reg);
 	}
 
 	/* if one pointer register is compared to another pointer
@@ -15264,7 +15376,7 @@  static int check_cond_jmp_op(struct bpf_verifier_env *env,
 	 * could be null even without PTR_MAYBE_NULL marking, so
 	 * only propagate nullness when neither reg is that type.
 	 */
-	if (!is_jmp32 && BPF_SRC(insn->code) == BPF_X &&
+	if (!is_jmp32 && has_src_reg &&
 	    __is_pointer_value(false, src_reg) && __is_pointer_value(false, dst_reg) &&
 	    type_may_be_null(src_reg->type) != type_may_be_null(dst_reg->type) &&
 	    base_type(src_reg->type) != PTR_TO_BTF_ID &&
@@ -17409,6 +17521,7 @@  static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
 			 * => unsafe memory access at 11 would not be caught.
 			 */
 			if (is_iter_next_insn(env, insn_idx)) {
+				update_loop_entry(cur, &sl->state);
 				if (states_equal(env, &sl->state, cur, RANGE_WITHIN)) {
 					struct bpf_func_state *cur_frame;
 					struct bpf_reg_state *iter_state, *iter_reg;
@@ -17425,18 +17538,15 @@  static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
 					 */
 					spi = __get_spi(iter_reg->off + iter_reg->var_off.value);
 					iter_state = &func(env, iter_reg)->stack[spi].spilled_ptr;
-					if (iter_state->iter.state == BPF_ITER_STATE_ACTIVE) {
-						update_loop_entry(cur, &sl->state);
+					if (iter_state->iter.state == BPF_ITER_STATE_ACTIVE)
 						goto hit;
-					}
 				}
 				goto skip_inf_loop_check;
 			}
 			if (is_may_goto_insn_at(env, insn_idx)) {
-				if (states_equal(env, &sl->state, cur, RANGE_WITHIN)) {
-					update_loop_entry(cur, &sl->state);
+				update_loop_entry(cur, &sl->state);
+				if (states_equal(env, &sl->state, cur, RANGE_WITHIN))
 					goto hit;
-				}
 				goto skip_inf_loop_check;
 			}
 			if (calls_callback(env, insn_idx)) {