[PATCH v5 bpf-next 1/3] bpf: Relax tuple len requirement for sk helpers.

[PATCH v5 bpf-next 1/3] bpf: Relax tuple len requirement for sk helpers.

[Alexei Starovoitov]

From: Alexei Starovoitov <ast@kernel.org>

__bpf_skc_lookup() safely handles incorrect values of tuple len,
hence we can allow zero to be passed as tuple len.
This patch alone doesn't make observable verifier difference, but if next
patch is tweaked to apply scalar widening logic to normal loops
then test_cls_redirect may benefit:

File                     Insns (A)  Insns (B)  Insns        (DIFF)
-----------------------  ---------  ---------  -------------------
test_cls_redirect.bpf.o      35423      29883      -5540 (-15.64%)

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
---
 net/core/filter.c | 24 ++++++++++++------------
 1 file changed, 12 insertions(+), 12 deletions(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index 7c46ecba3b01..cb133232a887 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -6815,7 +6815,7 @@ static const struct bpf_func_proto bpf_skc_lookup_tcp_proto = {
 	.ret_type	= RET_PTR_TO_SOCK_COMMON_OR_NULL,
 	.arg1_type	= ARG_PTR_TO_CTX,
 	.arg2_type	= ARG_PTR_TO_MEM | MEM_RDONLY,
-	.arg3_type	= ARG_CONST_SIZE,
+	.arg3_type	= ARG_CONST_SIZE_OR_ZERO,
 	.arg4_type	= ARG_ANYTHING,
 	.arg5_type	= ARG_ANYTHING,
 };
@@ -6834,7 +6834,7 @@ static const struct bpf_func_proto bpf_sk_lookup_tcp_proto = {
 	.ret_type	= RET_PTR_TO_SOCKET_OR_NULL,
 	.arg1_type	= ARG_PTR_TO_CTX,
 	.arg2_type	= ARG_PTR_TO_MEM | MEM_RDONLY,
-	.arg3_type	= ARG_CONST_SIZE,
+	.arg3_type	= ARG_CONST_SIZE_OR_ZERO,
 	.arg4_type	= ARG_ANYTHING,
 	.arg5_type	= ARG_ANYTHING,
 };
@@ -6853,7 +6853,7 @@ static const struct bpf_func_proto bpf_sk_lookup_udp_proto = {
 	.ret_type	= RET_PTR_TO_SOCKET_OR_NULL,
 	.arg1_type	= ARG_PTR_TO_CTX,
 	.arg2_type	= ARG_PTR_TO_MEM | MEM_RDONLY,
-	.arg3_type	= ARG_CONST_SIZE,
+	.arg3_type	= ARG_CONST_SIZE_OR_ZERO,
 	.arg4_type	= ARG_ANYTHING,
 	.arg5_type	= ARG_ANYTHING,
 };
@@ -6877,7 +6877,7 @@ static const struct bpf_func_proto bpf_tc_skc_lookup_tcp_proto = {
 	.ret_type	= RET_PTR_TO_SOCK_COMMON_OR_NULL,
 	.arg1_type	= ARG_PTR_TO_CTX,
 	.arg2_type	= ARG_PTR_TO_MEM | MEM_RDONLY,
-	.arg3_type	= ARG_CONST_SIZE,
+	.arg3_type	= ARG_CONST_SIZE_OR_ZERO,
 	.arg4_type	= ARG_ANYTHING,
 	.arg5_type	= ARG_ANYTHING,
 };
@@ -6901,7 +6901,7 @@ static const struct bpf_func_proto bpf_tc_sk_lookup_tcp_proto = {
 	.ret_type	= RET_PTR_TO_SOCKET_OR_NULL,
 	.arg1_type	= ARG_PTR_TO_CTX,
 	.arg2_type	= ARG_PTR_TO_MEM | MEM_RDONLY,
-	.arg3_type	= ARG_CONST_SIZE,
+	.arg3_type	= ARG_CONST_SIZE_OR_ZERO,
 	.arg4_type	= ARG_ANYTHING,
 	.arg5_type	= ARG_ANYTHING,
 };
@@ -6925,7 +6925,7 @@ static const struct bpf_func_proto bpf_tc_sk_lookup_udp_proto = {
 	.ret_type	= RET_PTR_TO_SOCKET_OR_NULL,
 	.arg1_type	= ARG_PTR_TO_CTX,
 	.arg2_type	= ARG_PTR_TO_MEM | MEM_RDONLY,
-	.arg3_type	= ARG_CONST_SIZE,
+	.arg3_type	= ARG_CONST_SIZE_OR_ZERO,
 	.arg4_type	= ARG_ANYTHING,
 	.arg5_type	= ARG_ANYTHING,
 };
@@ -6963,7 +6963,7 @@ static const struct bpf_func_proto bpf_xdp_sk_lookup_udp_proto = {
 	.ret_type       = RET_PTR_TO_SOCKET_OR_NULL,
 	.arg1_type      = ARG_PTR_TO_CTX,
 	.arg2_type      = ARG_PTR_TO_MEM | MEM_RDONLY,
-	.arg3_type      = ARG_CONST_SIZE,
+	.arg3_type      = ARG_CONST_SIZE_OR_ZERO,
 	.arg4_type      = ARG_ANYTHING,
 	.arg5_type      = ARG_ANYTHING,
 };
@@ -6987,7 +6987,7 @@ static const struct bpf_func_proto bpf_xdp_skc_lookup_tcp_proto = {
 	.ret_type       = RET_PTR_TO_SOCK_COMMON_OR_NULL,
 	.arg1_type      = ARG_PTR_TO_CTX,
 	.arg2_type      = ARG_PTR_TO_MEM | MEM_RDONLY,
-	.arg3_type      = ARG_CONST_SIZE,
+	.arg3_type      = ARG_CONST_SIZE_OR_ZERO,
 	.arg4_type      = ARG_ANYTHING,
 	.arg5_type      = ARG_ANYTHING,
 };
@@ -7011,7 +7011,7 @@ static const struct bpf_func_proto bpf_xdp_sk_lookup_tcp_proto = {
 	.ret_type       = RET_PTR_TO_SOCKET_OR_NULL,
 	.arg1_type      = ARG_PTR_TO_CTX,
 	.arg2_type      = ARG_PTR_TO_MEM | MEM_RDONLY,
-	.arg3_type      = ARG_CONST_SIZE,
+	.arg3_type      = ARG_CONST_SIZE_OR_ZERO,
 	.arg4_type      = ARG_ANYTHING,
 	.arg5_type      = ARG_ANYTHING,
 };
@@ -7031,7 +7031,7 @@ static const struct bpf_func_proto bpf_sock_addr_skc_lookup_tcp_proto = {
 	.ret_type	= RET_PTR_TO_SOCK_COMMON_OR_NULL,
 	.arg1_type	= ARG_PTR_TO_CTX,
 	.arg2_type	= ARG_PTR_TO_MEM | MEM_RDONLY,
-	.arg3_type	= ARG_CONST_SIZE,
+	.arg3_type	= ARG_CONST_SIZE_OR_ZERO,
 	.arg4_type	= ARG_ANYTHING,
 	.arg5_type	= ARG_ANYTHING,
 };
@@ -7050,7 +7050,7 @@ static const struct bpf_func_proto bpf_sock_addr_sk_lookup_tcp_proto = {
 	.ret_type	= RET_PTR_TO_SOCKET_OR_NULL,
 	.arg1_type	= ARG_PTR_TO_CTX,
 	.arg2_type	= ARG_PTR_TO_MEM | MEM_RDONLY,
-	.arg3_type	= ARG_CONST_SIZE,
+	.arg3_type	= ARG_CONST_SIZE_OR_ZERO,
 	.arg4_type	= ARG_ANYTHING,
 	.arg5_type	= ARG_ANYTHING,
 };
@@ -7069,7 +7069,7 @@ static const struct bpf_func_proto bpf_sock_addr_sk_lookup_udp_proto = {
 	.ret_type	= RET_PTR_TO_SOCKET_OR_NULL,
 	.arg1_type	= ARG_PTR_TO_CTX,
 	.arg2_type	= ARG_PTR_TO_MEM | MEM_RDONLY,
-	.arg3_type	= ARG_CONST_SIZE,
+	.arg3_type	= ARG_CONST_SIZE_OR_ZERO,
 	.arg4_type	= ARG_ANYTHING,
 	.arg5_type	= ARG_ANYTHING,
 };
-- 
2.43.0

[PATCH v5 bpf-next 2/3] bpf: Relax precision marking in open coded iters and may_goto loop.

[Alexei Starovoitov]

From: Alexei Starovoitov <ast@kernel.org>

v1->v2->v3:
- Algorithm changed completely between revisions:
  v1: https://lore.kernel.org/bpf/20240522024713.59136-1-alexei.starovoitov@gmail.com/
  v2: https://lore.kernel.org/bpf/20240523064219.42465-1-alexei.starovoitov@gmail.com/
v3->v4:
- Fixed widening for Rx < Ry case and added more tests
  v4: https://lore.kernel.org/bpf/20240601034211.63962-1-alexei.starovoitov@gmail.com/
v4->v5:
- Algorithm changed again:
. Widen either lower or upper scalar range instead of both. See widen_reg().
. Recognize predicted == or != and convert to <, > when possible
  and follow that branch only after propagating precision.
. Apply to scalar constant only.
These changes made big difference for arena progs. In v4 arena tests were in the noise.

Motivations for the patch
-------------------------
1.
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).

Note, i = zero workaround disables bounded loop logic.
Open coded iterator bpf_for(i, 0, 100) also disables bounded loop logic,
hence apply heuristic in this patch only for iters and may_goto.

2.
Arena based program spent significant amount of verification time
propagating precision due to predicted conditional branches,
but this precision is useless work, since arena access doesn't
require precision unlike regular map access.
The difference before/after:
File                    Insns (A)  Insns (B)  Insns     (DIFF)
----------------------  ---------  ---------  ----------------
arena_htab.bpf.o            18656        781  -17875 (-95.81%)
arena_htab_asm.bpf.o        18523        598  -17925 (-96.77%)
arena_list.bpf.o             1685       1780      +95 (+5.64%)

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. When equality
condition is predicted check whether dst is < or > than src. Example:
  r1 = 10
  goto L1
L2:
  arr[r1] = 1
  r1++
L1:
  if r1 != 100 goto L2

This branch will be predicted as fallthrough, check that r1 < 100
and if so, widen r1 = [10, 99] in fallthrough and
r1 = [100, UMAX] in other branch.

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

The tests with open coded iters see dramatic improvement. The rest are noise.
File                  Program                          Insns (A)  Insns (B)  Insns       (DIFF)  Verdict (A)  Verdict (B)
--------------------  -------------------------------  ---------  ---------  ------------------  -----------  -----------
iters_task_vma.bpf.o  iter_task_vma_for_each               22043        132    -21911 (-99.40%)  success      success
iters_task_vma.bpf.o  iter_task_vma_for_each_eq            22043        131    -21912 (-99.41%)  success      success
iters_task_vma.bpf.o  loop_inside_iter                   1000001        148   -999853 (-99.99%)  failure      success
iters_task_vma.bpf.o  loop_inside_iter_signed            1000001        148   -999853 (-99.99%)  failure      success
iters_task_vma.bpf.o  loop_inside_iter_subprog           1000001         64   -999937 (-99.99%)  failure      success
iters_task_vma.bpf.o  loop_inside_iter_volatile_limit    1000001        134   -999867 (-99.99%)  failure      success

The bottom 4 tests were unverifiable before due to limitations of bounded loop
logic.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
---
 kernel/bpf/verifier.c | 330 ++++++++++++++++++++++++++++++++++++++----
 1 file changed, 302 insertions(+), 28 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 81a3d2ced78d..79e356ac02ab 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2321,6 +2321,7 @@ static void __mark_reg_unknown(const struct bpf_verifier_env *env,
 	reg->precise = !env->bpf_capable;
 }
 
+
 static void mark_reg_unknown(struct bpf_verifier_env *env,
 			     struct bpf_reg_state *regs, u32 regno)
 {
@@ -14704,6 +14705,165 @@ static u8 rev_opcode(u8 opcode)
 	}
 }
 
+/* Similar to mark_reg_unknown() and should only be called from cap_bpf path */
+static void mark_unknown(struct bpf_reg_state *reg)
+{
+	u32 id = reg->id;
+
+	__mark_reg_unknown_imprecise(reg);
+	reg->id = id;
+}
+/*
+ * Similar to regs_refine_cond_op(), but instead of tightening the range
+ * widen the upper bound of reg1 based on reg2 and
+ * lower bound of reg2 based on reg1.
+ */
+static void widen_reg_bounds(struct bpf_reg_state *reg1,
+			     struct bpf_reg_state *reg2,
+			     u8 opcode, bool is_jmp32)
+{
+	switch (opcode) {
+	case BPF_JGE:
+	case BPF_JGT:
+	case BPF_JSGE:
+	case BPF_JSGT:
+		opcode = flip_opcode(opcode);
+		swap(reg1, reg2);
+		break;
+	default:
+		break;
+	}
+
+	switch (opcode) {
+	case BPF_JLE:
+		if (is_jmp32) {
+			reg1->u32_max_value = reg2->u32_max_value;
+			reg1->s32_max_value = S32_MAX;
+			reg1->umax_value = U64_MAX;
+			reg1->smax_value = S64_MAX;
+
+			reg2->u32_min_value = reg1->u32_min_value;
+			reg2->s32_min_value = S32_MIN;
+			reg2->umin_value = 0;
+			reg2->smin_value = S64_MIN;
+		} else {
+			reg1->umax_value = reg2->umax_value;
+			reg1->smax_value = S64_MAX;
+			reg1->u32_max_value = U32_MAX;
+			reg1->s32_max_value = S32_MAX;
+
+			reg2->umin_value = reg1->umin_value;
+			reg2->smin_value = S64_MIN;
+			reg2->u32_min_value = U32_MIN;
+			reg2->s32_min_value = S32_MIN;
+		}
+		reg1->var_off = tnum_unknown;
+		reg2->var_off = tnum_unknown;
+		break;
+	case BPF_JLT:
+		if (is_jmp32) {
+			reg1->u32_max_value = reg2->u32_max_value - 1;
+			reg1->s32_max_value = S32_MAX;
+			reg1->umax_value = U64_MAX;
+			reg1->smax_value = S64_MAX;
+
+			reg2->u32_min_value = reg1->u32_min_value + 1;
+			reg2->s32_min_value = S32_MIN;
+			reg2->umin_value = 0;
+			reg2->smin_value = S64_MIN;
+		} else {
+			reg1->umax_value = reg2->umax_value - 1;
+			reg1->smax_value = S64_MAX;
+			reg1->u32_max_value = U32_MAX;
+			reg1->s32_max_value = S32_MAX;
+
+			reg2->umin_value = reg1->umin_value + 1;
+			reg2->smin_value = S64_MIN;
+			reg2->u32_min_value = U32_MIN;
+			reg2->s32_min_value = S32_MIN;
+		}
+		reg1->var_off = tnum_unknown;
+		reg2->var_off = tnum_unknown;
+		break;
+	case BPF_JSLE:
+		if (is_jmp32) {
+			reg1->u32_max_value = U32_MAX;
+			reg1->s32_max_value = reg2->s32_max_value;
+			reg1->umax_value = U64_MAX;
+			reg1->smax_value = S64_MAX;
+
+			reg2->u32_min_value = U32_MIN;
+			reg2->s32_min_value = reg1->s32_min_value;
+			reg2->umin_value = 0;
+			reg2->smin_value = S64_MIN;
+		} else {
+			reg1->umax_value = U64_MAX;
+			reg1->smax_value = reg2->smax_value;
+			reg1->u32_max_value = U32_MAX;
+			reg1->s32_max_value = S32_MAX;
+
+			reg2->umin_value = 0;
+			reg2->smin_value = reg1->smin_value;
+			reg2->u32_min_value = U32_MIN;
+			reg2->s32_min_value = S32_MIN;
+		}
+		reg1->var_off = tnum_unknown;
+		reg2->var_off = tnum_unknown;
+		break;
+	case BPF_JSLT:
+		if (is_jmp32) {
+			reg1->u32_max_value = U32_MAX;
+			reg1->s32_max_value = reg2->s32_max_value - 1;
+			reg1->umax_value = U64_MAX;
+			reg1->smax_value = S64_MAX;
+
+			reg2->u32_min_value = U32_MIN;
+			reg2->s32_min_value = reg1->s32_min_value + 1;
+			reg2->umin_value = 0;
+			reg2->smin_value = S64_MIN;
+		} else {
+			reg1->umax_value = U64_MAX;
+			reg1->smax_value = reg2->smax_value - 1;
+			reg1->u32_max_value = U32_MAX;
+			reg1->s32_max_value = S32_MAX;
+
+			reg2->umin_value = 0;
+			reg2->smin_value = reg1->smin_value + 1;
+			reg2->u32_min_value = U32_MIN;
+			reg2->s32_min_value = S32_MIN;
+		}
+		reg1->var_off = tnum_unknown;
+		reg2->var_off = tnum_unknown;
+		break;
+	default:
+		break;
+	}
+}
+
+/*
+ * Widen reg bounds. Example:
+ *   r1 = 3
+ *   r2 = 100
+ *   if (r1 < r2)
+ * will produce
+ *   r1 = [3, 99] r2 = [100, UMAX]
+ */
+static int widen_reg(struct bpf_verifier_env *env,
+		     struct bpf_reg_state *reg1,
+		     struct bpf_reg_state *reg2,
+		     u8 opcode, bool is_jmp32, bool branch_taken)
+{
+	int err;
+
+	widen_reg_bounds(reg1, reg2, branch_taken ? opcode : rev_opcode(opcode), is_jmp32);
+	reg_bounds_sync(reg1);
+	reg_bounds_sync(reg2);
+
+	err = reg_bounds_sanity_check(env, reg1, "widen reg1");
+	err = err ?: reg_bounds_sanity_check(env, reg2, "widen reg2");
+	return err;
+}
+
 /* Refine range knowledge for <reg1> <op> <reg>2 conditional operation. */
 static void regs_refine_cond_op(struct bpf_reg_state *reg1, struct bpf_reg_state *reg2,
 				u8 opcode, bool is_jmp32)
@@ -15104,10 +15264,11 @@ 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 = NULL, *other_src_reg = NULL;
 	struct bpf_reg_state fake_reg = {};
 	u8 opcode = BPF_OP(insn->code);
-	bool is_jmp32;
+	bool is_jmp32, do_widen;
+	bool has_src_reg = false;
 	int pred = -1;
 	int err;
 
@@ -15159,6 +15320,7 @@ 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 +15339,78 @@ 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 ||
+	    /* Widen scalars only if they're constants */
+	    !is_reg_const(dst_reg, is_jmp32) || !is_reg_const(src_reg, is_jmp32))
+		do_widen = false;
+	else if (reg_const_value(dst_reg, is_jmp32) == reg_const_value(src_reg, is_jmp32))
+		/* And not equal */
+		do_widen = false;
+	else
+		do_widen = (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 (do_widen && ((opcode == BPF_JNE && pred == 1) ||
+			 (opcode == BPF_JEQ && pred == 0))) {
+		/*
+		 * != is too vague. let's try < and > and widen. Example:
+		 *
+		 * R6=2
+		 * 21: (15) if r6 == 0x3e8 goto pc+14
+		 * Predicted == not-taken, but < is also true
+		 * 21: R6=scalar(smin=umin=smin32=umin32=2,smax=umax=smax32=umax32=999,var_off=(0x0; 0x3ff))
+		 */
+		int refine_pred;
+		u8 opcode2 = BPF_JLT;
+
+		refine_pred = is_branch_taken(dst_reg, src_reg, BPF_JLT, is_jmp32);
+		if (refine_pred == 1) {
+			widen_reg(env, dst_reg, src_reg, BPF_JLT, is_jmp32, true);
+
+		} else {
+			opcode2 = BPF_JGT;
+			refine_pred = is_branch_taken(dst_reg, src_reg, BPF_JGT, is_jmp32);
+			if (refine_pred == 1)
+				widen_reg(env, dst_reg, src_reg, BPF_JGT, is_jmp32, true);
+		}
+
+		if (refine_pred == 1) {
+			if (dst_reg->id)
+				find_equal_scalars(this_branch, dst_reg);
+			if (env->log.level & BPF_LOG_LEVEL) {
+				verbose(env, "Predicted %s, but %s is also true\n",
+					opcode == BPF_JNE ? "!= taken" : "== not-taken",
+					opcode2 == BPF_JLT ? "<" : ">");
+				print_insn_state(env, this_branch->frame[this_branch->curframe]);
+			}
+			err = mark_chain_precision(env, insn->dst_reg);
+			if (err)
+				return err;
+			if (has_src_reg) {
+				err = mark_chain_precision(env, insn->src_reg);
+				if (err)
+					return err;
+			}
+			if (pred == 1)
+				*insn_idx += insn->off;
+			return 0;
+		}
+		/*
+		 * No luck. Predicted dst != src taken or dst == src not-taken,
+		 * but !(dst < src) and !(dst > src).
+		 * Constants must have been negative.
+		 */
+	}
+
+	if (do_widen && (opcode == BPF_JNE || opcode == BPF_JEQ || opcode == BPF_JSET))
+		/* widen_reg() algorithm works for <, <=, >, >= only */
+		do_widen = false;
+
+	if (pred >= 0 && !do_widen) {
 		/* If we get here with a dst_reg pointer type it is because
 		 * above is_branch_taken() special cased the 0 comparison.
 		 */
@@ -15189,6 +15421,60 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
 			err = mark_chain_precision(env, insn->src_reg);
 		if (err)
 			return err;
+	} else {
+		/*
+		 * The verifier has to propagate precision if it's going to
+		 * continue exploring only one branch of conditional jump.
+		 * Otherwise push_stack() to explore both branches.
+		 */
+		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 (do_widen && pred >= 0) {
+		/*
+		 * Widen predicted <, <=, >, >= comparison of constant scalars. Example:
+		 *
+		 * R7=0x186a0
+		 * 21: (25) if r7 > 0x1869f goto pc-10
+		 * Predicted branch taken
+		 * 21: R7=scalar(smin=smin32=0,smax=umax=smax32=umax32=0x1869f,var_off=(0x0; 0x1ffff))
+		 * other branch:
+		 * R7=0x186a0
+		 *
+		 * R7=2
+		 * 21: (25) if r7 > 0x1869f goto pc-10
+		 * Predicted branch not-taken
+		 * 21: R7=scalar(smin=umin=smin32=umin32=2,smax=umax=smax32=umax32=0x1869f,var_off=(0x0; 0x1ffff))
+		 * other branch:
+		 * R7=scalar(umin=0x186a0)
+		 */
+		if (pred == 1)
+			mark_unknown(dst_reg);
+		widen_reg(env, dst_reg, src_reg, opcode, is_jmp32, false);
+		if (!has_src_reg) {
+			other_src_reg = &fake_reg;
+			other_src_reg->type = SCALAR_VALUE;
+			__mark_reg_known(other_src_reg, insn->imm);
+		}
+		if (pred == 0)
+			mark_unknown(other_dst_reg);
+		widen_reg(env, other_dst_reg, other_src_reg, opcode, is_jmp32, true);
+
+		if (env->log.level & BPF_LOG_LEVEL) {
+			verbose(env, "Predicted branch %s\n", pred == 1 ? "taken" : "not-taken");
+			print_insn_state(env, this_branch->frame[this_branch->curframe]);
+			verbose(env, "other branch:\n");
+			mark_reg_scratched(env, insn->dst_reg);
+			print_verifier_state(env, other_branch->frame[other_branch->curframe], false);
+		}
+		goto skip_min_max;
 	}
 
 	if (pred == 1) {
@@ -15219,37 +15505,27 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
 		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],
+		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],
-				      src_reg /* fake one */,
+		err = reg_set_min_max(env, other_dst_reg, src_reg /* fake one */,
 				      dst_reg, src_reg /* same fake one */,
 				      opcode, is_jmp32);
 	}
 	if (err)
 		return err;
-
-	if (BPF_SRC(insn->code) == BPF_X &&
+skip_min_max:
+	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 +15540,7 @@ 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 +17685,7 @@ 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 +17702,15 @@ 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)) {
-- 
2.43.0

[PATCH v5 bpf-next 3/3] selftests/bpf: Remove i = zero workaround and add new tests.

[Alexei Starovoitov]

From: Alexei Starovoitov <ast@kernel.org>

. remove i = zero workaround from various tests
. improve arena based tests
. add asm test for this_branch_reg->id == other_branch_reg->id condition
. add several loop inside open coded iter tests

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
---
 .../testing/selftests/bpf/progs/arena_htab.c  |  16 ++-
 tools/testing/selftests/bpf/progs/iters.c     |  14 +-
 .../selftests/bpf/progs/iters_task_vma.c      | 130 ++++++++++++++++++
 .../bpf/progs/verifier_iterating_callbacks.c  |  23 ++--
 .../selftests/bpf/progs/verifier_reg_equal.c  |  29 ++++
 5 files changed, 185 insertions(+), 27 deletions(-)

diff --git a/tools/testing/selftests/bpf/progs/arena_htab.c b/tools/testing/selftests/bpf/progs/arena_htab.c
index 1e6ac187a6a0..1917f35b5963 100644
--- a/tools/testing/selftests/bpf/progs/arena_htab.c
+++ b/tools/testing/selftests/bpf/progs/arena_htab.c
@@ -18,25 +18,35 @@ void __arena *htab_for_user;
 bool skip = false;
 
 int zero = 0;
+char __arena arr1[100000];
+char arr2[1000];
 
 SEC("syscall")
 int arena_htab_llvm(void *ctx)
 {
 #if defined(__BPF_FEATURE_ADDR_SPACE_CAST) || defined(BPF_ARENA_FORCE_ASM)
 	struct htab __arena *htab;
+	char __arena *arr = arr1;
 	__u64 i;
 
 	htab = bpf_alloc(sizeof(*htab));
 	cast_kern(htab);
 	htab_init(htab);
 
+	cast_kern(arr);
+
 	/* first run. No old elems in the table */
-	for (i = zero; i < 1000; i++)
+	for (i = 0; i < 100000 && can_loop; i++) {
 		htab_update_elem(htab, i, i);
+		arr[i] = i;
+	}
 
-	/* should replace all elems with new ones */
-	for (i = zero; i < 1000; i++)
+	/* should replace some elems with new ones */
+	for (i = 0; i < 1000 && can_loop; i++) {
 		htab_update_elem(htab, i, i);
+		/* Access mem to make the verifier use bounded loop logic */
+		arr2[i] = i;
+	}
 	cast_user(htab);
 	htab_for_user = htab;
 #else
diff --git a/tools/testing/selftests/bpf/progs/iters.c b/tools/testing/selftests/bpf/progs/iters.c
index fe65e0952a1e..1a5adffae5d3 100644
--- a/tools/testing/selftests/bpf/progs/iters.c
+++ b/tools/testing/selftests/bpf/progs/iters.c
@@ -291,10 +291,7 @@ int iter_obfuscate_counter(const void *ctx)
 {
 	struct bpf_iter_num it;
 	int *v, sum = 0;
-	/* Make i's initial value unknowable for verifier to prevent it from
-	 * pruning if/else branch inside the loop body and marking i as precise.
-	 */
-	int i = zero;
+	int i = 0;
 
 	MY_PID_GUARD();
 
@@ -304,15 +301,6 @@ int iter_obfuscate_counter(const void *ctx)
 
 		i += 1;
 
-		/* If we initialized i as `int i = 0;` above, verifier would
-		 * track that i becomes 1 on first iteration after increment
-		 * above, and here verifier would eagerly prune else branch
-		 * and mark i as precise, ruining open-coded iterator logic
-		 * completely, as each next iteration would have a different
-		 * *precise* value of i, and thus there would be no
-		 * convergence of state. This would result in reaching maximum
-		 * instruction limit, no matter what the limit is.
-		 */
 		if (i == 1)
 			x = 123;
 		else
diff --git a/tools/testing/selftests/bpf/progs/iters_task_vma.c b/tools/testing/selftests/bpf/progs/iters_task_vma.c
index dc0c3691dcc2..31c4e7d9eaa3 100644
--- a/tools/testing/selftests/bpf/progs/iters_task_vma.c
+++ b/tools/testing/selftests/bpf/progs/iters_task_vma.c
@@ -28,6 +28,10 @@ int iter_task_vma_for_each(const void *ctx)
 		return 0;
 
 	bpf_for_each(task_vma, vma, task, 0) {
+		/*
+		 * Fast to verify, since 'seen' has the same range at every
+		 * loop iteration.
+		 */
 		if (bpf_cmp_unlikely(seen, >=, 1000))
 			break;
 
@@ -40,4 +44,130 @@ int iter_task_vma_for_each(const void *ctx)
 	return 0;
 }
 
+SEC("raw_tp/sys_enter")
+int iter_task_vma_for_each_eq(const void *ctx)
+{
+	struct task_struct *task = bpf_get_current_task_btf();
+	struct vm_area_struct *vma;
+	unsigned int seen = 0;
+
+	if (task->pid != target_pid)
+		return 0;
+
+	if (vmas_seen)
+		return 0;
+
+	bpf_for_each(task_vma, vma, task, 0) {
+		/*
+		 * Also fast, since the verifier recognizes
+		 * 0, 1, 2 != 1000 as [0, 999] range.
+		 */
+		if (bpf_cmp_unlikely(seen, ==, 1000))
+			break;
+
+		vm_ranges[seen].vm_start = vma->vm_start;
+		vm_ranges[seen].vm_end = vma->vm_end;
+		seen++;
+	}
+
+	vmas_seen = seen;
+	return 0;
+}
+
+#define ARR_SZ 100000
+char arr[ARR_SZ];
+
+SEC("socket")
+__success __flag(BPF_F_TEST_STATE_FREQ)
+int loop_inside_iter(const void *ctx)
+{
+	struct bpf_iter_num it;
+	int *v, sum = 0;
+	__u64 i = 0;
+
+	bpf_iter_num_new(&it, 0, ARR_SZ);
+	while ((v = bpf_iter_num_next(&it))) {
+		if (i < ARR_SZ)
+			sum += arr[i++];
+	}
+	bpf_iter_num_destroy(&it);
+	return sum;
+}
+
+SEC("socket")
+__success __flag(BPF_F_TEST_STATE_FREQ)
+int loop_inside_iter_signed(const void *ctx)
+{
+	struct bpf_iter_num it;
+	int *v, sum = 0;
+	long i = 0;
+
+	bpf_iter_num_new(&it, 0, ARR_SZ);
+	while ((v = bpf_iter_num_next(&it))) {
+		if (i < ARR_SZ && i >= 0)
+			sum += arr[i++];
+	}
+	bpf_iter_num_destroy(&it);
+	return sum;
+}
+
+volatile const int limit = ARR_SZ;
+
+SEC("socket")
+__success __flag(BPF_F_TEST_STATE_FREQ)
+int loop_inside_iter_volatile_limit(const void *ctx)
+{
+	struct bpf_iter_num it;
+	int *v, sum = 0;
+	__u64 i = 0;
+
+	bpf_iter_num_new(&it, 0, ARR_SZ);
+	while ((v = bpf_iter_num_next(&it))) {
+		if (i < limit)
+			sum += arr[i++];
+	}
+	bpf_iter_num_destroy(&it);
+	return sum;
+}
+
+__noinline
+static void touch_arr(int i)
+{
+	/*
+	 * Though 'i' is signed the verifier sees that 0
+	 * is the lowest number passed into static subprogram
+	 * and determines the range [0, ARR_SZ - 1].
+	 */
+	if (i >= ARR_SZ)
+		return;
+	arr[i] = i;
+}
+
+__noinline
+int touch_arr_global(__u32 i)
+{
+	/*
+	 * In global function the array index 'i' has to be unsigned,
+	 * otherwise the verifier will see unbounded min value.
+	 */
+	if (i >= ARR_SZ)
+		return 0;
+	arr[i] = i;
+	return 0;
+}
+
+SEC("socket")
+__success
+int loop_inside_iter_subprog(const void *ctx)
+{
+	long i;
+
+	for (i = 0; i <= 1000000 && can_loop; i++)
+		touch_arr(i);
+
+	for (i = 0; i <= 1000000 && can_loop; i++)
+		touch_arr_global(i);
+
+	return 0;
+}
 char _license[] SEC("license") = "GPL";
diff --git a/tools/testing/selftests/bpf/progs/verifier_iterating_callbacks.c b/tools/testing/selftests/bpf/progs/verifier_iterating_callbacks.c
index bd676d7e615f..78016294df95 100644
--- a/tools/testing/selftests/bpf/progs/verifier_iterating_callbacks.c
+++ b/tools/testing/selftests/bpf/progs/verifier_iterating_callbacks.c
@@ -308,7 +308,6 @@ int iter_limit_bug(struct __sk_buff *skb)
 }
 
 #define ARR_SZ 1000000
-int zero;
 char arr[ARR_SZ];
 
 SEC("socket")
@@ -318,9 +317,10 @@ int cond_break1(const void *ctx)
 	unsigned long i;
 	unsigned int sum = 0;
 
-	for (i = zero; i < ARR_SZ && can_loop; i++)
+	for (i = 0; i < ARR_SZ && can_loop; i++)
 		sum += i;
-	for (i = zero; i < ARR_SZ; i++) {
+
+	for (i = 0; i < ARR_SZ; i++) {
 		barrier_var(i);
 		sum += i + arr[i];
 		cond_break;
@@ -333,11 +333,11 @@ SEC("socket")
 __success __retval(999000000)
 int cond_break2(const void *ctx)
 {
-	int i, j;
+	long i, j;
 	int sum = 0;
 
-	for (i = zero; i < 1000 && can_loop; i++)
-		for (j = zero; j < 1000; j++) {
+	for (i = 0; i < 1000 && can_loop; i++)
+		for (j = 0; j < 1000; j++) {
 			sum += i + j;
 			cond_break;
 	}
@@ -346,9 +346,10 @@ int cond_break2(const void *ctx)
 
 static __noinline int loop(void)
 {
-	int i, sum = 0;
+	int sum = 0;
+	long i;
 
-	for (i = zero; i <= 1000000 && can_loop; i++)
+	for (i = 0; i <= 1000000 && can_loop; i++)
 		sum += i;
 
 	return sum;
@@ -365,7 +366,7 @@ SEC("socket")
 __success __retval(1)
 int cond_break4(const void *ctx)
 {
-	int cnt = zero;
+	int cnt = 0;
 
 	for (;;) {
 		/* should eventually break out of the loop */
@@ -378,7 +379,7 @@ int cond_break4(const void *ctx)
 
 static __noinline int static_subprog(void)
 {
-	int cnt = zero;
+	int cnt = 0;
 
 	for (;;) {
 		cond_break;
@@ -392,7 +393,7 @@ SEC("socket")
 __success __retval(1)
 int cond_break5(const void *ctx)
 {
-	int cnt1 = zero, cnt2;
+	int cnt1 = 0, cnt2;
 
 	for (;;) {
 		cond_break;
diff --git a/tools/testing/selftests/bpf/progs/verifier_reg_equal.c b/tools/testing/selftests/bpf/progs/verifier_reg_equal.c
index dc1d8c30fb0e..cc1e7e372daf 100644
--- a/tools/testing/selftests/bpf/progs/verifier_reg_equal.c
+++ b/tools/testing/selftests/bpf/progs/verifier_reg_equal.c
@@ -55,4 +55,33 @@ l1_%=:	exit;						\
 	: __clobber_all);
 }
 
+/*
+ * The tests checks that the verifier doesn't WARN_ON in:
+ * if (dst_reg->type == SCALAR_VALUE && dst_reg->id &&
+ *     !WARN_ON_ONCE(dst_reg->id != other_dst_reg->id)) {
+ */
+SEC("socket")
+__description("check this_branch_reg->id == other_branch_reg->id")
+__success
+__naked void reg_id(void)
+{
+	asm volatile ("					\
+	call %[bpf_ktime_get_ns];			\
+	.byte 0xe5; /* may_goto */			\
+	.byte 0; /* regs */				\
+	.short 5; /* off of l0_%=: */			\
+	.long 0; /* imm */				\
+	r0 &= 1;					\
+	r2 = r0;					\
+	/* is_branch_taken will predict fallthrough */	\
+	if r2 == 2 goto l0_%=;				\
+	r0 = 0;						\
+	exit;						\
+l0_%=:	r0 = 0;						\
+	exit;						\
+"	:
+	: __imm(bpf_ktime_get_ns)
+	: __clobber_all);
+}
+
 char _license[] SEC("license") = "GPL";
-- 
2.43.0

Re: [PATCH v5 bpf-next 2/3] bpf: Relax precision marking in open coded iters and may_goto loop.

[Eduard Zingerman]

On Wed, 2024-06-05 at 17:54 -0700, Alexei Starovoitov wrote:

[...]

This looks interesting, need a bit more time to think about it.
A few minor notes below.

[...]

> @@ -14704,6 +14705,165 @@ static u8 rev_opcode(u8 opcode)
>  	}
>  }
>  
> +/* Similar to mark_reg_unknown() and should only be called from cap_bpf path */
> +static void mark_unknown(struct bpf_reg_state *reg)
> +{
> +	u32 id = reg->id;
> +
> +	__mark_reg_unknown_imprecise(reg);
> +	reg->id = id;
> +}
> +/*
> + * Similar to regs_refine_cond_op(), but instead of tightening the range
> + * widen the upper bound of reg1 based on reg2 and
> + * lower bound of reg2 based on reg1.
> + */
> +static void widen_reg_bounds(struct bpf_reg_state *reg1,
> +			     struct bpf_reg_state *reg2,
> +			     u8 opcode, bool is_jmp32)
> +{
> +	switch (opcode) {
> +	case BPF_JGE:
> +	case BPF_JGT:
> +	case BPF_JSGE:
> +	case BPF_JSGT:
> +		opcode = flip_opcode(opcode);
> +		swap(reg1, reg2);
> +		break;
> +	default:
> +		break;
> +	}
> +
> +	switch (opcode) {
> +	case BPF_JLE:
> +		if (is_jmp32) {
> +			reg1->u32_max_value = reg2->u32_max_value;
> +			reg1->s32_max_value = S32_MAX;
> +			reg1->umax_value = U64_MAX;
> +			reg1->smax_value = S64_MAX;
> +
> +			reg2->u32_min_value = reg1->u32_min_value;
> +			reg2->s32_min_value = S32_MIN;
> +			reg2->umin_value = 0;
> +			reg2->smin_value = S64_MIN;
> +		} else {
> +			reg1->umax_value = reg2->umax_value;
> +			reg1->smax_value = S64_MAX;
> +			reg1->u32_max_value = U32_MAX;
> +			reg1->s32_max_value = S32_MAX;
> +
> +			reg2->umin_value = reg1->umin_value;
> +			reg2->smin_value = S64_MIN;
> +			reg2->u32_min_value = U32_MIN;
> +			reg2->s32_min_value = S32_MIN;
> +		}
> +		reg1->var_off = tnum_unknown;
> +		reg2->var_off = tnum_unknown;
> +		break;

Just a random thought: suppose that one of the registers in question
is used as an index int the array of ints, and compiler increments it
using += 4. Would it be interesting to preserve alignment info in the
var_off in such case? (in other words, preserve known trailing zeros).

[...]

> @@ -15177,8 +15339,78 @@ 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 ||
> +	    /* Widen scalars only if they're constants */
> +	    !is_reg_const(dst_reg, is_jmp32) || !is_reg_const(src_reg, is_jmp32))
> +		do_widen = false;
> +	else if (reg_const_value(dst_reg, is_jmp32) == reg_const_value(src_reg, is_jmp32))
> +		/* And not equal */
> +		do_widen = false;
> +	else
> +		do_widen = (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 (do_widen && ((opcode == BPF_JNE && pred == 1) ||
> +			 (opcode == BPF_JEQ && pred == 0))) {
> +		/*
> +		 * != is too vague. let's try < and > and widen. Example:
> +		 *
> +		 * R6=2
> +		 * 21: (15) if r6 == 0x3e8 goto pc+14
> +		 * Predicted == not-taken, but < is also true
> +		 * 21: R6=scalar(smin=umin=smin32=umin32=2,smax=umax=smax32=umax32=999,var_off=(0x0; 0x3ff))
> +		 */
> +		int refine_pred;
> +		u8 opcode2 = BPF_JLT;
> +
> +		refine_pred = is_branch_taken(dst_reg, src_reg, BPF_JLT, is_jmp32);
> +		if (refine_pred == 1) {
> +			widen_reg(env, dst_reg, src_reg, BPF_JLT, is_jmp32, true);
> +
> +		} else {

nit: would it be possible to avoid second call to is_branch_taken()
     by checking that refine_pred == 0 and assuming opcode2 == BPF_JGE?

> +			opcode2 = BPF_JGT;
> +			refine_pred = is_branch_taken(dst_reg, src_reg, BPF_JGT, is_jmp32);
> +			if (refine_pred == 1)
> +				widen_reg(env, dst_reg, src_reg, BPF_JGT, is_jmp32, true);
> +		}
> +
> +		if (refine_pred == 1) {
> +			if (dst_reg->id)
> +				find_equal_scalars(this_branch, dst_reg);

I think it is necessary to do find_equal_scalars() for src_reg as well,
since widen_reg() could change both registers.

> +			if (env->log.level & BPF_LOG_LEVEL) {
> +				verbose(env, "Predicted %s, but %s is also true\n",
> +					opcode == BPF_JNE ? "!= taken" : "== not-taken",
> +					opcode2 == BPF_JLT ? "<" : ">");
> +				print_insn_state(env, this_branch->frame[this_branch->curframe]);
> +			}
> +			err = mark_chain_precision(env, insn->dst_reg);
> +			if (err)
> +				return err;
> +			if (has_src_reg) {
> +				err = mark_chain_precision(env, insn->src_reg);
> +				if (err)
> +					return err;
> +			}
> +			if (pred == 1)
> +				*insn_idx += insn->off;
> +			return 0;
> +		}
> +		/*
> +		 * No luck. Predicted dst != src taken or dst == src not-taken,
> +		 * but !(dst < src) and !(dst > src).
> +		 * Constants must have been negative.
> +		 */
> +	}
> +
> +	if (do_widen && (opcode == BPF_JNE || opcode == BPF_JEQ || opcode == BPF_JSET))
> +		/* widen_reg() algorithm works for <, <=, >, >= only */
> +		do_widen = false;
> +
> +	if (pred >= 0 && !do_widen) {
>  		/* If we get here with a dst_reg pointer type it is because
>  		 * above is_branch_taken() special cased the 0 comparison.
>  		 */

[...]

Re: [PATCH v5 bpf-next 2/3] bpf: Relax precision marking in open coded iters and may_goto loop.

[Alexei Starovoitov]

[-- Attachment #1: Type: text/plain, Size: 6699 bytes --]

On Thu, Jun 6, 2024 at 4:08 AM Eduard Zingerman <eddyz87@gmail.com> wrote:
>
> On Wed, 2024-06-05 at 17:54 -0700, Alexei Starovoitov wrote:
>
> [...]
>
> This looks interesting, need a bit more time to think about it.
> A few minor notes below.
>
> [...]
>
> > @@ -14704,6 +14705,165 @@ static u8 rev_opcode(u8 opcode)
> >       }
> >  }
> >
> > +/* Similar to mark_reg_unknown() and should only be called from cap_bpf path */
> > +static void mark_unknown(struct bpf_reg_state *reg)
> > +{
> > +     u32 id = reg->id;
> > +
> > +     __mark_reg_unknown_imprecise(reg);
> > +     reg->id = id;
> > +}
> > +/*
> > + * Similar to regs_refine_cond_op(), but instead of tightening the range
> > + * widen the upper bound of reg1 based on reg2 and
> > + * lower bound of reg2 based on reg1.
> > + */
> > +static void widen_reg_bounds(struct bpf_reg_state *reg1,
> > +                          struct bpf_reg_state *reg2,
> > +                          u8 opcode, bool is_jmp32)
> > +{
> > +     switch (opcode) {
> > +     case BPF_JGE:
> > +     case BPF_JGT:
> > +     case BPF_JSGE:
> > +     case BPF_JSGT:
> > +             opcode = flip_opcode(opcode);
> > +             swap(reg1, reg2);
> > +             break;
> > +     default:
> > +             break;
> > +     }
> > +
> > +     switch (opcode) {
> > +     case BPF_JLE:
> > +             if (is_jmp32) {
> > +                     reg1->u32_max_value = reg2->u32_max_value;
> > +                     reg1->s32_max_value = S32_MAX;
> > +                     reg1->umax_value = U64_MAX;
> > +                     reg1->smax_value = S64_MAX;
> > +
> > +                     reg2->u32_min_value = reg1->u32_min_value;
> > +                     reg2->s32_min_value = S32_MIN;
> > +                     reg2->umin_value = 0;
> > +                     reg2->smin_value = S64_MIN;
> > +             } else {
> > +                     reg1->umax_value = reg2->umax_value;
> > +                     reg1->smax_value = S64_MAX;
> > +                     reg1->u32_max_value = U32_MAX;
> > +                     reg1->s32_max_value = S32_MAX;
> > +
> > +                     reg2->umin_value = reg1->umin_value;
> > +                     reg2->smin_value = S64_MIN;
> > +                     reg2->u32_min_value = U32_MIN;
> > +                     reg2->s32_min_value = S32_MIN;
> > +             }
> > +             reg1->var_off = tnum_unknown;
> > +             reg2->var_off = tnum_unknown;
> > +             break;
>
> Just a random thought: suppose that one of the registers in question
> is used as an index int the array of ints, and compiler increments it
> using += 4. Would it be interesting to preserve alignment info in the
> var_off in such case? (in other words, preserve known trailing zeros).

Well, the verifier cannot figure out which register is
an induction variable. Compiler can generate a code where
would be multiple such registers too.
But even if it was one rX += 4
it's nor clear how to figure out the size of the increment.

Also the above code is called at the time of comparison like "if 2 < 100".
I figured I will try a heuristic at that time.
See attached diff.
It computes alignment of LHS and RHS and
then heuristically adjusts the range.
After spending all morning on it and various heuristics
I'm convinced that this is a dead end.
It cannot be made to work with i += 2 loops.

> [...]
>
> > @@ -15177,8 +15339,78 @@ 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 ||
> > +         /* Widen scalars only if they're constants */
> > +         !is_reg_const(dst_reg, is_jmp32) || !is_reg_const(src_reg, is_jmp32))
> > +             do_widen = false;
> > +     else if (reg_const_value(dst_reg, is_jmp32) == reg_const_value(src_reg, is_jmp32))
> > +             /* And not equal */
> > +             do_widen = false;
> > +     else
> > +             do_widen = (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 (do_widen && ((opcode == BPF_JNE && pred == 1) ||
> > +                      (opcode == BPF_JEQ && pred == 0))) {
> > +             /*
> > +              * != is too vague. let's try < and > and widen. Example:
> > +              *
> > +              * R6=2
> > +              * 21: (15) if r6 == 0x3e8 goto pc+14
> > +              * Predicted == not-taken, but < is also true
> > +              * 21: R6=scalar(smin=umin=smin32=umin32=2,smax=umax=smax32=umax32=999,var_off=(0x0; 0x3ff))
> > +              */
> > +             int refine_pred;
> > +             u8 opcode2 = BPF_JLT;
> > +
> > +             refine_pred = is_branch_taken(dst_reg, src_reg, BPF_JLT, is_jmp32);
> > +             if (refine_pred == 1) {
> > +                     widen_reg(env, dst_reg, src_reg, BPF_JLT, is_jmp32, true);
> > +
> > +             } else {
>
> nit: would it be possible to avoid second call to is_branch_taken()
>      by checking that refine_pred == 0 and assuming opcode2 == BPF_JGE?

I considered it, but it's too fragile, since it will depend
on both sides being constant (thought the algorithm currently requires it)
and on both sides not being equal (though the algorithm also
checks that beforehand).

Much easier to reason about and experiment with the algorithm
when < and > are checked explicitly.
So I prefer to keep it this way.

> > +                     opcode2 = BPF_JGT;
> > +                     refine_pred = is_branch_taken(dst_reg, src_reg, BPF_JGT, is_jmp32);
> > +                     if (refine_pred == 1)
> > +                             widen_reg(env, dst_reg, src_reg, BPF_JGT, is_jmp32, true);
> > +             }
> > +
> > +             if (refine_pred == 1) {
> > +                     if (dst_reg->id)
> > +                             find_equal_scalars(this_branch, dst_reg);
>
> I think it is necessary to do find_equal_scalars() for src_reg as well,
> since widen_reg() could change both registers.

Kinda.
Doing find_equal_scalars(dst_reg) makes zero difference in tests.
I've added it for completeness,
but decided to avoid spending extra cycles on src_reg,
since most of the time it's BPF_K anyway.
We can add it later when there is clear evidence that it helps.

[-- Attachment #2: mask.diff --]
[-- Type: application/octet-stream, Size: 3701 bytes --]

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 79e356ac02ab..d6f64e24b015 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -14722,6 +14722,9 @@ static void widen_reg_bounds(struct bpf_reg_state *reg1,
 			     struct bpf_reg_state *reg2,
 			     u8 opcode, bool is_jmp32)
 {
+	int bit1 = ffs(reg_const_value(reg1, is_jmp32));
+	int bit2 = ffs(reg_const_value(reg2, is_jmp32));
+
 	switch (opcode) {
 	case BPF_JGE:
 	case BPF_JGT:
@@ -14838,6 +14841,67 @@ static void widen_reg_bounds(struct bpf_reg_state *reg1,
 	default:
 		break;
 	}
+
+	if (1) {
+		int bit = 64;
+
+		if (reg1->umax_value != U64_MAX) {
+			bit = min(bit, ffs(reg1->umax_value));
+			reg1->umax_value &= reg1->umax_value - 1;
+		}
+		if (reg1->u32_max_value != U32_MAX) {
+			bit = min(bit, ffs(reg1->u32_max_value));
+			reg1->u32_max_value &= reg1->u32_max_value - 1;
+		}
+		if (reg1->smax_value != S64_MAX) {
+			bit = min(bit, ffs(reg1->smax_value));
+			reg1->smax_value &= reg1->smax_value - 1;
+		}
+		if (reg1->s32_max_value != S32_MAX) {
+			bit = min(bit, ffs(reg1->s32_max_value));
+			reg1->s32_max_value &= reg1->s32_max_value - 1;
+		}
+
+		reg1->var_off = tnum_lshift(reg1->var_off, max(bit1, bit));
+		if (reg1->umax_value != U64_MAX)
+			reg1->umax_value &= reg1->var_off.mask;
+		if (reg1->u32_max_value != U32_MAX)
+			reg1->u32_max_value &= reg1->var_off.mask;
+		if (reg1->smax_value != S64_MAX)
+			reg1->smax_value &= reg1->var_off.mask;
+		if (reg1->s32_max_value != S32_MAX)
+			reg1->s32_max_value &= reg1->var_off.mask;
+	}
+	if (1) {
+		int bit = 64;
+
+		if (reg2->umin_value != U64_MAX) {
+			bit = min(bit, ffs(reg2->umin_value));
+			reg2->umin_value &= reg2->umin_value - 1;
+		}
+		if (reg2->u32_min_value != U32_MAX) {
+			bit = min(bit, ffs(reg2->u32_min_value));
+			reg2->u32_min_value &= reg2->u32_min_value - 1;
+		}
+		if (reg2->smin_value != S64_MAX) {
+			bit = min(bit, ffs(reg2->smin_value));
+			reg2->smin_value &= reg2->smin_value - 1;
+		}
+		if (reg2->s32_min_value != S32_MAX) {
+			bit = min(bit, ffs(reg2->s32_min_value));
+			reg2->s32_min_value &= reg2->s32_min_value - 1;
+		}
+
+		reg2->var_off = tnum_lshift(reg2->var_off, max(bit2, bit));
+		if (reg2->umin_value != U64_MAX)
+			reg2->umin_value &= reg2->var_off.mask;
+		if (reg2->u32_min_value != U32_MAX)
+			reg2->u32_min_value &= reg2->var_off.mask;
+		if (reg2->smin_value != S64_MAX)
+			reg2->smin_value &= reg2->var_off.mask;
+		if (reg2->s32_min_value != S32_MAX)
+			reg2->s32_min_value &= reg2->var_off.mask;
+	}
 }
 
 /*
diff --git a/tools/testing/selftests/bpf/progs/iters_task_vma.c b/tools/testing/selftests/bpf/progs/iters_task_vma.c
index 31c4e7d9eaa3..e49a0e59435a 100644
--- a/tools/testing/selftests/bpf/progs/iters_task_vma.c
+++ b/tools/testing/selftests/bpf/progs/iters_task_vma.c
@@ -88,7 +88,7 @@ int loop_inside_iter(const void *ctx)
 	bpf_iter_num_new(&it, 0, ARR_SZ);
 	while ((v = bpf_iter_num_next(&it))) {
 		if (i < ARR_SZ)
-			sum += arr[i++];
+			sum += arr[i += 4];
 	}
 	bpf_iter_num_destroy(&it);
 	return sum;
@@ -158,7 +158,7 @@ int touch_arr_global(__u32 i)
 
 SEC("socket")
 __success
-int loop_inside_iter_subprog(const void *ctx)
+int may_goto_subprog(const void *ctx)
 {
 	long i;
 
@@ -170,4 +170,26 @@ int loop_inside_iter_subprog(const void *ctx)
 
 	return 0;
 }
+
+__noinline
+static void touch_arr_multi(int i)
+{
+	if (i >= ARR_SZ - 4)
+		return;
+	arr[i] = i;
+	arr[i + 1] = i;
+	arr[i + 2] = i;
+	arr[i + 3] = i;
+}
+
+SEC("socket")
+__success
+int may_goto_subprog_mask(const void *ctx)
+{
+	long i;
+
+	for (i = 0; i <= 1000000 && can_loop; i += 4)
+		touch_arr_multi(i);
+	return 0;
+}
 char _license[] SEC("license") = "GPL";

Re: [PATCH v5 bpf-next 2/3] bpf: Relax precision marking in open coded iters and may_goto loop.

[Eduard Zingerman]

On Thu, 2024-06-06 at 13:02 -0700, Alexei Starovoitov wrote:

[...]

> > > +             reg1->var_off = tnum_unknown;
> > > +             reg2->var_off = tnum_unknown;
> > > +             break;
> > 
> > Just a random thought: suppose that one of the registers in question
> > is used as an index int the array of ints, and compiler increments it
> > using += 4. Would it be interesting to preserve alignment info in the
> > var_off in such case? (in other words, preserve known trailing zeros).
> 
> Well, the verifier cannot figure out which register is
> an induction variable. Compiler can generate a code where
> would be multiple such registers too.
> But even if it was one rX += 4
> it's nor clear how to figure out the size of the increment.
> 
> Also the above code is called at the time of comparison like "if 2 < 100".
> I figured I will try a heuristic at that time.
> See attached diff.
> It computes alignment of LHS and RHS and
> then heuristically adjusts the range.
> After spending all morning on it and various heuristics
> I'm convinced that this is a dead end.
> It cannot be made to work with i += 2 loops.

Summary of off-list discussion below.
For the following C code:

    long arr1[1024];

    SEC("socket")
    __success
    int test1(const void *ctx)
    {
        long i;
    
        for (i = 0; i < 1024 && can_loop; i++)
                arr1[i] = i;
        return 0;
    }
    
clang generates the following BPF code:

0000000000000340 <test1>:
     104:       r1 = 0x0
     105:       r2 = 0x0 ll

0000000000000358 <LBB28_1>:
     107:       may_goto +0x4 <LBB28_3>
     108:       *(u64 *)(r2 + 0x0) = r1
     109:       r2 += 0x8
     110:       r1 += 0x1
     111:       if r1 != 0x400 goto -0x5 <LBB28_1>

0000000000000380 <LBB28_3>:
     112:       w0 = 0x0
     113:       exit

Here r2 is used as an index and r1 as a counter.
Since r2 is never compared it is never widened.
Hence my point about preserving trailing zeros
for widened values is moot, as it won't really
help for real-life programs.

Re: [PATCH v5 bpf-next 2/3] bpf: Relax precision marking in open coded iters and may_goto loop.

[Eduard Zingerman]

On Thu, 2024-06-06 at 15:19 -0700, Eduard Zingerman wrote:

[...]

> For the following C code:
> 
>     long arr1[1024];
> 
>     SEC("socket")
>     __success
>     int test1(const void *ctx)
>     {
>         long i;
>     
>         for (i = 0; i < 1024 && can_loop; i++)
>                 arr1[i] = i;
>         return 0;
>     }
>     
> clang generates the following BPF code:
> 
> 0000000000000340 <test1>:
>      104:       r1 = 0x0
>      105:       r2 = 0x0 ll
> 
> 0000000000000358 <LBB28_1>:
>      107:       may_goto +0x4 <LBB28_3>
>      108:       *(u64 *)(r2 + 0x0) = r1
>      109:       r2 += 0x8
>      110:       r1 += 0x1
>      111:       if r1 != 0x400 goto -0x5 <LBB28_1>
> 
> 0000000000000380 <LBB28_3>:
>      112:       w0 = 0x0
>      113:       exit

[...]

I've also took a look why the same program could be verified w/o
can_loop, but fails with can_loop (with this series applied):

0000000000000358 <LBB28_1>:
     107:       may_goto +0x4 <LBB28_3>
     108:       *(u64 *)(r2 + 0x0) = r1
     109:       r2 += 0x8
     110:       r1 += 0x1
     111:       if r1 != 0x400 goto -0x5 <LBB28_1>
                   ^^
    r1 is no longer marked precise,
    thus maybe_widen_reg() forgets the range for it
    when widen_imprecise_scalars() is called from
    may_goto processing logic.
    
As a result, verifier enumerates states
{r2=0P,r1=scalar()}, {r2=8P,r1=scalar()}, {r2=16P,r1=scalar()}, ...
eventually hitting the value of r2 that does not fit in 'arr1' bounds.

Re: [PATCH v5 bpf-next 2/3] bpf: Relax precision marking in open coded iters and may_goto loop.

[Alexei Starovoitov]

On Thu, Jun 6, 2024 at 4:39 PM Eduard Zingerman <eddyz87@gmail.com> wrote:
>
> On Thu, 2024-06-06 at 15:19 -0700, Eduard Zingerman wrote:
>
> [...]
>
> > For the following C code:
> >
> >     long arr1[1024];
> >
> >     SEC("socket")
> >     __success
> >     int test1(const void *ctx)
> >     {
> >         long i;
> >
> >         for (i = 0; i < 1024 && can_loop; i++)
> >                 arr1[i] = i;
> >         return 0;
> >     }
> >
> > clang generates the following BPF code:
> >
> > 0000000000000340 <test1>:
> >      104:       r1 = 0x0
> >      105:       r2 = 0x0 ll
> >
> > 0000000000000358 <LBB28_1>:
> >      107:       may_goto +0x4 <LBB28_3>
> >      108:       *(u64 *)(r2 + 0x0) = r1
> >      109:       r2 += 0x8
> >      110:       r1 += 0x1
> >      111:       if r1 != 0x400 goto -0x5 <LBB28_1>
> >
> > 0000000000000380 <LBB28_3>:
> >      112:       w0 = 0x0
> >      113:       exit
>
> [...]
>
> I've also took a look why the same program could be verified w/o
> can_loop, but fails with can_loop (with this series applied):
>
> 0000000000000358 <LBB28_1>:
>      107:       may_goto +0x4 <LBB28_3>
>      108:       *(u64 *)(r2 + 0x0) = r1
>      109:       r2 += 0x8
>      110:       r1 += 0x1
>      111:       if r1 != 0x400 goto -0x5 <LBB28_1>
>                    ^^
>     r1 is no longer marked precise,
>     thus maybe_widen_reg() forgets the range for it
>     when widen_imprecise_scalars() is called from
>     may_goto processing logic.
>
> As a result, verifier enumerates states
> {r2=0P,r1=scalar()}, {r2=8P,r1=scalar()}, {r2=16P,r1=scalar()}, ...
> eventually hitting the value of r2 that does not fit in 'arr1' bounds.

That's correct.
As I was arguing couple emails ago the existing
maybe_widen_reg() logic is just as damaging heuristic
as this new widen_reg() logic.

With this hack:
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 79e356ac02ab..23892759f05e 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -7928,7 +7928,7 @@ static void maybe_widen_reg(struct bpf_verifier_env *env,
                return;
        if (rold->precise || rcur->precise || regs_exact(rold, rcur, idmap))
                return;
-       __mark_reg_unknown(env, rcur);
+//     __mark_reg_unknown(env, rcur);
 }

the above test is passing.
Though widen_reg() widened the range the bounded loop logic still works.
The loop now looks like
r1=[1,999] r2=8
r1=[2,999] r2=16
..
and eventually the verifier goes to check all 1k iterations of the loop
and passes.

Should I disable old maybe_widen_reg() when new widen_reg() kicks in? ;)

My point is that we had good and bad heuristics. The new one
helps arena. Is it perfect? No. But based on this particular
test it's arguable which one is worse.

Anyway, I think I'm going to tighten it a bit more.

pw-bot: cr