[PATCH bpf-next v4 0/2] bpf: Skip bounds adjustment for conditional jumps on same scalar register

[PATCH bpf-next v4 0/2] bpf: Skip bounds adjustment for conditional jumps on same scalar register

[KaFai Wan]

This small patchset is about avoid verifier bug warning when conditional
 jumps on same register when the register holds a scalar with range.

v4:
  - make code better. (Alexei) 

v3:
  https://lore.kernel.org/bpf/20251031154107.403054-1-kafai.wan@linux.dev/
  - Enhance is_scalar_branch_taken() to handle scalar case. (Eduard)
  - Update the selftest to cover all conditional jump opcodes. (Eduard)

v2:
  https://lore.kernel.org/bpf/20251025053017.2308823-1-kafai.wan@linux.dev/
 - Enhance is_branch_taken() and is_scalar_branch_taken() to handle
   branch direction computation for same register. (Eduard and Alexei)
 - Update the selftest.
 

v1:
  https://lore.kernel.org/bpf/20251022164457.1203756-1-kafai.wan@linux.dev/
---
KaFai Wan (2):
  bpf: Skip bounds adjustment for conditional jumps on same scalar
    register
  selftests/bpf: Add test for conditional jumps on same scalar register

 kernel/bpf/verifier.c                         |  31 ++++
 .../selftests/bpf/progs/verifier_bounds.c     | 154 ++++++++++++++++++
 2 files changed, 185 insertions(+)

-- 
2.43.0

[PATCH bpf-next v4 1/2] bpf: Skip bounds adjustment for conditional jumps on same scalar register

[KaFai Wan]

When conditional jumps are performed on the same scalar register
(e.g., r0 <= r0, r0 > r0, r0 < r0), the BPF verifier incorrectly
attempts to adjust the register's min/max bounds. This leads to
invalid range bounds and triggers a BUG warning.

The problematic BPF program:
   0: call bpf_get_prandom_u32
   1: w8 = 0x80000000
   2: r0 &= r8
   3: if r0 > r0 goto <exit>

The instruction 3 triggers kernel warning:
   3: if r0 > r0 goto <exit>
   true_reg1: range bounds violation u64=[0x1, 0x0] s64=[0x1, 0x0] u32=[0x1, 0x0] s32=[0x1, 0x0] var_off=(0x0, 0x0)
   true_reg2: const tnum out of sync with range bounds u64=[0x0, 0xffffffffffffffff] s64=[0x8000000000000000, 0x7fffffffffffffff] var_off=(0x0, 0x0)

Comparing a register with itself should not change its bounds and
for most comparison operations, comparing a register with itself has
a known result (e.g., r0 == r0 is always true, r0 < r0 is always false).

Fix this by:
1. Enhance is_scalar_branch_taken() to properly handle branch direction
   computation for same register comparisons across all BPF jump operations
2. Adds early return in reg_set_min_max() to avoid bounds adjustment
   for unknown branch directions (e.g., BPF_JSET) on the same register

The fix ensures that unnecessary bounds adjustments are skipped, preventing
the verifier bug while maintaining correct branch direction analysis.

Reported-by: Kaiyan Mei <M202472210@hust.edu.cn>
Reported-by: Yinhao Hu <dddddd@hust.edu.cn>
Closes: https://lore.kernel.org/all/1881f0f5.300df.199f2576a01.Coremail.kaiyanm@hust.edu.cn/
Signed-off-by: KaFai Wan <kafai.wan@linux.dev>
---
 kernel/bpf/verifier.c | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 542e23fb19c7..e4928846e763 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -15993,6 +15993,30 @@ static int is_scalar_branch_taken(struct bpf_reg_state *reg1, struct bpf_reg_sta
 	s64 smin2 = is_jmp32 ? (s64)reg2->s32_min_value : reg2->smin_value;
 	s64 smax2 = is_jmp32 ? (s64)reg2->s32_max_value : reg2->smax_value;
 
+	if (reg1 == reg2) {
+		switch (opcode) {
+		case BPF_JGE:
+		case BPF_JLE:
+		case BPF_JSGE:
+		case BPF_JSLE:
+		case BPF_JEQ:
+			return 1;
+		case BPF_JGT:
+		case BPF_JLT:
+		case BPF_JSGT:
+		case BPF_JSLT:
+		case BPF_JNE:
+			return 0;
+		case BPF_JSET:
+			if (tnum_is_const(t1))
+				return t1.value != 0;
+			else
+				return (smin1 <= 0 && smax1 >= 0) ? -1 : 1;
+		default:
+			return -1;
+		}
+	}
+
 	switch (opcode) {
 	case BPF_JEQ:
 		/* constants, umin/umax and smin/smax checks would be
@@ -16439,6 +16463,13 @@ static int reg_set_min_max(struct bpf_verifier_env *env,
 	if (false_reg1->type != SCALAR_VALUE || false_reg2->type != SCALAR_VALUE)
 		return 0;
 
+	/* We compute branch direction for same SCALAR_VALUE registers in
+	 * is_scalar_branch_taken(). For unknown branch directions (e.g., BPF_JSET)
+	 * on the same registers, we don't need to adjust the min/max values.
+	 */
+	if (false_reg1 == false_reg2)
+		return 0;
+
 	/* fallthrough (FALSE) branch */
 	regs_refine_cond_op(false_reg1, false_reg2, rev_opcode(opcode), is_jmp32);
 	reg_bounds_sync(false_reg1);
-- 
2.43.0

Re: [PATCH bpf-next v4 1/2] bpf: Skip bounds adjustment for conditional jumps on same scalar register

[Eduard Zingerman]

On Mon, 2025-11-03 at 14:31 +0800, KaFai Wan wrote:
> When conditional jumps are performed on the same scalar register
> (e.g., r0 <= r0, r0 > r0, r0 < r0), the BPF verifier incorrectly
> attempts to adjust the register's min/max bounds. This leads to
> invalid range bounds and triggers a BUG warning.
> 
> The problematic BPF program:
>    0: call bpf_get_prandom_u32
>    1: w8 = 0x80000000
>    2: r0 &= r8
>    3: if r0 > r0 goto <exit>
> 
> The instruction 3 triggers kernel warning:
>    3: if r0 > r0 goto <exit>
>    true_reg1: range bounds violation u64=[0x1, 0x0] s64=[0x1, 0x0] u32=[0x1, 0x0] s32=[0x1, 0x0] var_off=(0x0, 0x0)
>    true_reg2: const tnum out of sync with range bounds u64=[0x0, 0xffffffffffffffff] s64=[0x8000000000000000, 0x7fffffffffffffff] var_off=(0x0, 0x0)
> 
> Comparing a register with itself should not change its bounds and
> for most comparison operations, comparing a register with itself has
> a known result (e.g., r0 == r0 is always true, r0 < r0 is always false).
> 
> Fix this by:
> 1. Enhance is_scalar_branch_taken() to properly handle branch direction
>    computation for same register comparisons across all BPF jump operations
> 2. Adds early return in reg_set_min_max() to avoid bounds adjustment
>    for unknown branch directions (e.g., BPF_JSET) on the same register
> 
> The fix ensures that unnecessary bounds adjustments are skipped, preventing
> the verifier bug while maintaining correct branch direction analysis.
> 
> Reported-by: Kaiyan Mei <M202472210@hust.edu.cn>
> Reported-by: Yinhao Hu <dddddd@hust.edu.cn>
> Closes: https://lore.kernel.org/all/1881f0f5.300df.199f2576a01.Coremail.kaiyanm@hust.edu.cn/
> Signed-off-by: KaFai Wan <kafai.wan@linux.dev>
> ---

Acked-by: Eduard Zingerman <eddyz87@gmail.com>

[...]

[PATCH bpf-next v4 2/2] selftests/bpf: Add test for conditional jumps on same scalar register

[KaFai Wan]

Add test cases to verify the correctness of the BPF verifier's branch analysis
when conditional jumps are performed on the same scalar register. And make sure
that JGT does not trigger verifier BUG.

Signed-off-by: KaFai Wan <kafai.wan@linux.dev>
---
 .../selftests/bpf/progs/verifier_bounds.c     | 154 ++++++++++++++++++
 1 file changed, 154 insertions(+)

diff --git a/tools/testing/selftests/bpf/progs/verifier_bounds.c b/tools/testing/selftests/bpf/progs/verifier_bounds.c
index 0a72e0228ea9..e975dc285db6 100644
--- a/tools/testing/selftests/bpf/progs/verifier_bounds.c
+++ b/tools/testing/selftests/bpf/progs/verifier_bounds.c
@@ -1709,4 +1709,158 @@ __naked void jeq_disagreeing_tnums(void *ctx)
 	: __clobber_all);
 }
 
+SEC("socket")
+__description("conditional jump on same register, branch taken")
+__not_msg("20: (b7) r0 = 1 {{.*}} R0=1")
+__success __log_level(2)
+__retval(0) __flag(BPF_F_TEST_REG_INVARIANTS)
+__naked void condition_jump_on_same_register(void *ctx)
+{
+	asm volatile("			\
+	call %[bpf_get_prandom_u32];	\
+	w8 = 0x80000000;		\
+	r0 &= r8;			\
+	if r0 == r0 goto +1;		\
+	goto l1_%=;			\
+	if r0 >= r0 goto +1;		\
+	goto l1_%=;			\
+	if r0 s>= r0 goto +1;		\
+	goto l1_%=;			\
+	if r0 <= r0 goto +1;		\
+	goto l1_%=;			\
+	if r0 s<= r0 goto +1;		\
+	goto l1_%=;			\
+	if r0 != r0 goto l1_%=;		\
+	if r0 >  r0 goto l1_%=;		\
+	if r0 s> r0 goto l1_%=;		\
+	if r0 <  r0 goto l1_%=;		\
+	if r0 s< r0 goto l1_%=;		\
+l0_%=:	r0 = 0;				\
+	exit;				\
+l1_%=:	r0 = 1;				\
+	exit;				\
+"	:
+	: __imm(bpf_get_prandom_u32)
+	: __clobber_all);
+}
+
+SEC("socket")
+__description("jset on same register, constant value branch taken")
+__not_msg("7: (b7) r0 = 1 {{.*}} R0=1")
+__success __log_level(2)
+__retval(0) __flag(BPF_F_TEST_REG_INVARIANTS)
+__naked void jset_on_same_register_1(void *ctx)
+{
+	asm volatile("			\
+	r0 = 0;				\
+	if r0 & r0 goto l1_%=;		\
+	r0 = 1;				\
+	if r0 & r0 goto +1;		\
+	goto l1_%=;			\
+l0_%=:	r0 = 0;				\
+	exit;				\
+l1_%=:	r0 = 1;				\
+	exit;				\
+"	:
+	: __imm(bpf_get_prandom_u32)
+	: __clobber_all);
+}
+
+SEC("socket")
+__description("jset on same register, scalar value branch taken")
+__not_msg("12: (b7) r0 = 1 {{.*}} R0=1")
+__success __log_level(2)
+__retval(0) __flag(BPF_F_TEST_REG_INVARIANTS)
+__naked void jset_on_same_register_2(void *ctx)
+{
+	asm volatile("			\
+	/* range [1;2] */		\
+	call %[bpf_get_prandom_u32];	\
+	r0 &= 0x1;			\
+	r0 += 1;			\
+	if r0 & r0 goto +1;		\
+	goto l1_%=;			\
+	/* range [-2;-1] */		\
+	call %[bpf_get_prandom_u32];	\
+	r0 &= 0x1;			\
+	r0 -= 2;			\
+	if r0 & r0 goto +1;		\
+	goto l1_%=;			\
+l0_%=:	r0 = 0;				\
+	exit;				\
+l1_%=:	r0 = 1;				\
+	exit;				\
+"	:
+	: __imm(bpf_get_prandom_u32)
+	: __clobber_all);
+}
+
+SEC("socket")
+__description("jset on same register, scalar value unknown branch 1")
+__msg("3: (b7) r0 = 0 {{.*}} R0=0")
+__msg("5: (b7) r0 = 1 {{.*}} R0=1")
+__success __log_level(2)
+__flag(BPF_F_TEST_REG_INVARIANTS)
+__naked void jset_on_same_register_3(void *ctx)
+{
+	asm volatile("			\
+	/* range [0;1] */		\
+	call %[bpf_get_prandom_u32];	\
+	r0 &= 0x1;			\
+	if r0 & r0 goto l1_%=;		\
+l0_%=:	r0 = 0;				\
+	exit;				\
+l1_%=:	r0 = 1;				\
+	exit;				\
+"	:
+	: __imm(bpf_get_prandom_u32)
+	: __clobber_all);
+}
+
+SEC("socket")
+__description("jset on same register, scalar value unknown branch 2")
+__msg("4: (b7) r0 = 0 {{.*}} R0=0")
+__msg("6: (b7) r0 = 1 {{.*}} R0=1")
+__success __log_level(2)
+__flag(BPF_F_TEST_REG_INVARIANTS)
+__naked void jset_on_same_register_4(void *ctx)
+{
+	asm volatile("			\
+	/* range [-1;0] */		\
+	call %[bpf_get_prandom_u32];	\
+	r0 &= 0x1;			\
+	r0 -= 1;			\
+	if r0 & r0 goto l1_%=;		\
+l0_%=:	r0 = 0;				\
+	exit;				\
+l1_%=:	r0 = 1;				\
+	exit;				\
+"	:
+	: __imm(bpf_get_prandom_u32)
+	: __clobber_all);
+}
+
+SEC("socket")
+__description("jset on same register, scalar value unknown branch 3")
+__msg("4: (b7) r0 = 0 {{.*}} R0=0")
+__msg("6: (b7) r0 = 1 {{.*}} R0=1")
+__success __log_level(2)
+__flag(BPF_F_TEST_REG_INVARIANTS)
+__naked void jset_on_same_register_5(void *ctx)
+{
+	asm volatile("			\
+	/* range [-1;-1] */		\
+	call %[bpf_get_prandom_u32];	\
+	r0 &= 0x2;			\
+	r0 -= 1;			\
+	if r0 & r0 goto l1_%=;		\
+l0_%=:	r0 = 0;				\
+	exit;				\
+l1_%=:	r0 = 1;				\
+	exit;				\
+"	:
+	: __imm(bpf_get_prandom_u32)
+	: __clobber_all);
+}
+
 char _license[] SEC("license") = "GPL";
-- 
2.43.0

Re: [PATCH bpf-next v4 2/2] selftests/bpf: Add test for conditional jumps on same scalar register

[Eduard Zingerman]

On Mon, 2025-11-03 at 14:31 +0800, KaFai Wan wrote:
> Add test cases to verify the correctness of the BPF verifier's branch analysis
> when conditional jumps are performed on the same scalar register. And make sure
> that JGT does not trigger verifier BUG.
> 
> Signed-off-by: KaFai Wan <kafai.wan@linux.dev>
> ---

Thank you for adding these.

Acked-by: Eduard Zingerman <eddyz87@gmail.com>

(but a comment needs a fix)

[...]

> +SEC("socket")
> +__description("jset on same register, scalar value unknown branch 3")
> +__msg("4: (b7) r0 = 0 {{.*}} R0=0")
> +__msg("6: (b7) r0 = 1 {{.*}} R0=1")
> +__success __log_level(2)
> +__flag(BPF_F_TEST_REG_INVARIANTS)
> +__naked void jset_on_same_register_5(void *ctx)
> +{
> +	asm volatile("			\
> +	/* range [-1;-1] */		\
                     ^^
   Typo, should be [-1;1].

> +	call %[bpf_get_prandom_u32];	\
> +	r0 &= 0x2;			\
> +	r0 -= 1;			\
> +	if r0 & r0 goto l1_%=;		\
> +l0_%=:	r0 = 0;				\
> +	exit;				\
> +l1_%=:	r0 = 1;				\
> +	exit;				\
> +"	:
> +	: __imm(bpf_get_prandom_u32)
> +	: __clobber_all);
> +}
> +
>  char _license[] SEC("license") = "GPL";

Re: [PATCH bpf-next v4 2/2] selftests/bpf: Add test for conditional jumps on same scalar register

[Alexei Starovoitov]

On Mon, Nov 3, 2025 at 10:14 AM Eduard Zingerman <eddyz87@gmail.com> wrote:
>
> On Mon, 2025-11-03 at 14:31 +0800, KaFai Wan wrote:
> > Add test cases to verify the correctness of the BPF verifier's branch analysis
> > when conditional jumps are performed on the same scalar register. And make sure
> > that JGT does not trigger verifier BUG.
> >
> > Signed-off-by: KaFai Wan <kafai.wan@linux.dev>
> > ---
>
> Thank you for adding these.
>
> Acked-by: Eduard Zingerman <eddyz87@gmail.com>
>
> (but a comment needs a fix)
>
> [...]
>
> > +SEC("socket")
> > +__description("jset on same register, scalar value unknown branch 3")
> > +__msg("4: (b7) r0 = 0 {{.*}} R0=0")
> > +__msg("6: (b7) r0 = 1 {{.*}} R0=1")
> > +__success __log_level(2)
> > +__flag(BPF_F_TEST_REG_INVARIANTS)
> > +__naked void jset_on_same_register_5(void *ctx)
> > +{
> > +     asm volatile("                  \
> > +     /* range [-1;-1] */             \
>                      ^^
>    Typo, should be [-1;1].

Eagle eye.

Fixed while applying.

Re: [PATCH bpf-next v4 0/2] bpf: Skip bounds adjustment for conditional jumps on same scalar register

[patchwork-bot+netdevbpf]

Hello:

This series was applied to bpf/bpf-next.git (master)
by Alexei Starovoitov <ast@kernel.org>:

On Mon,  3 Nov 2025 14:31:06 +0800 you wrote:
> This small patchset is about avoid verifier bug warning when conditional
>  jumps on same register when the register holds a scalar with range.
> 
> v4:
>   - make code better. (Alexei)
> 
> v3:
>   https://lore.kernel.org/bpf/20251031154107.403054-1-kafai.wan@linux.dev/
>   - Enhance is_scalar_branch_taken() to handle scalar case. (Eduard)
>   - Update the selftest to cover all conditional jump opcodes. (Eduard)
> 
> [...]

Here is the summary with links:
  - [bpf-next,v4,1/2] bpf: Skip bounds adjustment for conditional jumps on same scalar register
    https://git.kernel.org/bpf/bpf-next/c/d43ad9da8052
  - [bpf-next,v4,2/2] selftests/bpf: Add test for conditional jumps on same scalar register
    (no matching commit)

You are awesome, thank you!
-- 
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