[RFC PATCH 6.1.y 2/2] bpf: make the verifier tracks the "not equal" for regs
From: Zhenzhong Wu <hidden>
Date: 2026-06-01 18:07:49
Also in:
bpf, lkml
Subsystem:
bpf [core], bpf [general] (safe dynamic programs and tools), the rest · Maintainers:
Alexei Starovoitov, Daniel Borkmann, Andrii Nakryiko, Eduard Zingerman, Kumar Kartikeya Dwivedi, Linus Torvalds
[ Upstream commit d028f87517d6775dccff4ddbca2740826f9e53f1 ]
We can derive some new information for BPF_JNE in regs_refine_cond_op().
Take following code for example:
/* The type of "a" is u32 */
if (a > 0 && a < 100) {
/* the range of the register for a is [0, 99], not [1, 99],
* and will cause the following error:
*
* invalid zero-sized read
*
* as a can be 0.
*/
bpf_skb_store_bytes(skb, xx, xx, a, 0);
}
In the code above, "a > 0" will be compiled to "jmp xxx if a == 0". In the
TRUE branch, the dst_reg will be marked as known to 0. However, in the
fallthrough(FALSE) branch, the dst_reg will not be handled, which makes
the [min, max] for a is [0, 99], not [1, 99].
For BPF_JNE, we can reduce the range of the dst reg if the src reg is a
const and is exactly the edge of the dst reg.
Signed-off-by: Menglong Dong <redacted>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Shung-Hsi Yu <redacted>
Link: https://lore.kernel.org/r/20231219134800.1550388-2-menglong8.dong@gmail.com (local)
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
[ zhenzhong: adapt to 6.1.y reg_set_min_max() layout. The upstream
change lives in regs_refine_cond_op(); 6.1.y still refines true/false
branch states in reg_set_min_max(), so apply the not-equal range
exclusion to BPF_JEQ's false_reg and BPF_JNE's true_reg there. ]
Signed-off-by: Zhenzhong Wu <redacted>
---
kernel/bpf/verifier.c | 32 ++++++++++++++++++++++++++++++++
1 file changed, 32 insertions(+)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 5e029d1..e51f44b 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c@@ -9954,18 +9954,50 @@ static void reg_set_min_max(struct bpf_reg_state *true_reg, if (is_jmp32) { __mark_reg32_known(true_reg, val32); true_32off = tnum_subreg(true_reg->var_off); + if (false_reg->u32_min_value == val32) + false_reg->u32_min_value++; + if (false_reg->u32_max_value == val32) + false_reg->u32_max_value--; + if (false_reg->s32_min_value == sval32) + false_reg->s32_min_value++; + if (false_reg->s32_max_value == sval32) + false_reg->s32_max_value--; } else { ___mark_reg_known(true_reg, val); true_64off = true_reg->var_off; + if (false_reg->umin_value == val) + false_reg->umin_value++; + if (false_reg->umax_value == val) + false_reg->umax_value--; + if (false_reg->smin_value == sval) + false_reg->smin_value++; + if (false_reg->smax_value == sval) + false_reg->smax_value--; } break; case BPF_JNE: if (is_jmp32) { __mark_reg32_known(false_reg, val32); false_32off = tnum_subreg(false_reg->var_off); + if (true_reg->u32_min_value == val32) + true_reg->u32_min_value++; + if (true_reg->u32_max_value == val32) + true_reg->u32_max_value--; + if (true_reg->s32_min_value == sval32) + true_reg->s32_min_value++; + if (true_reg->s32_max_value == sval32) + true_reg->s32_max_value--; } else { ___mark_reg_known(false_reg, val); false_64off = false_reg->var_off; + if (true_reg->umin_value == val) + true_reg->umin_value++; + if (true_reg->umax_value == val) + true_reg->umax_value--; + if (true_reg->smin_value == sval) + true_reg->smin_value++; + if (true_reg->smax_value == sval) + true_reg->smax_value--; } break; case BPF_JSET:
--
2.43.0