Merge branch 'bpf-recognize-special-arithmetic-shift-in-the-verifier'

Puranjay Mohan says:

====================
bpf: Recognize special arithmetic shift in the verifier

v3: https://lore.kernel.org/all/20260103022310.935686-1-puranjay@kernel.org/
Changes in v3->v4:
- Fork verifier state while processing BPF_OR when src_reg has [-1,0]
  range and 2nd operand is a constant. This is to detect the following pattern:
	i32 X > -1 ? C1 : -1 --> (X >>s 31) | C1
- Add selftests for above.
- Remove __description("s>>=63") (Eduard in another patchset)

v2: https://lore.kernel.org/bpf/20251115022611.64898-1-alexei.starovoitov@gmail.com/
Changes in v2->v3:
- fork verifier state while processing BPF_AND when src_reg has [-1,0]
  range and 2nd operand is a constant.

v1->v2:
Use __mark_reg32_known() or __mark_reg_known() for zero too.
Add comment to selftest.

v1:
https://lore.kernel.org/bpf/20251114031039.63852-1-alexei.starovoitov@gmail.com/
====================

Link: https://patch.msgid.link/20260112201424.816836-1-puranjay@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 39 insertions, 0 deletions

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 02a43cafbb25..9d3ad2876d8f 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -15491,6 +15491,35 @@ static bool is_safe_to_compute_dst_reg_range(struct bpf_insn *insn,
 	}
 }
 
+static int maybe_fork_scalars(struct bpf_verifier_env *env, struct bpf_insn *insn,
+			      struct bpf_reg_state *dst_reg)
+{
+	struct bpf_verifier_state *branch;
+	struct bpf_reg_state *regs;
+	bool alu32;
+
+	if (dst_reg->smin_value == -1 && dst_reg->smax_value == 0)
+		alu32 = false;
+	else if (dst_reg->s32_min_value == -1 && dst_reg->s32_max_value == 0)
+		alu32 = true;
+	else
+		return 0;
+
+	branch = push_stack(env, env->insn_idx + 1, env->insn_idx, false);
+	if (IS_ERR(branch))
+		return PTR_ERR(branch);
+
+	regs = branch->frame[branch->curframe]->regs;
+	if (alu32) {
+		__mark_reg32_known(&regs[insn->dst_reg], 0);
+		__mark_reg32_known(dst_reg, -1ull);
+	} else {
+		__mark_reg_known(&regs[insn->dst_reg], 0);
+		__mark_reg_known(dst_reg, -1ull);
+	}
+	return 0;
+}
+
 /* WARNING: This function does calculations on 64-bit values, but the actual
  * execution may occur on 32-bit values. Therefore, things like bitshifts
  * need extra checks in the 32-bit case.
@@ -15553,11 +15582,21 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env,
 		scalar_min_max_mul(dst_reg, &src_reg);
 		break;
 	case BPF_AND:
+		if (tnum_is_const(src_reg.var_off)) {
+			ret = maybe_fork_scalars(env, insn, dst_reg);
+			if (ret)
+				return ret;
+		}
 		dst_reg->var_off = tnum_and(dst_reg->var_off, src_reg.var_off);
 		scalar32_min_max_and(dst_reg, &src_reg);
 		scalar_min_max_and(dst_reg, &src_reg);
 		break;
 	case BPF_OR:
+		if (tnum_is_const(src_reg.var_off)) {
+			ret = maybe_fork_scalars(env, insn, dst_reg);
+			if (ret)
+				return ret;
+		}
 		dst_reg->var_off = tnum_or(dst_reg->var_off, src_reg.var_off);
 		scalar32_min_max_or(dst_reg, &src_reg);
 		scalar_min_max_or(dst_reg, &src_reg);