selftests/bpf: Add test for BPF_END register ID reset

Add a test case to ensure that BPF_END operations correctly break
register's scalar ID ties.

The test creates a scenario where r1 is a copy of r0, r0 undergoes a
byte swap, and then r0 is checked against a constant.

- Without the fix in the verifier, the bounds learned from r0 are
  incorrectly propagated to r1, making the verifier believe r1 is
  bounded and wrongly allowing subsequent pointer arithmetic.

- With the fix, r1 remains an unbounded scalar, and the verifier
  correctly rejects the arithmetic operation between the frame pointer
  and the unbounded register.

Co-developed-by: Tianci Cao <ziye@zju.edu.cn>
Signed-off-by: Tianci Cao <ziye@zju.edu.cn>
Co-developed-by: Shenghao Yuan <shenghaoyuan0928@163.com>
Signed-off-by: Shenghao Yuan <shenghaoyuan0928@163.com>
Signed-off-by: Yazhou Tang <tangyazhou518@outlook.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20260304083228.142016-3-tangyazhou@zju.edu.cn
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/progs/verifier_bswap.c

@@ -91,6 +91,28 @@ BSWAP_RANGE_TEST(le32_range, "le32", 0x3f00, 0x3f0000)
 BSWAP_RANGE_TEST(le64_range, "le64", 0x3f00, 0x3f000000000000)
 #endif
 
+SEC("socket")
+__description("BSWAP, reset reg id")
+__failure __msg("math between fp pointer and register with unbounded min value is not allowed")
+__naked void bswap_reset_reg_id(void)
+{
+	asm volatile ("					\
+	call %[bpf_ktime_get_ns];			\
+	r1 = r0;					\
+	r0 = be16 r0;					\
+	if r0 != 1 goto l0_%=;				\
+	r2 = r10;					\
+	r2 += -512;					\
+	r2 += r1;					\
+	*(u8 *)(r2 + 0) = 0;				\
+l0_%=:							\
+	r0 = 0;						\
+	exit;						\
+"	:
+	: __imm(bpf_ktime_get_ns)
+	: __clobber_all);
+}
+
 #else
 
 SEC("socket")