selftests/bpf: Add cmp_map_pointer_with_const test

Add a test for CONST_PTR_TO_MAP comparison with a non-0 constant. A
BPF program with this code must not pass verification in unpriv.

Signed-off-by: Ihor Solodrai <isolodrai@meta.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250609183024.359974-3-isolodrai@meta.com

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

@@ -624,7 +624,6 @@ __retval(0)
 __naked void cmp_map_pointer_with_zero(void)
 {
 	asm volatile ("					\
-	r1 = 0;						\
 	r1 = %[map_hash_8b] ll;				\
 	if r1 == 0 goto l0_%=;				\
 l0_%=:	r0 = 0;						\
@@ -634,6 +633,22 @@ l0_%=:	r0 = 0;						\
 	: __clobber_all);
 }
 
+SEC("socket")
+__description("unpriv: cmp map pointer with const")
+__success __failure_unpriv __msg_unpriv("R1 pointer comparison prohibited")
+__retval(0)
+__naked void cmp_map_pointer_with_const(void)
+{
+	asm volatile ("					\
+	r1 = %[map_hash_8b] ll;				\
+	if r1 == 0x0000beef goto l0_%=;			\
+l0_%=:	r0 = 0;						\
+	exit;						\
+"	:
+	: __imm_addr(map_hash_8b)
+	: __clobber_all);
+}
+
 SEC("socket")
 __description("unpriv: write into frame pointer")
 __failure __msg("frame pointer is read only")