selftests/bpf: Add test for truncation after sign extension in coerce_reg_to_size_sx()

Add test that checks whether unsigned ranges deduced by the verifier for
sign extension instruction is correct. Without previous patch that
fixes truncation in coerce_reg_to_size_sx() this test fails.
Acked-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
Signed-off-by: Dimitar Kanaliev <dimitar.kanaliev@siteground.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20241014121155.92887-3-dimitar.kanaliev@siteground.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

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

@@ -287,6 +287,26 @@ l0_%=:							\
 	: __clobber_all);
 }
 
+SEC("socket")
+__description("MOV64SX, S8, unsigned range_check")
+__success __retval(0)
+__naked void mov64sx_s8_range_check(void)
+{
+	asm volatile ("					\
+	call %[bpf_get_prandom_u32];			\
+	r0 &= 0x1;					\
+	r0 += 0xfe;					\
+	r0 = (s8)r0;					\
+	if r0 < 0xfffffffffffffffe goto label_%=;	\
+	r0 = 0;						\
+	exit;						\
+label_%=:						\
+	exit;						\
+"	:
+	: __imm(bpf_get_prandom_u32)
+	: __clobber_all);
+}
+
 #else
 
 SEC("socket")