add osmo_mobile_identity API

Implement better API around 3GPP TS 24.008 Mobile Identity coding.

struct osmo_mobile_identity is a decoded representation of the raw Mobile
Identity, with a string representation as well as dedicated raw uint32_t TMSI.
The aim is to remove all uncertainty about decoded buffer sizes / data types.

I have patches ready for all osmo programs, completely replacing the Mobile
Identity coding with this new API. Hence deprecate the old MI API.

New API functions provide properly size-checking implementations of:
- decoding a raw MI from a bunch of MI octets;
- locating and decoding MI from a full 3GPP TS 24.008 Complete Layer 3 msgb;
- encoding to a buffer;
- encoding to the end of a msgb.

Other than the old gsm48_generate_mid(), omit a TLV tag and length from
encoding. Many callers manually stripped the tag and value after calling
gsm48_generate_mid(). The aim is to leave writing a TL to the caller entirely,
especially since some callers need to use a TvL, i.e. support a variable-size
length of 8 or 16 bit.

New validity checks so far not implemented anywhere else:
- stricter validation of number of digits of IMSI, IMEI, IMEI-SV MI.
- stricter on filler nibbles to be 0xf.

Rationale:

While implementing osmo-bsc's MSC pooling feature in osmo-bsc, this API will be
used to reduce the number of times a Mobile Identity is extracted from a raw
RSL message.

Extracting the Mobile Identity from messages has numerous duplicate
implementations across our code with various levels of specialization.
https://xkcd.com/927/

To name a few:
- libosmocore: gsm48_mi_to_string(), osmo_mi_name_buf()
- osmo-bsc: extract_sub()
- osmo-msc: mm_rx_loc_upd_req(), cm_serv_reuse_conn(), gsm48_rx_mm_serv_req(),
  vlr_proc_acc_req()

We have existing functions to produce a human readable string from a Mobile
Identity, more or less awkward:
- gsm48_mi_to_string() decodes a TMSI as a decimal number. These days we use
  hexadecimal TMSI everywhere.
- osmo_mi_name_buf() decodes the BCD digits from a raw MI every time, so we'd
  need to pass around the raw message bytes. Also, osmo_mi_name_buf() has the
  wrong signature, it should return a length like snprintf().
- osmo-bsc's extract_sub() first uses gsm48_mi_to_string() which encodes the
  raw uint32_t TMSI to a string, and then calls strtoul() via
  tmsi_from_string() to code those back to a raw uint32_t.

Each of the above implementations employ their own size overflow checks, each
invoke osmo_bcd2str() and implement their own TMSI osmo_load32be() handling.
Too much code dup, let's hope that each and every one is correct.

In osmo-bsc, I am now implementing MSC pooling, and need to extract NRI bits
from a TMSI Mobile Identity. Since none of the above functions are general
enough to be re-used, I found myself again copy-pasting Mobile Identity code:
locating the MI in a 24.008 message with proper size checks, decoding MI
octets.

This time I would like it to become a generally re-usable API.

Change-Id: Ic3f969e739654c1e8c387aedeeba5cce07fe2307

2 files changed, 21 insertions, 0 deletions

diff --git a/tests/utils/utils_test.c b/tests/utils/utils_test.c
index e87cb22f..e15cf5f6 100644
--- a/tests/utils/utils_test.c
+++ b/tests/utils/utils_test.c
@@ -487,6 +487,7 @@ static void bcd2str_test(void)
 {
 	int i;
 	uint8_t bcd[64];
+	uint8_t bcd2[64];
 	int rc;
 
 	printf("\nTesting bcd to string conversion\n");
@@ -511,6 +512,12 @@ static void bcd2str_test(void)
 			printf("    ERROR: expected rc=%d\n", t->expect_rc);
 		if (strcmp(str, t->expect_str))
 			printf("    ERROR: expected result %s\n", osmo_quote_str(t->expect_str, -1));
+
+		memset(bcd2, 0xff, sizeof(bcd2));
+		rc = osmo_str2bcd(bcd2, sizeof(bcd2), str, t->start_nibble, -1, t->allow_hex);
+		printf("osmo_str2bcd(start_nibble=%d) -> rc=%d\n", t->start_nibble, rc);
+		if (rc > 0)
+			printf(" = %s\n", osmo_hexdump(bcd2, rc));
 	}
 
 	printf("- zero output buffer\n");
diff --git a/tests/utils/utils_test.ok b/tests/utils/utils_test.ok
index baa708ee..cbab72a9 100644
--- a/tests/utils/utils_test.ok
+++ b/tests/utils/utils_test.ok
@@ -181,27 +181,41 @@ Testing bcd to string conversion
 - BCD-input='1a 32 54 76 98 f0' nibbles=[1..11[ str_size=64
   rc=10
   -> "1234567890"
+osmo_str2bcd(start_nibble=1) -> rc=6
+ = 1f 32 54 76 98 f0 
 - BCD-input='1a 32 a4 cb 9d f0' nibbles=[1..11[ str_size=64
   rc=-22
   -> "1234ABCD90"
+osmo_str2bcd(start_nibble=1) -> rc=-22
 - BCD-input='1a 32 a4 cb 9d f0' nibbles=[1..11[ str_size=64
   rc=10
   -> "1234ABCD90"
+osmo_str2bcd(start_nibble=1) -> rc=6
+ = 1f 32 a4 cb 9d f0 
 - BCD-input='1a 32 54 76 98 f0' nibbles=[1..12[ str_size=64
   rc=-22
   -> "1234567890F"
+osmo_str2bcd(start_nibble=1) -> rc=-22
 - BCD-input='1a 32 54 76 98 f0' nibbles=[1..12[ str_size=64
   rc=11
   -> "1234567890F"
+osmo_str2bcd(start_nibble=1) -> rc=6
+ = 1f 32 54 76 98 f0 
 - BCD-input='1a 32 54 76 98 f0' nibbles=[0..12[ str_size=64
   rc=12
   -> "A1234567890F"
+osmo_str2bcd(start_nibble=0) -> rc=6
+ = 1a 32 54 76 98 f0 
 - BCD-input='1a 32 54 76 98 f0' nibbles=[1..12[ str_size=5
   rc=11
   -> "1234"
+osmo_str2bcd(start_nibble=1) -> rc=3
+ = 1f 32 f4 
 - BCD-input='' nibbles=[1..1[ str_size=64
   rc=0
   -> ""
+osmo_str2bcd(start_nibble=1) -> rc=1
+ = ff 
 - zero output buffer
   bcd2str(NULL, ...) -> -12
   bcd2str(dst, 0, ...) -> -12