bts.cpp: Fix overloading ambiguity

Fix error introduced in 1275a3f91a744e011b0dba82b09124d249c7abb5 by
using signed 32 bit integer which is enough for Frame Number in
GSM. Also, mark parameter constraints more explicitly:
- add assert for expected FN values
- don't perform computation for non-relative FN

The error was:
bts.cpp: In member function ‘uint32_t BTS::rfn_to_fn(uint32_t)’:
bts.cpp:554:25: error: call of overloaded ‘abs(uint32_t)’ is ambiguous
  if (abs(rfn - m_cur_rfn) > RFN_THRESHOLD) {
                         ^
In file included from /usr/include/c++/6/cstdlib:75:0,
                 from /usr/include/c++/6/stdlib.h:36,
                 from /usr/include/osmocom/core/linuxrbtree.h:97,
                 from /usr/include/osmocom/core/timer.h:35,
                 from ./bts.h:29,
                 from bts.cpp:21:
/usr/include/stdlib.h:735:12: note: candidate: int abs(int)
 extern int abs (int __x) __THROW __attribute__ ((__const__)) __wur;
            ^~~
In file included from /usr/include/c++/6/stdlib.h:36:0,
                 from /usr/include/osmocom/core/linuxrbtree.h:97,
                 from /usr/include/osmocom/core/timer.h:35,
                 from ./bts.h:29,
                 from bts.cpp:21:
/usr/include/c++/6/cstdlib:185:3: note: candidate: __int128 std::abs(__int128)
   abs(__GLIBCXX_TYPE_INT_N_0 __x) { return __x >= 0 ? __x : -__x; }
   ^~~
/usr/include/c++/6/cstdlib:180:3: note: candidate: long long int std::abs(long long int)
   abs(long long __x) { return __builtin_llabs (__x); }
   ^~~
/usr/include/c++/6/cstdlib:172:3: note: candidate: long int std::abs(long int)
   abs(long __i) { return __builtin_labs(__i); }

Change-Id: Ib6d895a97aa35414f245ea4406c6e78f1b4fb5b8

2 files changed, 11 insertions, 6 deletions

diff --git a/src/bts.cpp b/src/bts.cpp
index 548d0009..c36647ec 100644
--- a/src/bts.cpp
+++ b/src/bts.cpp
@@ -521,11 +521,15 @@ int BTS::rcv_imm_ass_cnf(const uint8_t *data, uint32_t fn)
 }
 
 /* Determine the full frame number from a relative frame number */
-uint32_t BTS::rfn_to_fn(uint32_t rfn)
+uint32_t BTS::rfn_to_fn(int32_t rfn)
 {
-	uint32_t m_cur_rfn;
-	uint32_t fn;
-	uint32_t fn_rounded;
+	int32_t m_cur_rfn;
+	int32_t fn;
+	int32_t fn_rounded;
+
+	/* double-check that relative FN is not negative and fits into int32_t */
+	OSMO_ASSERT(rfn < GSM_MAX_FN);
+	OSMO_ASSERT(rfn >= 0);
 
 	/* Note: If a BTS is sending in a rach request it will be fully aware
 	 * of the frame number. If the PCU is used in a BSC-co-located setup.
@@ -536,7 +540,8 @@ uint32_t BTS::rfn_to_fn(uint32_t rfn)
 
 	/* Ensure that all following calculations are performed with the
 	 * relative frame number */
-	rfn = rfn % 42432;
+	if (rfn >= RFN_MODULUS)
+		return rfn;
 
 	/* Compute an internal relative frame number from the full internal
 	   frame number */
diff --git a/src/bts.h b/src/bts.h
index 5d8dc592..78ed002b 100644
--- a/src/bts.h
+++ b/src/bts.h
@@ -350,7 +350,7 @@ public:
 	uint8_t is_single_block(uint16_t ra, enum ph_burst_type burst_type,
 		uint8_t is_11bit, uint16_t *ms_class, uint16_t *priority);
 
-	uint32_t rfn_to_fn(uint32_t rfn);
+	uint32_t rfn_to_fn(int32_t rfn);
 	int rcv_rach(uint16_t ra, uint32_t Fn, int16_t qta, uint8_t is_11bit,
 		enum ph_burst_type burst_type);