Fix handling of overlapping strncpy() in QVMs

macOS (targeting newer SDKs) and glibc 2.37 changes broke overlapping
strncpy() used by OpenArena 0.8.8 QVMs. (Technically it was undefined
behavior.)

Add Q_strncpy() by Eugene C. from Quake3e that allows overlapping dest
and src for QVM strncpy syscalls.

I made Q_strncpyz() use it for consistency and made QVMs remap the
Q_strncpy() call to the strncpy syscall in case there is a better idea
of how to handle strncpy in the future. Try to handle strncpy the same
in QVMs and DLLs.

code/cgame/cg_syscalls.asm

@@ -94,6 +94,7 @@ equ trap_FS_Seek			-90
 equ	memset						-101
 equ	memcpy						-102
 equ	strncpy						-103
+equ	Q_strncpy					-103 ; alternate name
 equ	sin							-104
 equ	cos							-105
 equ	atan2						-106

code/client/cl_cgame.c

@@ -618,7 +618,7 @@ intptr_t CL_CgameSystemCalls( intptr_t *args ) {
 		Com_Memcpy( VMA(1), VMA(2), args[3] );
 		return 0;
 	case CG_STRNCPY:
-		strncpy( VMA(1), VMA(2), args[3] );
+		Q_strncpy( VMA(1), VMA(2), args[3] );
 		return args[1];
 	case CG_SIN:
 		return FloatAsInt( sin( VMF(1) ) );

code/client/cl_ui.c

@@ -1005,7 +1005,7 @@ intptr_t CL_UISystemCalls( intptr_t *args ) {
 		return 0;
 
 	case UI_STRNCPY:
-		strncpy( VMA(1), VMA(2), args[3] );
+		Q_strncpy( VMA(1), VMA(2), args[3] );
 		return args[1];
 
 	case UI_SIN:

code/game/g_syscalls.asm

@@ -50,6 +50,7 @@ equ trap_FS_Seek -46
 equ	memset					-101
 equ	memcpy					-102
 equ	strncpy					-103
+equ	Q_strncpy				-103 ; alternate name
 equ	sin						-104
 equ	cos						-105
 equ	atan2					-106

code/qcommon/q_shared.c

@@ -802,6 +802,31 @@ int Q_vsnprintf(char *str, size_t size, const char *format, va_list ap)
 }
 #endif
 
+#ifndef Q3_VM
+/*
+=============
+Q_strncpy
+
+strncpy that allows overlapping dest and src
+
+QVMs uses the strncpy syscall to call this
+=============
+*/
+char *Q_strncpy( char *dest, const char *src, int destsize ) {
+	char *start = dest;
+
+	while ( destsize > 0 && (*dest++ = *src++) != '\0' ) {
+		--destsize;
+	}
+
+	while ( --destsize > 0 ) {
+		*dest++ = '\0';
+	}
+
+	return start;
+}
+#endif
+
 /*
 =============
 Q_strncpyz
@@ -820,7 +845,7 @@ void Q_strncpyz( char *dest, const char *src, int destsize ) {
 		Com_Error(ERR_FATAL,"Q_strncpyz: destsize < 1" ); 
 	}
 
-	strncpy( dest, src, destsize-1 );
+	Q_strncpy( dest, src, destsize-1 );
   dest[destsize-1] = 0;
 }
 

code/qcommon/q_shared.h

@@ -827,6 +827,11 @@ char	*Q_strlwr( char *s1 );
 char	*Q_strupr( char *s1 );
 const char	*Q_stristr( const char *s, const char *find);
 
+#ifndef Q3_VM
+// allows overlapping dest and src
+char	*Q_strncpy( char *dest, const char *src, int destsize );
+#endif
+
 // buffer size safe library replacements
 void	Q_strncpyz( char *dest, const char *src, int destsize );
 void	Q_strcat( char *dest, int size, const char *src );

code/server/sv_game.c

@@ -809,7 +809,7 @@ intptr_t SV_GameSystemCalls( intptr_t *args ) {
 		return 0;
 
 	case TRAP_STRNCPY:
-		strncpy( VMA(1), VMA(2), args[3] );
+		Q_strncpy( VMA(1), VMA(2), args[3] );
 		return args[1];
 
 	case TRAP_SIN:

code/ui/ui_syscalls.asm

@@ -92,6 +92,7 @@ equ trap_SetPbClStatus -88
 equ	memset						-101
 equ	memcpy						-102
 equ	strncpy						-103
+equ	Q_strncpy					-103 ; alternate name
 equ	sin							-104
 equ	cos							-105
 equ	atan2						-106