Validate seek offsets in cfseek

Invalid values previously corrupted the internal position counter which
caused crashes on some platforms because the position counter would
overflow in the next read operation. I added some assertions to track
the position counter corruption and it's probably a good idea to leave
them in to catch possible future errors early.

The actual fix was to just ensure that the seek position it always valid
for the file.

This fixes #1059.

Author: asarium

code/cfile/cfilearchive.cpp

@@ -161,10 +161,15 @@ int cfseek( CFILE *cfile, int offset, int where )
 	default:
 		Int3();
 		return 1;
-	}	
+	}
+
+	// Make sure we don't seek beyond the end of the file
+	CAP(goal_position, cb->lib_offset, cb->lib_offset + cb->size);
 
 	int result = fseek(cb->fp, (long)goal_position, SEEK_SET );
+	Assertion(goal_position >= cb->lib_offset, "Invalid offset values detected while seeking! Goal was " SIZE_T_ARG ", lib_offset is " SIZE_T_ARG ".", goal_position, cb->lib_offset);
 	cb->raw_position = goal_position - cb->lib_offset;
+	Assertion(cb->raw_position <= cb->size, "Invalid raw_position value detected!");
 
 	#if defined(CHECK_POSITION) && !defined(NDEBUG)
 	auto tmp_offset = ftell(cb->fp) - cb->lib_offset;
@@ -200,6 +205,7 @@ int cfread(void *buf, int elsize, int nelem, CFILE *cfile)
 	}
 
 	if ( (cb->raw_position+size) > cb->size ) {
+		Assertion(cb->raw_position <= cb->size, "Invalid raw_position value detected!");
 		size = cb->size - cb->raw_position;
 		if ( size < 1 ) {
 			return 0;
@@ -219,6 +225,7 @@ int cfread(void *buf, int elsize, int nelem, CFILE *cfile)
 	size_t bytes_read = fread( buf, 1, size, cb->fp );
 	if ( bytes_read > 0 )	{
 		cb->raw_position += bytes_read;
+		Assertion(cb->raw_position <= cb->size, "Invalid raw_position value detected!");
 	}		
 
 	#if defined(CHECK_POSITION) && !defined(NDEBUG)
@@ -249,7 +256,8 @@ int cfread_lua_number(double *buf, CFILE *cfile)
 
 	long orig_pos = ftell(cb->fp);
 	int items_read = fscanf(cb->fp, LUA_NUMBER_SCAN, buf);
-	cb->raw_position += ftell(cb->fp)-orig_pos;		
+	cb->raw_position += ftell(cb->fp)-orig_pos;
+	Assertion(cb->raw_position <= cb->size, "Invalid raw_position value detected!");
 
 	#if defined(CHECK_POSITION) && !defined(NDEBUG)
 	auto tmp_offset = ftell(cb->fp) - cb->lib_offset;

code/libs/ffmpeg/FFmpegContext.cpp

@@ -43,7 +43,7 @@ int64_t cfileSeek(void* ptr, int64_t offset, int whence) {
 
 	auto ret = cfseek(cfile, intOff, op);
 
-	if (ret < 0) {
+	if (ret != 0) {
 		// Error
 		return -1;
 	}