Q: Given the following code slice:
```
1 static int __uvcg_iter_item_entries(const char *page, size_t len,
2 				    int (*fun)(char *, void *, unsigned int),
3 				    void *priv, unsigned int size)
6 	unsigned int bufsize = 1 + size * 8 + 1 + 1;
7 	const char *pg = page;
9 	char *buf;
11 	if (!fun)
14 	buf = kzalloc(bufsize, GFP_KERNEL);
15 	if (!buf)
18 	while (pg - page < len) {
19 		i = 0;
20 		while (i < sizeof(buf) && (pg - page < len) &&
22 			buf[i++] = *pg++;
23 		if (i == sizeof(buf)) {
29 		buf[i] = '\0';
30 		ret = fun(buf, priv, size);
31 		if (ret)```
which has a CWE-787 vulnerability at line:
```
20 		while (i < sizeof(buf) && (pg - page < len) &&
```
Please analyze the root cause of the vulnerability.