pppd: fix memcpy overlap (#579)

memcpy() with overlapping src and dest buffers is an undefined behavior
in C. In the current code, a ConfRej response is generated by copying
input data in-place, where the dest address is lower than the src.
This happens to work in practice because memcpy() forward-copies data,
matching the behavior of memmove() in this case.

However, if FORTIFY_SOURCE or Address Sanitizer is enabled, memcpy()
will detect the overlap at run time and abort the program.

Replace the memcpy() with memmove() to ensure a well-defined behavior.

Reported-by: Filippo Carletti <filippo.carletti@gmail.com>
Closes: #576

Signed-off-by: Qingfang Deng <dqfext@gmail.com>

Author: LGA1150

pppd/pppd-private.h

@@ -531,7 +531,7 @@ int parse_dotted_ip(char *, u_int32_t *);
 #define TIMEOUT(r, f, t)	ppp_timeout((r), (f), (t), 0)
 #define UNTIMEOUT(r, f)		ppp_untimeout((r), (f))
 
-#define BCOPY(s, d, l)		memcpy(d, s, l)
+#define BCOPY(s, d, l)		memmove(d, s, l)
 #define BZERO(s, n)		memset(s, 0, n)
 #define	BCMP(s1, s2, l)		memcmp(s1, s2, l)