[PATCH] kbuild: fix buffer overflow in modpost

Jiri Benc <jbenc@suse.cz> reported that modpost would stop with SIGABRT if
used with long filepaths.
The error looked like:
>   Building modules, stage 2.
>   MODPOST
> *** glibc detected *** scripts/mod/modpost: realloc(): invalid next size:
+0x0809f588 ***
> [...]

Fix this by allocating at least the required memory + SZ bytes each time.
Before we sometimes ended up allocating too little memory resuting in the
glibc detected bug above.  Based on patch originally submitted by: Jiri
Benc <jbenc@suse.cz>

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

scripts/mod/modpost.c

@@ -508,20 +508,15 @@ buf_printf(struct buffer *buf, const char *fmt, ...)
 
 	va_start(ap, fmt);
 	len = vsnprintf(tmp, SZ, fmt, ap);
-	if (buf->size - buf->pos < len + 1) {
-		buf->size += 128;
-		buf->p = realloc(buf->p, buf->size);
-	}
-	strncpy(buf->p + buf->pos, tmp, len + 1);
-	buf->pos += len;
+	buf_write(buf, tmp, len);
 	va_end(ap);
 }
 
 void
 buf_write(struct buffer *buf, const char *s, int len)
 {
 	if (buf->size - buf->pos < len) {
-		buf->size += len;
+		buf->size += len + SZ;
 		buf->p = realloc(buf->p, buf->size);
 	}
 	strncpy(buf->p + buf->pos, s, len);