coredump: cn_vprintf() has no reason to call vsnprintf() twice

cn_vprintf() looks really overcomplicated and sub-optimal.  We do not need
vsnprintf(NULL) to calculate the size we need, we can simply try to print
into the current buffer and expand/retry only if necessary.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Colin Walters <walters@verbum.org>
Cc: Denys Vlasenko <vda.linux@googlemail.com>
Cc: Jiri Slaby <jslaby@suse.cz>
Cc: Lennart Poettering <mzxreary@0pointer.de>
Cc: Lucas De Marchi <lucas.de.marchi@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/coredump.c

@@ -71,26 +71,20 @@ static int expand_corename(struct core_name *cn)
 
 static int cn_vprintf(struct core_name *cn, const char *fmt, va_list arg)
 {
-	char *cur;
-	int need;
-	int ret;
-
-	need = vsnprintf(NULL, 0, fmt, arg);
-	if (likely(need < cn->size - cn->used - 1))
-		goto out_printf;
-
-	ret = expand_corename(cn);
-	if (ret)
-		goto expand_fail;
+	int free, need;
+
+again:
+	free = cn->size - cn->used;
+	need = vsnprintf(cn->corename + cn->used, free, fmt, arg);
+	if (need < free) {
+		cn->used += need;
+		return 0;
+	}
 
-out_printf:
-	cur = cn->corename + cn->used;
-	vsnprintf(cur, need + 1, fmt, arg);
-	cn->used += need;
-	return 0;
+	if (!expand_corename(cn))
+		goto again;
 
-expand_fail:
-	return ret;
+	return -ENOMEM;
 }
 
 static int cn_printf(struct core_name *cn, const char *fmt, ...)