[RFC] printk: Fix print_time length computation when no buffer is given

[Sylvain Munaut <s.munaut@whatever-company.com>]

The "%5lu" part of the sprintf only guarantees a minimum length,
not a maximum one. This patch should make it correct for any
possible timestamp.

Signed-off-by: Sylvain Munaut <s.munaut@whatever-company.com>
---
 kernel/printk.c |   13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/kernel/printk.c b/kernel/printk.c
index 66a2ea3..2ceceea 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -847,8 +847,17 @@ static size_t print_time(u64 ts, char *buf)
 	if (!printk_time)
 		return 0;
 
-	if (!buf)
-		return 15;
+	if (!buf) {
+		size_t len = 15;
+		u64 ts_limit = (u64)100000 * (u64)1000000000;
+
+		while ((ts > ts_limit) && (len < 21)) {
+			len++;
+			ts_limit = 10 * ts_limit;
+		}
+
+		return len;
+	}
 
 	rem_nsec = do_div(ts, 1000000000);
 	return sprintf(buf, "[%5lu.%06lu] ",
-- 
1.7.10.4