[Devel] [PATCH rh9 v2] kmsg: Virtualize timestamps in /dev/ksmg output

[Konstantin Khorenko]

Internally log messages contain the timestamps relative to the Host boot
time, while Container could start much later than the Host boots and the
timestamps should be shifted accordingly.

Otherwise "dmesg -H" reports data from the future.

Note:
 * time_ns->offsets.boottime is (ve_uptime - host_uptime),
   i.e. negative for Containers created on this Host
 * ring buffer contains timestamps relative to the Host boot

 => we have to add the .boottime offset to the timestamp of Host
    to get the msg entry timestamp relative to Container boottime.

Note 2: we cannot use timespec64_to_ns() in ve_timens_sub_boottime_ns()
	to convert .boottime to nsec because timespec64_to_ns() is not
	designed to handle negative offsets, it tries to avoid the
	overflow and returns KTIME_MAX instead for negative offsets.

Based on the vz7 commit
  e20414197aef ("kmsg: Virtualize timestamps in /dev/ksmg output")

https://jira.sw.ru/browse/PSBM-145313

Signed-off-by: Konstantin Khorenko <khorenko at virtuozzo.com>

v2 changes:
  * ve_timens_add_boottime_ns() should return "nsec", not 0 in case
    Container is not started yet

---
 kernel/printk/printk.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index c68e3b468ab7..57a8b708ecb3 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -600,6 +600,25 @@ static void append_char(char **pp, char *e, char c)
 		*(*pp)++ = c;
 }
 
+static inline u64 ve_timens_add_boottime_ns(u64 nsec)
+{
+	struct time_namespace *ve_time_ns;
+	struct timens_offsets *ns_offsets;
+
+	ve_time_ns = ve_get_time_ns(get_exec_env());
+	if (unlikely(!ve_time_ns)) {
+		/* container not yet started */
+		return nsec;
+	}
+
+	ns_offsets = &ve_time_ns->offsets;
+	nsec += ns_offsets->boottime.tv_sec * NSEC_PER_SEC;
+	nsec += ns_offsets->boottime.tv_nsec;
+	put_time_ns(ve_time_ns);
+
+	return nsec;
+}
+
 static ssize_t info_print_ext_header(char *buf, size_t size,
 				     struct printk_info *info)
 {
@@ -614,6 +633,8 @@ static ssize_t info_print_ext_header(char *buf, size_t size,
 	caller[0] = '\0';
 #endif
 
+	/* shift the timestamp on the Container uptime value */
+	ts_usec = ve_timens_add_boottime_ns(ts_usec);
 	do_div(ts_usec, 1000);
 
 	return scnprintf(buf, size, "%u,%llu,%llu,%c%s;",
-- 
2.31.1