pstore: Use ktime_get_real_fast_ns() instead of __getnstimeofday()
authorArnd Bergmann <arnd@arndb.de>
Fri, 10 Nov 2017 15:25:04 +0000 (16:25 +0100)
committerThomas Gleixner <tglx@linutronix.de>
Sun, 12 Nov 2017 14:05:52 +0000 (15:05 +0100)
commitdf27067e6040b51188184876253d93da002433aa
tree86cc33ed2a85cd5a971f19962e4f5e842da96da8
parentb3fe565bf47de3f79bb5dff9bee1112b46ba5434
pstore: Use ktime_get_real_fast_ns() instead of __getnstimeofday()

__getnstimeofday() is a rather odd interface, with a number of quirks:

- The caller may come from NMI context, but the implementation is not NMI safe,
  one way to get there from NMI is

      NMI handler:
        something bad
          panic()
            kmsg_dump()
              pstore_dump()
                 pstore_record_init()
                   __getnstimeofday()

- The calling conventions are different from any other timekeeping functions,
  to deal with returning an error code during suspended timekeeping.

Address the above issues by using a completely different method to get the
time: ktime_get_real_fast_ns() is NMI safe and has a reasonable behavior
when timekeeping is suspended: it returns the time at which it got
suspended. As Thomas Gleixner explained, this is safe, as
ktime_get_real_fast_ns() does not call into the clocksource driver that
might be suspended.

The result can easily be transformed into a timespec structure. Since
ktime_get_real_fast_ns() was not exported to modules, add the export.

The pstore behavior for the suspended case changes slightly, as it now
stores the timestamp at which timekeeping was suspended instead of storing
a zero timestamp.

This change is not addressing y2038-safety, that's subject to a more
complex follow up patch.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Anton Vorontsov <anton@enomsg.org>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Colin Cross <ccross@android.com>
Link: https://lkml.kernel.org/r/20171110152530.1926955-1-arnd@arndb.de
fs/pstore/platform.c
kernel/time/timekeeping.c