Two interfaces keep reporting the remaining time of the previous setting after a timer was disarmed with a zero it_value, until the old expiry time has passed:
- timerfd_gettime() for timerfds on CLOCK_REALTIME_ALARM and CLOCK_BOOTTIME_ALARM (patch 1). The timerfds on the hrtimer based clocks report zero. - timer_gettime() for SIGEV_NONE timers on CLOCK_MONOTONIC, CLOCK_BOOTTIME, CLOCK_REALTIME and the alarm clocks (patch 2). Timers with signal delivery and the CPU time clocks report zero. In both cases the stale expiry time of a timer which is not queued is used to compute the remaining time. Patch 1 resets it in alarm_init(), patch 2 moves it into the past when a SIGEV_NONE timer is disarmed. Patch 3 adds a test for the second case to the posix_timers selftest. The first problem was found by reading fs/timerfd.c, the second one when the test program for the first was extended to POSIX timers. Both are old: the first exists since timerfd got alarm clock support, the second is also there in a v6.8 based distribution kernel. Patch 2 uses the timer_arm() callback with an expiry time of 0 to reset the stored expiry time, because that needs no new callback. If a dedicated callback or a different place is preferred, I can redo it that way. Tested in qemu (x86_64) on top of mainline a5ebb76233b7 (v7.3-rc6+) with a program which runs the arm/disarm/gettime sequence on all five clocks for timerfd and for POSIX timers with SIGEV_SIGNAL and SIGEV_NONE: 9 wrong results without the series, none with it. The posix_timers selftest with patch 3 fails the new CLOCK_MONOTONIC test without patch 2 and passes all 21 tests with it. The patches also apply to next-20261009. Shashank Mohan Jain (3): alarmtimer: Reset the expiry time in alarm_init() posix-timers: Report a disarmed SIGEV_NONE timer as disarmed selftests/timers: Test timer_gettime() on a disarmed SIGEV_NONE timer kernel/time/alarmtimer.c | 1 + kernel/time/posix-timers.c | 12 +++++- tools/testing/selftests/timers/posix_timers.c | 38 ++++++++++++++++++- 3 files changed, 48 insertions(+), 3 deletions(-) base-commit: a5ebb76233b79db01e82e061173cb73c6d2b5c6b -- 2.43.0
