kernel.git/kernel/time/alarmtimer.c, branch linux-5.10.y

alarmtimer: Prevent starvation by small intervals and SIG_IGN

2023-02-22T11:55:59Z

commit d125d1349abeb46945dc5e98f7824bf688266f13 upstream. syzbot reported a RCU stall which is caused by setting up an alarmtimer with a very small interval and ignoring the signal. The reproducer arms the alarm timer with a relative expiry of 8ns and an interval of 9ns. Not a problem per se, but that's an issue when the signal is ignored because then the timer is immediately rearmed because there is no way to delay that rearming to the signal delivery path. See posix_timer_fn() and commit 58229a189942 ("posix-timers: Prevent softirq starvation by small intervals and SIG_IGN") for details. The reproducer does not set SIG_IGN explicitely, but it sets up the timers signal with SIGCONT. That has the same effect as explicitely setting SIG_IGN for a signal as SIGCONT is ignored if there is no handler set and the task is not ptraced. The log clearly shows that: [pid 5102] --- SIGCONT {si_signo=SIGCONT, si_code=SI_TIMER, si_timerid=0, si_overrun=316014, si_int=0, si_ptr=NULL} --- It works because the tasks are traced and therefore the signal is queued so the tracer can see it, which delays the restart of the timer to the signal delivery path. But then the tracer is killed: [pid 5087] kill(-5102, SIGKILL ... ./strace-static-x86_64: Process 5107 detached and after it's gone the stall can be observed: syzkaller login: [ 79.439102][ C0] hrtimer: interrupt took 68471 ns [ 184.460538][ C1] rcu: INFO: rcu_preempt detected stalls on CPUs/tasks: ... [ 184.658237][ C1] rcu: Stack dump where RCU GP kthread last ran: [ 184.664574][ C1] Sending NMI from CPU 1 to CPUs 0: [ 184.669821][ C0] NMI backtrace for cpu 0 [ 184.669831][ C0] CPU: 0 PID: 5108 Comm: syz-executor192 Not tainted 6.2.0-rc6-next-20230203-syzkaller #0 ... [ 184.670036][ C0] Call Trace: [ 184.670041][ C0] [ 184.670045][ C0] alarmtimer_fired+0x327/0x670 posix_timer_fn() prevents that by checking whether the interval for timers which have the signal ignored is smaller than a jiffie and artifically delay it by shifting the next expiry out by a jiffie. That's accurate vs. the overrun accounting, but slightly inaccurate vs. timer_gettimer(2). The comment in that function says what needs to be done and there was a fix available for the regular userspace induced SIG_IGN mechanism, but that did not work due to the implicit ignore for SIGCONT and similar signals. This needs to be worked on, but for now the only available workaround is to do exactly what posix_timer_fn() does: Increase the interval of self-rearming timers, which have their signal ignored, to at least a jiffie. Interestingly this has been fixed before via commit ff86bf0c65f1 ("alarmtimer: Rate limit periodic intervals") already, but that fix got lost in a later rework. Reported-by: syzbot+b9564ba6e8e00694511b@syzkaller.appspotmail.com Fixes: f2c45807d399 ("alarmtimer: Switch over to generic set/get/rearm routine") Signed-off-by: Thomas Gleixner Acked-by: John Stultz Cc: stable@vger.kernel.org Link: https://lore.kernel.org/r/87k00q1no2.ffs@tglx Signed-off-by: Greg Kroah-Hartman

kernel, fs: Introduce and use set_restart_fn() and arch_set_restart_data()

2021-03-25T08:04:16Z

commit 5abbe51a526253b9f003e9a0a195638dc882d660 upstream. Preparation for fixing get_nr_restart_syscall() on X86 for COMPAT. Add a new helper which sets restart_block->fn and calls a dummy arch_set_restart_data() helper. Fixes: 609c19a385c8 ("x86/ptrace: Stop setting TS_COMPAT in ptrace code") Signed-off-by: Oleg Nesterov Signed-off-by: Thomas Gleixner Cc: stable@vger.kernel.org Link: https://lore.kernel.org/r/20210201174641.GA17871@redhat.com Signed-off-by: Greg Kroah-Hartman

alarmtimer: Convert comma to semicolon

2020-08-25T10:45:53Z

Replace a comma between expression statements by a semicolon. Signed-off-by: Xu Wang Signed-off-by: Thomas Gleixner Reviewed-by: Stephen Boyd Link: https://lore.kernel.org/r/20200818062651.21680-1-vulab@iscas.ac.cn

time: Delete repeated words in comments

2020-08-10T20:14:07Z

Drop repeated words in kernel/time/. {when, one, into} Signed-off-by: Randy Dunlap Signed-off-by: Thomas Gleixner Acked-by: John Stultz Link: https://lore.kernel.org/r/20200807033248.8452-1-rdunlap@infradead.org

alarmtimer: Make alarmtimer_get_rtcdev() a stub when CONFIG_RTC_CLASS=n

2020-01-24T20:03:53Z

The stubbed version of alarmtimer_get_rtcdev() is not exported. so this won't work if this function is used in a module when CONFIG_RTC_CLASS=n. Move the stub function to the header file and make it inline so that callers don't have to worry about linking against this symbol. rtcdev isn't used outside of this ifdef so it's not required to be redefined to NULL. Drop that while touching this area. Signed-off-by: Stephen Boyd Signed-off-by: Thomas Gleixner Link: https://lore.kernel.org/r/20200124055849.154411-4-swboyd@chromium.org

alarmtimer: Use wakeup source from alarmtimer platform device

2020-01-24T20:00:21Z

Use the wakeup source that can be associated with the 'alarmtimer' platform device instead of registering another one by hand. Signed-off-by: Stephen Boyd Signed-off-by: Thomas Gleixner Reviewed-by: Douglas Anderson Link: https://lore.kernel.org/r/20200124055849.154411-3-swboyd@chromium.org

alarmtimer: Make alarmtimer platform device child of RTC device

2020-01-24T20:00:20Z

The alarmtimer_suspend() function will fail if an RTC device is on a bus such as SPI or i2c and that RTC device registers and probes after alarmtimer_init() registers and probes the 'alarmtimer' platform device. This is because system wide suspend suspends devices in the reverse order of their probe. When alarmtimer_suspend() attempts to program the RTC for a wakeup it will try to program an RTC device on a bus that has already been suspended. Move the alarmtimer device registration to happen when the RTC which is used for wakeup is registered. Register the 'alarmtimer' platform device as a child of the RTC device too, so that it can be guaranteed that the RTC device won't be suspended when alarmtimer_suspend() is called. Reported-by: Douglas Anderson Signed-off-by: Stephen Boyd Signed-off-by: Thomas Gleixner Reviewed-by: Douglas Anderson Link: https://lore.kernel.org/r/20200124055849.154411-2-swboyd@chromium.org

alarmtimer: Update alarmtimer_get_rtcdev() docs to reflect reality

2020-01-24T20:00:20Z

This function doesn't do anything like this comment says when an RTC device hasn't been chosen. It looks like we used to do something like that before commit 8bc0dafb5cf3 ("alarmtimers: Rework RTC device selection using class interface") but that's long gone now. Remove this sentence to avoid confusing the reader. Signed-off-by: Stephen Boyd Signed-off-by: Thomas Gleixner Link: https://lore.kernel.org/r/20200124055849.154411-5-swboyd@chromium.org

alarmtimer: Unregister wakeup source when module get fails

2020-01-15T10:16:54Z

The alarmtimer_rtc_add_device() function creates a wakeup source and then tries to grab a module reference. If that fails the function returns early with an error code, but fails to remove the wakeup source. Cleanup this exit path so there is no dangling wakeup source, which is named 'alarmtime' left allocated which will conflict with another RTC device that may be registered later. Fixes: 51218298a25e ("alarmtimer: Ensure RTC module is not unloaded") Signed-off-by: Stephen Boyd Signed-off-by: Thomas Gleixner Reviewed-by: Douglas Anderson Cc: stable@vger.kernel.org Link: https://lore.kernel.org/r/20200109155910.907-2-swboyd@chromium.org

alarmtimer: Make nanosleep() time namespace aware

2020-01-14T11:20:55Z

clock_nanosleep() accepts absolute values of expiration time when the TIMER_ABSTIME flag is set. This absolute value is inside the task's time namespace and has to be converted to the host's time. Co-developed-by: Dmitry Safonov Signed-off-by: Andrei Vagin Signed-off-by: Dmitry Safonov Signed-off-by: Thomas Gleixner Link: https://lore.kernel.org/r/20191112012724.250792-16-dima@arista.com