[PATCH v2] pstore/ram: no timekeeping calls when unavailable

[PATCH v2] pstore/ram: no timekeeping calls when unavailable

[Kees Cook]

We must not call timekeeping functions unless they are available. If we dump
before they have resumed, avoid a WARN_ON by setting the timestamp to 0.

Since the "ram" pstore driver can be a module, we must have
timekeeping_suspended exported.

Reported-by: Doug Anderson <dianders@chromium.org>
Cc: Anton Vorontsov <cbouatmailru@gmail.com>
Cc: John Stultz <johnstul@us.ibm.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
---
v2:
 - export needed for timekeeping_suspended (thanks to Fengguang Wu).
---
 fs/pstore/ram.c           |    8 +++++++-
 kernel/time/timekeeping.c |    1 +
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/fs/pstore/ram.c b/fs/pstore/ram.c
index 1a4f6da..6d014e0 100644
--- a/fs/pstore/ram.c
+++ b/fs/pstore/ram.c
@@ -171,7 +171,13 @@ static size_t ramoops_write_kmsg_hdr(struct persistent_ram_zone *prz)
 	struct timeval timestamp;
 	size_t len;
 
-	do_gettimeofday(&timestamp);
+	/* Handle dumping before timekeeping has resumed. */
+	if (unlikely(timekeeping_suspended)) {
+		timestamp.tv_sec = 0;
+		timestamp.tv_usec = 0;
+	} else
+		do_gettimeofday(&timestamp);
+
 	hdr = kasprintf(GFP_ATOMIC, RAMOOPS_KERNMSG_HDR "%lu.%lu\n",
 		(long)timestamp.tv_sec, (long)timestamp.tv_usec);
 	WARN_ON_ONCE(!hdr);
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index e424970..24ea968 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -33,6 +33,7 @@ __cacheline_aligned_in_smp DEFINE_SEQLOCK(xtime_lock);
 
 /* flag for if timekeeping is suspended */
 int __read_mostly timekeeping_suspended;
+EXPORT_SYMBOL_GPL(timekeeping_suspended);
 
 static inline void tk_normalize_xtime(struct timekeeper *tk)
 {
-- 
1.7.9.5


-- 
Kees Cook
Chrome OS Security

Re: [PATCH v2] pstore/ram: no timekeeping calls when unavailable

[John Stultz]

On 11/05/2012 02:00 PM, Kees Cook wrote:
> We must not call timekeeping functions unless they are available. If we dump
> before they have resumed, avoid a WARN_ON by setting the timestamp to 0.
>
> Since the "ram" pstore driver can be a module, we must have
> timekeeping_suspended exported.
>
> Reported-by: Doug Anderson <dianders@chromium.org>
> Cc: Anton Vorontsov <cbouatmailru@gmail.com>
> Cc: John Stultz <johnstul@us.ibm.com>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
> v2:
>   - export needed for timekeeping_suspended (thanks to Fengguang Wu).
> ---
>   fs/pstore/ram.c           |    8 +++++++-
>   kernel/time/timekeeping.c |    1 +
>   2 files changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/fs/pstore/ram.c b/fs/pstore/ram.c
> index 1a4f6da..6d014e0 100644
> --- a/fs/pstore/ram.c
> +++ b/fs/pstore/ram.c
> @@ -171,7 +171,13 @@ static size_t ramoops_write_kmsg_hdr(struct persistent_ram_zone *prz)
>   	struct timeval timestamp;
>   	size_t len;
>
> -	do_gettimeofday(&timestamp);
> +	/* Handle dumping before timekeeping has resumed. */
> +	if (unlikely(timekeeping_suspended)) {
> +		timestamp.tv_sec = 0;
> +		timestamp.tv_usec = 0;
> +	} else
> +		do_gettimeofday(&timestamp);
> +
Would nulling out the timestamp be better done in do_gettimeofday()?  
That way we don't have to export timekeeping internals and users would 
get something more sane for this corner case.

thanks
-john

Re: [PATCH v2] pstore/ram: no timekeeping calls when unavailable

[Kees Cook]

On Fri, Nov 9, 2012 at 4:56 PM, John Stultz <johnstul@us.ibm.com> wrote:
> On 11/05/2012 02:00 PM, Kees Cook wrote:
>>
>> We must not call timekeeping functions unless they are available. If we
>> dump
>> before they have resumed, avoid a WARN_ON by setting the timestamp to 0.
>>
>> Since the "ram" pstore driver can be a module, we must have
>> timekeeping_suspended exported.
>>
>> Reported-by: Doug Anderson <dianders@chromium.org>
>> Cc: Anton Vorontsov <cbouatmailru@gmail.com>
>> Cc: John Stultz <johnstul@us.ibm.com>
>> Signed-off-by: Kees Cook <keescook@chromium.org>
>> ---
>> v2:
>>   - export needed for timekeeping_suspended (thanks to Fengguang Wu).
>> ---
>>   fs/pstore/ram.c           |    8 +++++++-
>>   kernel/time/timekeeping.c |    1 +
>>   2 files changed, 8 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/pstore/ram.c b/fs/pstore/ram.c
>> index 1a4f6da..6d014e0 100644
>> --- a/fs/pstore/ram.c
>> +++ b/fs/pstore/ram.c
>> @@ -171,7 +171,13 @@ static size_t ramoops_write_kmsg_hdr(struct
>> persistent_ram_zone *prz)
>>         struct timeval timestamp;
>>         size_t len;
>>
>> -       do_gettimeofday(&timestamp);
>> +       /* Handle dumping before timekeeping has resumed. */
>> +       if (unlikely(timekeeping_suspended)) {
>> +               timestamp.tv_sec = 0;
>> +               timestamp.tv_usec = 0;
>> +       } else
>> +               do_gettimeofday(&timestamp);
>> +
>
> Would nulling out the timestamp be better done in do_gettimeofday()?  That
> way we don't have to export timekeeping internals and users would get
> something more sane for this corner case.

Well... I'm not sure. If we don't want to expose the
timekeeping_suspended variable, maybe we need a function to check
this? I think it's probably better to find the users of timekeeping
that could call it when suspended. That's why I figured the BUG was
there. Very very few things should be attempting to call gettimeofday
in a place where it might be suspended. As such, it seems like those
things should be able to determine how to handle it. Maybe not
everything would be sensible to get back 0s.

In this particular case, I'm fine with removing the BUG and returning
0 instead, since that's fine for ramoops. :)

-Kees

-- 
Kees Cook
Chrome OS Security

Re: [PATCH v2] pstore/ram: no timekeeping calls when unavailable

[Anton Vorontsov]

On Fri, Nov 09, 2012 at 05:26:53PM -0800, Kees Cook wrote:
[....]
> >> @@ -171,7 +171,13 @@ static size_t ramoops_write_kmsg_hdr(struct
> >> persistent_ram_zone *prz)
> >>         struct timeval timestamp;
> >>         size_t len;
> >>
> >> -       do_gettimeofday(&timestamp);
> >> +       /* Handle dumping before timekeeping has resumed. */
> >> +       if (unlikely(timekeeping_suspended)) {
> >> +               timestamp.tv_sec = 0;
> >> +               timestamp.tv_usec = 0;
> >> +       } else
> >> +               do_gettimeofday(&timestamp);
> >> +
> >
> > Would nulling out the timestamp be better done in do_gettimeofday()?  That
> > way we don't have to export timekeeping internals and users would get
> > something more sane for this corner case.
> 
> Well... I'm not sure. If we don't want to expose the
> timekeeping_suspended variable, maybe we need a function to check
> this? I think it's probably better to find the users of timekeeping
> that could call it when suspended. That's why I figured the BUG was
> there. Very very few things should be attempting to call gettimeofday
> in a place where it might be suspended. As such, it seems like those
> things should be able to determine how to handle it. Maybe not
> everything would be sensible to get back 0s.
> 
> In this particular case, I'm fine with removing the BUG and returning
> 0 instead, since that's fine for ramoops. :)

In the lack of agreement on kernel/time/timekeeping.c change, I can't
apply the patch. And personally I tend to agree that doing this workaround
in the pstore code is odd. How about introducing ___do_gettimeofday() that
is safe to call when suspened, and the func would have good kernel doc
comments explaining the purpose of it?

Thanks,
Anton.

Re: [PATCH v2] pstore/ram: no timekeeping calls when unavailable

[John Stultz]

On 11/16/2012 06:53 PM, Anton Vorontsov wrote:
> On Fri, Nov 09, 2012 at 05:26:53PM -0800, Kees Cook wrote:
> [....]
>>>> @@ -171,7 +171,13 @@ static size_t ramoops_write_kmsg_hdr(struct
>>>> persistent_ram_zone *prz)
>>>>          struct timeval timestamp;
>>>>          size_t len;
>>>>
>>>> -       do_gettimeofday(&timestamp);
>>>> +       /* Handle dumping before timekeeping has resumed. */
>>>> +       if (unlikely(timekeeping_suspended)) {
>>>> +               timestamp.tv_sec = 0;
>>>> +               timestamp.tv_usec = 0;
>>>> +       } else
>>>> +               do_gettimeofday(&timestamp);
>>>> +
>>> Would nulling out the timestamp be better done in do_gettimeofday()?  That
>>> way we don't have to export timekeeping internals and users would get
>>> something more sane for this corner case.
>> Well... I'm not sure. If we don't want to expose the
>> timekeeping_suspended variable, maybe we need a function to check
>> this? I think it's probably better to find the users of timekeeping
>> that could call it when suspended. That's why I figured the BUG was
>> there. Very very few things should be attempting to call gettimeofday
>> in a place where it might be suspended. As such, it seems like those
>> things should be able to determine how to handle it. Maybe not
>> everything would be sensible to get back 0s.
>>
>> In this particular case, I'm fine with removing the BUG and returning
>> 0 instead, since that's fine for ramoops. :)
> In the lack of agreement on kernel/time/timekeeping.c change, I can't
> apply the patch. And personally I tend to agree that doing this workaround
> in the pstore code is odd. How about introducing ___do_gettimeofday() that
> is safe to call when suspened, and the func would have good kernel doc
> comments explaining the purpose of it?
Yea, I wanted to revisit this, because it is an odd case.

We don't want to call getnstimeofday() while the timekeeping code is 
suspended, since the clocksource cycle_last value may be invalid if the 
hardware was reset during suspend.  Kees is correct,  the WARN_ONs were 
there to make sure no one tries to use the timekeeping core before its 
resumed, so removing them is problematic.

Your sugggestion of having the __do_gettimeofday() internal accessor 
that maybe returns an error if timekeeping has been suspended could work.

The other possibility is depending on the needs for accuracy with the 
timestamp, current_kernel_time() might be a better interface to use, 
since it will return the time at the last tick, and doesn't require 
accessing the clocksource hardware.  Might that be a simpler solution? 
Or is sub-tick granularity necessary?

thanks
-john

Re: [PATCH v2] pstore/ram: no timekeeping calls when unavailable

[Kees Cook]

On Fri, Nov 16, 2012 at 7:16 PM, John Stultz <johnstul@us.ibm.com> wrote:
> Yea, I wanted to revisit this, because it is an odd case.
>
> We don't want to call getnstimeofday() while the timekeeping code is
> suspended, since the clocksource cycle_last value may be invalid if the
> hardware was reset during suspend.  Kees is correct,  the WARN_ONs were
> there to make sure no one tries to use the timekeeping core before its
> resumed, so removing them is problematic.
>
> Your sugggestion of having the __do_gettimeofday() internal accessor that
> maybe returns an error if timekeeping has been suspended could work.
>
> The other possibility is depending on the needs for accuracy with the
> timestamp, current_kernel_time() might be a better interface to use, since
> it will return the time at the last tick, and doesn't require accessing the
> clocksource hardware.  Might that be a simpler solution? Or is sub-tick
> granularity necessary?

I think it's only useful to have this to the same granularity as
sched_clock(), so things can be correlated to dmesg output. If it's
the same, I'd be fine to switch to using current_kernel_time().

-Kees

-- 
Kees Cook
Chrome OS Security

Re: [PATCH v2] pstore/ram: no timekeeping calls when unavailable

[John Stultz]

On 11/18/2012 12:09 PM, Kees Cook wrote:
> On Fri, Nov 16, 2012 at 7:16 PM, John Stultz <johnstul@us.ibm.com> wrote:
>> Yea, I wanted to revisit this, because it is an odd case.
>>
>> We don't want to call getnstimeofday() while the timekeeping code is
>> suspended, since the clocksource cycle_last value may be invalid if the
>> hardware was reset during suspend.  Kees is correct,  the WARN_ONs were
>> there to make sure no one tries to use the timekeeping core before its
>> resumed, so removing them is problematic.
>>
>> Your sugggestion of having the __do_gettimeofday() internal accessor that
>> maybe returns an error if timekeeping has been suspended could work.
>>
>> The other possibility is depending on the needs for accuracy with the
>> timestamp, current_kernel_time() might be a better interface to use, since
>> it will return the time at the last tick, and doesn't require accessing the
>> clocksource hardware.  Might that be a simpler solution? Or is sub-tick
>> granularity necessary?
> I think it's only useful to have this to the same granularity as
> sched_clock(), so things can be correlated to dmesg output. If it's
> the same, I'd be fine to switch to using current_kernel_time().
Oof.  That's another can of worms,   sched_clock() resolution isn't tied 
to getnstimeofday(), since you may have cases where the TSC is invalid 
for timekeeping (so we use something slow like the ACPI PM) but ok for 
scheduling, etc.

Even so, its current_kernel_time() is just tick granularity, so that 
probably won't work for you.

So I'm leaning towards Anton's suggestion of adding a new internal 
accessor that returns an error if we're suspended.

Thomas, what do you think of something like what's below?

thanks
-john


diff --git a/include/linux/time.h b/include/linux/time.h
index 4d358e9..0015aea 100644
--- a/include/linux/time.h
+++ b/include/linux/time.h
@@ -158,6 +158,7 @@ extern int do_setitimer(int which, struct itimerval *value,
  			struct itimerval *ovalue);
  extern unsigned int alarm_setitimer(unsigned int seconds);
  extern int do_getitimer(int which, struct itimerval *value);
+extern int __getnstimeofday(struct timespec *tv);
  extern void getnstimeofday(struct timespec *tv);
  extern void getrawmonotonic(struct timespec *ts);
  extern void getnstime_raw_and_real(struct timespec *ts_raw,
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index e424970..bb2638c 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -220,19 +220,20 @@ static void timekeeping_forward_now(struct timekeeper *tk)
  }
  
  /**
- * getnstimeofday - Returns the time of day in a timespec
+ * __getnstimeofday - Returns the time of day in a timespec
   * @ts:		pointer to the timespec to be set
   *
- * Returns the time of day in a timespec.
+ * Returns -1 if timekeeping is suspended.
+ * Returns 0 on success.
   */
-void getnstimeofday(struct timespec *ts)
+int __getnstimeofday(struct timespec *ts)
  {
  	struct timekeeper *tk = &timekeeper;
  	unsigned long seq;
  	s64 nsecs = 0;
  
-	WARN_ON(timekeeping_suspended);
-
+	if (unlikely(timekeeping_suspended));
+		return -1;
  	do {
  		seq = read_seqbegin(&tk->lock);
  
@@ -243,6 +244,19 @@ void getnstimeofday(struct timespec *ts)
  
  	ts->tv_nsec = 0;
  	timespec_add_ns(ts, nsecs);
+	return 0;
+}
+EXPORT_SYMBOL(__getnstimeofday);
+
+/**
+ * getnstimeofday - Returns the time of day in a timespec
+ * @ts:		pointer to the timespec to be set
+ *
+ * Returns the time of day in a timespec.
+ */
+void getnstimeofday(struct timespec *ts)
+{
+	WARN_ON(__getnstimeofday(ts));
  }
  EXPORT_SYMBOL(getnstimeofday);
  

Re: [PATCH v2] pstore/ram: no timekeeping calls when unavailable

[Kees Cook]

On Mon, Nov 19, 2012 at 9:23 AM, John Stultz <johnstul@us.ibm.com> wrote:
> On 11/18/2012 12:09 PM, Kees Cook wrote:
>>
>> On Fri, Nov 16, 2012 at 7:16 PM, John Stultz <johnstul@us.ibm.com> wrote:
>>>
>>> Yea, I wanted to revisit this, because it is an odd case.
>>>
>>> We don't want to call getnstimeofday() while the timekeeping code is
>>> suspended, since the clocksource cycle_last value may be invalid if the
>>> hardware was reset during suspend.  Kees is correct,  the WARN_ONs were
>>> there to make sure no one tries to use the timekeeping core before its
>>> resumed, so removing them is problematic.
>>>
>>> Your sugggestion of having the __do_gettimeofday() internal accessor that
>>> maybe returns an error if timekeeping has been suspended could work.
>>>
>>> The other possibility is depending on the needs for accuracy with the
>>> timestamp, current_kernel_time() might be a better interface to use,
>>> since
>>> it will return the time at the last tick, and doesn't require accessing
>>> the
>>> clocksource hardware.  Might that be a simpler solution? Or is sub-tick
>>> granularity necessary?
>>
>> I think it's only useful to have this to the same granularity as
>> sched_clock(), so things can be correlated to dmesg output. If it's
>> the same, I'd be fine to switch to using current_kernel_time().
>
> Oof.  That's another can of worms,   sched_clock() resolution isn't tied to
> getnstimeofday(), since you may have cases where the TSC is invalid for
> timekeeping (so we use something slow like the ACPI PM) but ok for
> scheduling, etc.
>
> Even so, its current_kernel_time() is just tick granularity, so that
> probably won't work for you.
>
> So I'm leaning towards Anton's suggestion of adding a new internal accessor
> that returns an error if we're suspended.
>
> Thomas, what do you think of something like what's below?
>
> thanks
> -john
>
>
> diff --git a/include/linux/time.h b/include/linux/time.h
> index 4d358e9..0015aea 100644
> --- a/include/linux/time.h
> +++ b/include/linux/time.h
> @@ -158,6 +158,7 @@ extern int do_setitimer(int which, struct itimerval
> *value,
>                         struct itimerval *ovalue);
>  extern unsigned int alarm_setitimer(unsigned int seconds);
>  extern int do_getitimer(int which, struct itimerval *value);
> +extern int __getnstimeofday(struct timespec *tv);
>  extern void getnstimeofday(struct timespec *tv);
>  extern void getrawmonotonic(struct timespec *ts);
>  extern void getnstime_raw_and_real(struct timespec *ts_raw,
> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> index e424970..bb2638c 100644
> --- a/kernel/time/timekeeping.c
> +++ b/kernel/time/timekeeping.c
> @@ -220,19 +220,20 @@ static void timekeeping_forward_now(struct timekeeper
> *tk)
>  }
>   /**
> - * getnstimeofday - Returns the time of day in a timespec
> + * __getnstimeofday - Returns the time of day in a timespec
>   * @ts:                pointer to the timespec to be set
>   *
> - * Returns the time of day in a timespec.
> + * Returns -1 if timekeeping is suspended.
> + * Returns 0 on success.
>   */
> -void getnstimeofday(struct timespec *ts)
> +int __getnstimeofday(struct timespec *ts)
>  {
>         struct timekeeper *tk = &timekeeper;
>         unsigned long seq;
>         s64 nsecs = 0;
>  -      WARN_ON(timekeeping_suspended);
> -
> +       if (unlikely(timekeeping_suspended));
> +               return -1;

Is it useful to make this -EAGAIN or something, just for clarity?
Also, this technically changes the semantics of callers that were
hitting WARN (there should be none) in that the timespec isn't updated
at all. In the prior code, a WARN would be emitted, but it would still
fill out the timespec and return.

When I looked at implementing this error path, I actually moved the
return -EAGAIN to the end of the function to keep the results the
same. All that said, this is much cleaner and more correct if not
touching the timespec on error is tolerable.

>         do {
>                 seq = read_seqbegin(&tk->lock);
>  @@ -243,6 +244,19 @@ void getnstimeofday(struct timespec *ts)
>         ts->tv_nsec = 0;
>         timespec_add_ns(ts, nsecs);
> +       return 0;
> +}
> +EXPORT_SYMBOL(__getnstimeofday);
> +
> +/**
> + * getnstimeofday - Returns the time of day in a timespec
> + * @ts:                pointer to the timespec to be set
> + *
> + * Returns the time of day in a timespec.
> + */
> +void getnstimeofday(struct timespec *ts)
> +{
> +       WARN_ON(__getnstimeofday(ts));
>  }
>  EXPORT_SYMBOL(getnstimeofday);
>

-Kees


-- 
Kees Cook
Chrome OS Security

Re: [PATCH v2] pstore/ram: no timekeeping calls when unavailable

[John Stultz]

On 11/19/2012 09:45 AM, Kees Cook wrote:
> On Mon, Nov 19, 2012 at 9:23 AM, John Stultz <johnstul@us.ibm.com> wrote:
>> On 11/18/2012 12:09 PM, Kees Cook wrote:
>>> On Fri, Nov 16, 2012 at 7:16 PM, John Stultz <johnstul@us.ibm.com> wrote:
>>>> Yea, I wanted to revisit this, because it is an odd case.
>>>>
>>>> We don't want to call getnstimeofday() while the timekeeping code is
>>>> suspended, since the clocksource cycle_last value may be invalid if the
>>>> hardware was reset during suspend.  Kees is correct,  the WARN_ONs were
>>>> there to make sure no one tries to use the timekeeping core before its
>>>> resumed, so removing them is problematic.
>>>>
>>>> Your sugggestion of having the __do_gettimeofday() internal accessor that
>>>> maybe returns an error if timekeeping has been suspended could work.
>>>>
>>>> The other possibility is depending on the needs for accuracy with the
>>>> timestamp, current_kernel_time() might be a better interface to use,
>>>> since
>>>> it will return the time at the last tick, and doesn't require accessing
>>>> the
>>>> clocksource hardware.  Might that be a simpler solution? Or is sub-tick
>>>> granularity necessary?
>>> I think it's only useful to have this to the same granularity as
>>> sched_clock(), so things can be correlated to dmesg output. If it's
>>> the same, I'd be fine to switch to using current_kernel_time().
>> Oof.  That's another can of worms,   sched_clock() resolution isn't tied to
>> getnstimeofday(), since you may have cases where the TSC is invalid for
>> timekeeping (so we use something slow like the ACPI PM) but ok for
>> scheduling, etc.
>>
>> Even so, its current_kernel_time() is just tick granularity, so that
>> probably won't work for you.
>>
>> So I'm leaning towards Anton's suggestion of adding a new internal accessor
>> that returns an error if we're suspended.
>>
>> Thomas, what do you think of something like what's below?
>>
>> thanks
>> -john
>>
>>
>> diff --git a/include/linux/time.h b/include/linux/time.h
>> index 4d358e9..0015aea 100644
>> --- a/include/linux/time.h
>> +++ b/include/linux/time.h
>> @@ -158,6 +158,7 @@ extern int do_setitimer(int which, struct itimerval
>> *value,
>>                          struct itimerval *ovalue);
>>   extern unsigned int alarm_setitimer(unsigned int seconds);
>>   extern int do_getitimer(int which, struct itimerval *value);
>> +extern int __getnstimeofday(struct timespec *tv);
>>   extern void getnstimeofday(struct timespec *tv);
>>   extern void getrawmonotonic(struct timespec *ts);
>>   extern void getnstime_raw_and_real(struct timespec *ts_raw,
>> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
>> index e424970..bb2638c 100644
>> --- a/kernel/time/timekeeping.c
>> +++ b/kernel/time/timekeeping.c
>> @@ -220,19 +220,20 @@ static void timekeeping_forward_now(struct timekeeper
>> *tk)
>>   }
>>    /**
>> - * getnstimeofday - Returns the time of day in a timespec
>> + * __getnstimeofday - Returns the time of day in a timespec
>>    * @ts:                pointer to the timespec to be set
>>    *
>> - * Returns the time of day in a timespec.
>> + * Returns -1 if timekeeping is suspended.
>> + * Returns 0 on success.
>>    */
>> -void getnstimeofday(struct timespec *ts)
>> +int __getnstimeofday(struct timespec *ts)
>>   {
>>          struct timekeeper *tk = &timekeeper;
>>          unsigned long seq;
>>          s64 nsecs = 0;
>>   -      WARN_ON(timekeeping_suspended);
>> -
>> +       if (unlikely(timekeeping_suspended));
>> +               return -1;
> Is it useful to make this -EAGAIN or something, just for clarity?
> Also, this technically changes the semantics of callers that were
> hitting WARN (there should be none) in that the timespec isn't updated
> at all. In the prior code, a WARN would be emitted, but it would still
> fill out the timespec and return.
>
> When I looked at implementing this error path, I actually moved the
> return -EAGAIN to the end of the function to keep the results the
> same. All that said, this is much cleaner and more correct if not
> touching the timespec on error is tolerable.

Yea, although really, the value returned if the WARN_ON is emitted is 
really junk. If the clocksource was reset under us, you could see really 
crazy time values.

Thinking some more, a better solution could be to simply return the time 
when timekeeping was suspended, which would be sane.

This wouldn't catch cases where folks were accessing the time while 
timekeeping is suspended, but they would get as valid a value as we can 
provide.

The only downside, is that accessors could see a flat-spot in time where 
they just get the same value over and over while timekeeping is 
suspended.  So could be possible for bad behavior if something was doing 
something dumb like spinning in a loop waiting for some time to pass and 
was preventing suspend from completing.

Untested patch below. Any thoughts?

thanks
-john

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index e424970..84593ef 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -146,6 +146,9 @@ static inline s64 timekeeping_get_ns(struct timekeeper *tk)
  	struct clocksource *clock;
  	s64 nsec;
  
+	if (unlikely(timekeeping_suspended))
+		return tk->xtime_nsec >> tk->shift;
+
  	/* read clocksource: */
  	clock = tk->clock;
  	cycle_now = clock->read(clock);
@@ -166,6 +169,9 @@ static inline s64 timekeeping_get_ns_raw(struct timekeeper *tk)
  	struct clocksource *clock;
  	s64 nsec;
  
+	if (unlikely(timekeeping_suspended))
+		return 0;
+
  	/* read clocksource: */
  	clock = tk->clock;
  	cycle_now = clock->read(clock);
@@ -231,8 +237,6 @@ void getnstimeofday(struct timespec *ts)
  	unsigned long seq;
  	s64 nsecs = 0;
  
-	WARN_ON(timekeeping_suspended);
-
  	do {
  		seq = read_seqbegin(&tk->lock);
  
@@ -252,8 +256,6 @@ ktime_t ktime_get(void)
  	unsigned int seq;
  	s64 secs, nsecs;
  
-	WARN_ON(timekeeping_suspended);
-
  	do {
  		seq = read_seqbegin(&tk->lock);
  		secs = tk->xtime_sec + tk->wall_to_monotonic.tv_sec;
@@ -283,8 +285,6 @@ void ktime_get_ts(struct timespec *ts)
  	s64 nsec;
  	unsigned int seq;
  
-	WARN_ON(timekeeping_suspended);
-
  	do {
  		seq = read_seqbegin(&tk->lock);
  		ts->tv_sec = tk->xtime_sec;
@@ -316,8 +316,6 @@ void getnstime_raw_and_real(struct timespec *ts_raw, struct timespec *ts_real)
  	unsigned long seq;
  	s64 nsecs_raw, nsecs_real;
  
-	WARN_ON_ONCE(timekeeping_suspended);
-
  	do {
  		seq = read_seqbegin(&tk->lock);
  
@@ -1203,8 +1201,6 @@ void get_monotonic_boottime(struct timespec *ts)
  	s64 nsec;
  	unsigned int seq;
  
-	WARN_ON(timekeeping_suspended);
-
  	do {
  		seq = read_seqbegin(&tk->lock);
  		ts->tv_sec = tk->xtime_sec;

Re: [PATCH v2] pstore/ram: no timekeeping calls when unavailable

[Kees Cook]

On Mon, Nov 19, 2012 at 10:57 AM, John Stultz <johnstul@us.ibm.com> wrote:
> On 11/19/2012 09:45 AM, Kees Cook wrote:
>>
>> On Mon, Nov 19, 2012 at 9:23 AM, John Stultz <johnstul@us.ibm.com> wrote:
>>>
>>> On 11/18/2012 12:09 PM, Kees Cook wrote:
>>>>
>>>> On Fri, Nov 16, 2012 at 7:16 PM, John Stultz <johnstul@us.ibm.com>
>>>> wrote:
>>>>>
>>>>> Yea, I wanted to revisit this, because it is an odd case.
>>>>>
>>>>> We don't want to call getnstimeofday() while the timekeeping code is
>>>>> suspended, since the clocksource cycle_last value may be invalid if the
>>>>> hardware was reset during suspend.  Kees is correct,  the WARN_ONs were
>>>>> there to make sure no one tries to use the timekeeping core before its
>>>>> resumed, so removing them is problematic.
>>>>>
>>>>> Your sugggestion of having the __do_gettimeofday() internal accessor
>>>>> that
>>>>> maybe returns an error if timekeeping has been suspended could work.
>>>>>
>>>>> The other possibility is depending on the needs for accuracy with the
>>>>> timestamp, current_kernel_time() might be a better interface to use,
>>>>> since
>>>>> it will return the time at the last tick, and doesn't require accessing
>>>>> the
>>>>> clocksource hardware.  Might that be a simpler solution? Or is sub-tick
>>>>> granularity necessary?
>>>>
>>>> I think it's only useful to have this to the same granularity as
>>>> sched_clock(), so things can be correlated to dmesg output. If it's
>>>> the same, I'd be fine to switch to using current_kernel_time().
>>>
>>> Oof.  That's another can of worms,   sched_clock() resolution isn't tied
>>> to
>>> getnstimeofday(), since you may have cases where the TSC is invalid for
>>> timekeeping (so we use something slow like the ACPI PM) but ok for
>>> scheduling, etc.
>>>
>>> Even so, its current_kernel_time() is just tick granularity, so that
>>> probably won't work for you.
>>>
>>> So I'm leaning towards Anton's suggestion of adding a new internal
>>> accessor
>>> that returns an error if we're suspended.
>>>
>>> Thomas, what do you think of something like what's below?
>>>
>>> thanks
>>> -john
>>>
>>>
>>> diff --git a/include/linux/time.h b/include/linux/time.h
>>> index 4d358e9..0015aea 100644
>>> --- a/include/linux/time.h
>>> +++ b/include/linux/time.h
>>> @@ -158,6 +158,7 @@ extern int do_setitimer(int which, struct itimerval
>>> *value,
>>>                          struct itimerval *ovalue);
>>>   extern unsigned int alarm_setitimer(unsigned int seconds);
>>>   extern int do_getitimer(int which, struct itimerval *value);
>>> +extern int __getnstimeofday(struct timespec *tv);
>>>   extern void getnstimeofday(struct timespec *tv);
>>>   extern void getrawmonotonic(struct timespec *ts);
>>>   extern void getnstime_raw_and_real(struct timespec *ts_raw,
>>> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
>>> index e424970..bb2638c 100644
>>> --- a/kernel/time/timekeeping.c
>>> +++ b/kernel/time/timekeeping.c
>>> @@ -220,19 +220,20 @@ static void timekeeping_forward_now(struct
>>> timekeeper
>>> *tk)
>>>   }
>>>    /**
>>> - * getnstimeofday - Returns the time of day in a timespec
>>> + * __getnstimeofday - Returns the time of day in a timespec
>>>    * @ts:                pointer to the timespec to be set
>>>    *
>>> - * Returns the time of day in a timespec.
>>> + * Returns -1 if timekeeping is suspended.
>>> + * Returns 0 on success.
>>>    */
>>> -void getnstimeofday(struct timespec *ts)
>>> +int __getnstimeofday(struct timespec *ts)
>>>   {
>>>          struct timekeeper *tk = &timekeeper;
>>>          unsigned long seq;
>>>          s64 nsecs = 0;
>>>   -      WARN_ON(timekeeping_suspended);
>>> -
>>> +       if (unlikely(timekeeping_suspended));
>>> +               return -1;
>>
>> Is it useful to make this -EAGAIN or something, just for clarity?
>> Also, this technically changes the semantics of callers that were
>> hitting WARN (there should be none) in that the timespec isn't updated
>> at all. In the prior code, a WARN would be emitted, but it would still
>> fill out the timespec and return.
>>
>> When I looked at implementing this error path, I actually moved the
>> return -EAGAIN to the end of the function to keep the results the
>> same. All that said, this is much cleaner and more correct if not
>> touching the timespec on error is tolerable.
>
>
> Yea, although really, the value returned if the WARN_ON is emitted is really
> junk. If the clocksource was reset under us, you could see really crazy time
> values.
>
> Thinking some more, a better solution could be to simply return the time
> when timekeeping was suspended, which would be sane.
>
> This wouldn't catch cases where folks were accessing the time while
> timekeeping is suspended, but they would get as valid a value as we can
> provide.
>
> The only downside, is that accessors could see a flat-spot in time where
> they just get the same value over and over while timekeeping is suspended.
> So could be possible for bad behavior if something was doing something dumb
> like spinning in a loop waiting for some time to pass and was preventing
> suspend from completing.
>
> Untested patch below. Any thoughts?

Hrm, yeah, the dead-lock condition is a bit worrisome. It seems like
it'd be better to keep the WARN_ONs and still define __getnstimeofday?
Then callers that don't care about a flat-spot can report "most
recent" time, and callers not expecting to run when suspended can
still be detected.

>
> thanks
> -john
>
> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> index e424970..84593ef 100644
> --- a/kernel/time/timekeeping.c
> +++ b/kernel/time/timekeeping.c
> @@ -146,6 +146,9 @@ static inline s64 timekeeping_get_ns(struct timekeeper
> *tk)
>         struct clocksource *clock;
>         s64 nsec;
>  +      if (unlikely(timekeeping_suspended))
> +               return tk->xtime_nsec >> tk->shift;
> +
>         /* read clocksource: */
>         clock = tk->clock;
>         cycle_now = clock->read(clock);
> @@ -166,6 +169,9 @@ static inline s64 timekeeping_get_ns_raw(struct
> timekeeper *tk)
>         struct clocksource *clock;
>         s64 nsec;
>  +      if (unlikely(timekeeping_suspended))
> +               return 0;
> +
>         /* read clocksource: */
>         clock = tk->clock;
>         cycle_now = clock->read(clock);

Then fs/pstore/ram.c wouldn't have to set things to zero; it would
just report the flat spot, which is better than reporting 0.

-Kees

-- 
Kees Cook
Chrome OS Security