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SubjectRe: [Y2038] [PATCH v2 08/10] fix get_timespec64() for y2038 safe compat interfaces
On Fri, Dec 15, 2017 at 4:02 AM, Arnd Bergmann <arnd@arndb.de> wrote:
> On Fri, Dec 15, 2017 at 1:21 AM, Ben Hutchings
> <ben.hutchings@codethink.co.uk> wrote:
>> On Mon, 2017-11-27 at 11:30 -0800, Deepa Dinamani wrote:
>>> get/put_timespec64() interfaces will eventually be used for
>>> conversions between the new y2038 safe struct __kernel_timespec
>>> and struct timespec64.
>>>
>>> The new y2038 safe syscalls have a common entry for native
>>> and compat interfaces.
>>> On compat interfaces, the high order bits of nanoseconds
>>> should be zeroed out. This is because the application code
>>> or the libc do not garuntee zeroing of these. If used without
>>
>> Spelling: "guarantee"
>>
>> [...]
>>> --- a/kernel/time/time.c
>>> +++ b/kernel/time/time.c
>> [...]
>>> @@ -851,6 +851,11 @@ int get_timespec64(struct timespec64 *ts,
>>> return -EFAULT;
>>>
>>> ts->tv_sec = kts.tv_sec;
>>> +
>>> + /* Zero out the padding for 32 bit systems or in compat mode */
>>> + if (IS_ENABLED(CONFIG_64BIT_TIME) || !IS_ENABLED(CONFIG_64BIT) || in_compat_syscall())
>>> + kts.tv_nsec &= 0xFFFFFFFFUL;
>> [...]
>>
>> I don't understand the condition here. Suppose we're building for an
>> architecture that enables the new syscalls and selects ARCH_64BIT_TIME,
>> but we also enable 64BIT. Then the above condition ends up as:
>> if (1 || 0 || in_compat_syscall())
>> so it's always true.
>>
>> Should the condition be:
>> if (!IS_ENABLED(CONFIG_64BIT) || in_compat_syscall())
>> or is your intent that architectures only select ARCH_64BIT_TIME if
>> 64BIT is not enabled?
>
> My understanding was that we always enable CONFIG_64BIT_TIME
> when 64BIT is enabled.
>
> For a 64-bit architecture, we must not clear the upper 32 bits of tv_nsec,
> but instead need later check them for being nonzero. I think the
> correct condition would be
>
> if ((IS_ENABLED(CONFIG_64BIT_TIME) && !IS_ENABLED(CONFIG_64BIT)) ||
> in_compat_syscall())

I haven't enabled this by default on all 64 bit architectures.
The reason I have the condition this way is that I haven't decided how
I want to handle 64 bit time on x32, and x86 is the first architecture
I plan to enable this for.
At that time, this will be reworked based on whatever solution we
agree on. I did not want to depend on COMPAT_USE_64BIT_TIME yet.

But, I did mean to do

if (IS_ENABLED(CONFIG_64BIT_TIME) && (!IS_ENABLED(CONFIG_64BIT)) ||
in_compat_syscall()))

I will update this.

> Two more thoughts:
>
> - The temporary variable here is defined as 'struct timespec', this must be
> changed to __kernel_timespec for the function to work correctly once we
> switch a 32-bit architecture over. Doing it in this patch is probably the best
> time for that change.

Thanks, will do. I will post an update.

> - I had an idea to handle the copying of timespec/timeval with a
> one-size-fits-all
> function and multiple wrappers around it, such as
>
> enum user_ts_type {
> USER_TS_TIMEVAL = 1,
> USER_TS_32 = 2,
> USER_TS_CLEARNSEC = 4,
> USER_TS_NOCHECK = 8,
> };
>
> /* native handlers want to check on 64-bit but zero on 32-bit */
> #define USER_TS_NATIVE (IS_ENABLED(CONFIG_64BIT) ? 0 : USER_TS_CLEARNSEC)
>
> /* compat handlers accessing 64-bit time structs always want to clear
> the upper half */
> #define USER_TS_COMPAT64 USER_TS_CLEARNSEC
>
> /* on x32, we always use 64-bit time_t but want to clear the upper half */
> #define USER_TS_COMPAT32 (COMPAT_USE_64BIT_TIME) ? USER_TS_CLEARNSEC :
> USER_TS_32)
>
> int get_timestruct(struct timespec64 *ts, const void __user *uts,
> enum user_ts_type flags)
> {
> int ret;
>
> if (flags & USER_TS_32) {
> struct compat_timespec ts32;
> ret = copy_from_user(&ts32, uts, sizeof(ts32));
> if (ret)
> return -EFAULT;
> ts->tv_sec = ts32.tv_sec;
> ts->tv_nsec = ts32.tv_nsec;
> } else {
> ret = copy_from_user(&ts, uts, sizeof(*ts));
> if (ret)
> return -EFAULT;
> if (flags & USER_TS_CLEARNSEC)
> ts->tv_nsec &= 0xFFFFFFFFUL;
> }
>
> if (flags & USER_TS_TIMEVAL) {
> if (!(flags & USER_TS_NOCHECK) &&
> ts->tv_nsec >= USEC_PER_SEC)
> return -EINVAL;
>
> ts->tv_nsec *= NSEC_PER_USEC;
> } else {
> if (!(flags & USER_TS_NOCHECK) &&
> ts->tv_nsec >= NSEC_PER_SEC)
> return -EINVAL;
> }
>
> return 0;
> }
>
> int get_timespec64(struct timespec64 *ts, const struct compat_timespec
> __user *uts)
> {
> return get_timestruct(ts. uts, USER_TS_NATIVE);
> }
>
> int get_compat_timespec32(struct timespec64 *ts, const struct
> compat_timespec __user *uts)
> {
> return get_timestruct(ts. uts, USER_TS_COMPAT32);
> }
>
> int get_compat_timespec64(struct timespec64 *ts, const struct
> __kernel_timespec __user *uts)
> {
> return get_timestruct(ts. uts, USER_TS_COMPAT64);
> }
>
> While working on the driver patches I encountered lots of different
> combinations of
> those that might be interesting here, so we could have wrappers for
> the most common
> ones and call get_timestruct() and put_timestruct() directly for the less common
> variations. Am I taking it too far here, or would that make sense?

I think this is a little confusing.
nanosleep_copyout() uses a similar strategy. But, I think it makes
sense in that case as it uses common functions for all versions of
nanosleep and because of the union.

-Deepa

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