On Fri, Sep 22, 2017 at 12:49 PM, Takashi Iwai tiwai@suse.de wrote:
On Fri, 22 Sep 2017 12:14:55 +0200, Arnd Bergmann wrote:
The kernel uses timespec64 internally, which is defined as "{ s64 tv_sec; long tv_nsec };", so this has the padding in a different place on big-endian architectures, and has a different alignment and size on i386. We plan to introduce a 'struct __kernel_timespec' that is compatible with the __64_BIT_TIME_T version of the user timespec, but that doesn't exist yet.
If you prefer, we can probably introduce it now with Baolin's series, I think Deepa was planning to post a patch to add it soon anyway.
Yes, this sounds like a saner solution than defining the own timespec at each place individually. Then we can have better conversion macros, too, I suppose.
Thinking about it again, we unfortunately can't use __kernel_timespec until after all 32-bit architectures have been converted to use the new syscalls that we still need to introduce: In the meantime the plan is that '__kernel_timespec' is an alias for the usual 'timespec' in user space and may still be 32-bit wide.
I definitely agree that open-coding 'struct { s64 tv_sec; s64 tv_nsec}' in a dozen locations is not overly helpful.
I suggested a different alternative in my reply to patch 3/7. Can you have a look at that? The idea would be that we just flatten all the structures in the ioctl implementation and make the structure definition very explicit using u32/s32/u64/s64 members with no implied padding or architecture-specific types.
And, if we have kernel_timespec (or kernel_timespec64 or such), can this deprecate the existing timespec64 usages, too? I see that timespec64 is internal only, so consolidation would be beneficial for code simplification.
Our current longterm plan is to only use __kernel_timespec on the ABI side, where we have to watch out for the tricky conversion of tv_nsec: Any timespec copied from a 32-bit process into the kernel must ignore the upper half of the nanoseconds, while copying the same structure from a 64-bit process must return an error if the 64-bit nanoseconds are larger than 999999999. When copying a timespec into user space, we have to be careful to zero the upper half of tv_nsec to avoid leaking uninitialized kernel data.
Inside of the kernel, we can ignore those constraints, so I'd keep using the timespec64. We certainly don't want to use the 64-bit nanoseconds field for internal uses on 32-bit kernels, as that would introduce expensive 64-bit arithmetic in a lot of places that don't need it.
My hope is also that we can eventually deprecate any use of the plain 'timespec' in the kernel: all internal users should migrate to timespec64 (one at a time, so we can properly review the changes), and the uapi uses should either have the 64-bit version of __kernel_timespec, or use compat_timespec once that becomes usable on 32-bit architectures.
Arnd