Re: [PATCH v2 29/29] y2038: add 64-bit time_t syscalls to all 32-bit architectures

[Andy Lutomirski <luto@kernel.org>]

On Fri, Jan 18, 2019 at 8:25 AM Arnd Bergmann <arnd@arndb.de> wrote:
>
> This adds 21 new system calls on each ABI that has 32-bit time_t
> today. All of these have the exact same semantics as their existing
> counterparts, and the new ones all have macro names that end in 'time64'
> for clarification.
>
> This gets us to the point of being able to safely use a C library
> that has 64-bit time_t in user space. There are still a couple of
> loose ends to tie up in various areas of the code, but this is the
> big one, and should be entirely uncontroversial at this point.
>
> In particular, there are four system calls (getitimer, setitimer,
> waitid, and getrusage) that don't have a 64-bit counterpart yet,
> but these can all be safely implemented in the C library by wrapping
> around the existing system calls because the 32-bit time_t they
> pass only counts elapsed time, not time since the epoch. They
> will be dealt with later.
>
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
> The one point that still needs to be agreed on is the actual
> number assignment. Following the earlier patch that added
> the sysv IPC calls with common numbers where possible, I also
> tried the same here, using consistent numbers on all 32-bit
> architectures.
>
> There are a couple of minor issues with this:
>
> - On asm-generic, we now leave the numbers from 295 to 402
>   unassigned, which wastes a small amount of kernel .data
>   segment. Originally I had asm-generic start at 300 and
>   everyone else start at 400 here, which was also not
>   perfect, and we have gone beyond 400 already, so I ended
>   up just using the same numbers as the rest here.
>
> - Once we get to 512, we clash with the x32 numbers (unless
>   we remove x32 support first), and probably have to skip
>   a few more. I also considered using the 512..547 space
>   for 32-bit-only calls (which never clash with x32), but
>   that also seems to add a bit of complexity.

I have a patch that I'll send soon to make x32 use its own table.  As
far as I'm concerned, 547 is *it*.  548 is just a normal number and is
not special.  But let's please not reuse 512..547 for other purposes
on x86 variants -- that way lies even more confusion, IMO.

--Andy