Re: [PATCH v4 00/24] ILP32 for ARM64

Dr. Philipp Tomsich
2015-04-14T15:29:49Z

> On 14 Apr 2015, at 16:47, Catalin Marinas <catalin.marinas@arm.com> wrote:
>
>> I mainly want to avoid accidentally creating new ABIs for syscalls and ioctls:
>> we have many drivers that today use ioctls with data structures derived from
>> '__kernel_ulong_t' in some form, often by including a timespec or time_t in
>> their own data structures. These are almost all broken today, because the
>> data structures are a mix of the aarch32 and aarch64 variants, while the
>> ioctl() system call in ilp32 always uses the aarch32 format by default.
>>
>> An example here would be
>>
>> struct cyclades_idle_stats {
>> __kernel_time_t in_use; /* Time device has been in use (secs) */
>> __kernel_time_t recv_idle; /* Time since last char received (secs) */
>> __kernel_time_t xmit_idle; /* Time since last char transmitted (secs) */
>> unsigned long recv_bytes; /* Bytes received */
>> unsigned long xmit_bytes; /* Bytes transmitted */
>> unsigned long overruns; /* Input overruns */
>> unsigned long frame_errs; /* Input framing errors */
>> unsigned long parity_errs; /* Input parity errors */
>> };
>>
>> for a random ancient driver. Introducing a third set of data structures
>> and syscalls for aarch64-ilp32 means that any driver doing something like
>> this needs to be modified to support existing user space source code.
>
> That's indeed a problem as ILP32 doesn't look like any of the other
> options (the siginfo structure is another case that doesn't fit in any
> of the ABI as long as time_t is 64-bit).

I believe we’ve already arrived at the conclusion that timespec needs to be
changed from what Andrew and I had submitted.

Let’s go back to the underlying definition of timespec:
"The range and precision of times representable in clock_t and time_t are
implementation-defined. The timespec structure shall contain at least the
following members, in any order.

time_t tv_sec; // whole seconds -- >= 0
long tv_nsec; // nanoseconds -- [0, 999999999]”

So tv_nsec needs to be 32bit on ILP32, as we would otherwise break the C
language. Any program that assumes that tv_nsec is sizeof(long) would be
correct and it would be unexpected and surprising behaviour [even though it
would be consider a good programming style] if one would need to explicitly
ask for the sizeof(ts.tv_nsec). Having the same problem on x32 doesn’t seem
like a good justification to do the same.

For time_t, I don’t see the need to have a 32bit type yet.
As long as the the type is properly exposed through header files (and user
programs can thus recreate the kernel’s data model), we should be safe.

Cases like the above data structure from an ioctl are clearly non-portable
and would break today on any architecture that supports ABIs with different
data models (say ILP32 and LP64)… so I would consider any attempt to
support this as trying to remain “bug-compatible”.

There are plenty of good examples in the uapi that will be nicely portable
between ILP32 and LP64. Let’s take aio_abi.h (I’ve intentionally chosen this,
as the userspace library libaio uses a broken redefinition instead of the
kernel header file) as an example:
> /*
> * we always use a 64bit off_t when communicating
> * with userland. its up to libraries to do the
> * proper padding and aio_error abstraction
> */
>
> struct iocb {
> /* these are internal to the kernel/libc. */
> __u64 aio_data; /* data to be returned in event's data */
> __u32 PADDED(aio_key, aio_reserved1);
> /* the kernel sets aio_key to the req # */
>
> /* common fields */
> __u16 aio_lio_opcode; /* see IOCB_CMD_ above */
> __s16 aio_reqprio;
> __u32 aio_fildes;
>
> __u64 aio_buf;
> __u64 aio_nbytes;
> __s64 aio_offset;
>
> /* extra parameters */
> __u64 aio_reserved2; /* TODO: use this for a (struct sigevent *) */
>
> /* flags for the "struct iocb" */
> __u32 aio_flags;
>
> /*
> * if the IOCB_FLAG_RESFD flag of "aio_flags" is set, this is an
> * eventfd to signal AIO readiness to
> */
> __u32 aio_resfd;
> }; /* 64 bytes */

The key to any design decision should be that we
(a) don’t break C11, POSIX or the Single UNIX Specification
(b) remain true to the definitions from the the AArch64 ILP32 ELF ABI
(which defines 64bit values transferable in registers to callees)

Can we thus agree on the following for the next revision of the patch-set:
(1) We retain a 64bit time_t, but implement different sizes (between ILP32 and
LP64) for ‘tv_nsec' in 'struct timespec’?
(2) We use the 64bit system calls whereever possible (i.e. no register splitting).

Best,
Phil.