Re: [PATCH v2 3/3] fuse: add COPY_FILE_RANGE_64 that allows large copies

[Florian Weimer <fweimer@redhat.com>]

* Joanne Koong:

> On Wed, Aug 13, 2025 at 8:24 AM Miklos Szeredi <mszeredi@redhat.com> wrote:
>>
>> The FUSE protocol uses struct fuse_write_out to convey the return value of
>> copy_file_range, which is restricted to uint32_t.  But the COPY_FILE_RANGE
>> interface supports a 64-bit size copies and there's no reason why copies
>> should be limited to 32-bit.
>>
>> Introduce a new op COPY_FILE_RANGE_64, which is identical, except the
>> number of bytes copied is returned in a 64-bit value.
>>
>> If the fuse server does not support COPY_FILE_RANGE_64, fall back to
>> COPY_FILE_RANGE.
>
> Is it unacceptable to add a union in struct fuse_write_out that
> accepts a uint64_t bytes_copied?
> struct fuse_write_out {
>     union {
>         struct {
>             uint32_t size;
>             uint32_t padding;
>         };
>         uint64_t bytes_copied;
>     };
> };
>
> Maybe a little ugly but that seems backwards-compatible to me and
> would prevent needing a new FUSE_COPY_FILE_RANGE64.

Even with a capability flag, it encourages the presence of bugs that
manifest only on big-endian systems.

Thanks,
Florian