On Thu, Aug 14, 2025 at 10:05 AM Darrick J. Wong <djwong@xxxxxxxxxx> wrote:
>
> On Wed, Aug 13, 2025 at 10:03:17AM -0700, Joanne Koong wrote:
> > On Wed, Aug 13, 2025 at 8:24 AM Miklos Szeredi <mszeredi@xxxxxxxxxx> 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.
>
> I wonder, does fuse_args::out_argvar==1 imply that you could create a
> new 64-bit fuse_write64_out:
>
> struct fuse_write64_out {
> uint64_t size;
> uint64_t padding;
> };
>
> and then fuse_copy_file_range declares a union:
>
> union fuse_cfr_out {
> struct fuse_write_out out;
> struct fuse_write64_out out64;
> };
>
> passes that into fuse_args:
>
> union fuse_cfr_out outarg;
>
> args.out_argvar = 1;
> args.out_numargs = 1;
> args.out_args[0].size = sizeof(outarg);
> args.out_args[0].value = &outarg;
>
> and then we can switch on the results:
>
> if (args.out_args[0].size == sizeof(fuse_write64_out))
> /* 64-bit return */
> else if (args.out_args[0].size == sizeof(fuse_write_out))
> /* 32-bit return */
> else
> /* error */
>
> I guess the problem is that userspace has to know that the kernel will
> accept a fuse_write64_out, because on an old kernel it'll get -EINVAL
> and ... then what? I think an error return ends the request and the
> fuse server can't just try again with fuse_write_out.
>
I think this would also need the feature flag sent in the init call
which Miklos didn't like
> <shrug> Maybe I'm speculating stupi^Wwildly. ;)
>
> --D
>
