Re: [fuse-devel] copy_file_range return value on FUSE

On Mon, 4 Aug 2025 at 11:42, Florian Weimer via fuse-devel
[off-list ref] 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 copy operation.  Given that
copy_file_range is expected to clone huge files, large copies are not
unexpected, so this appears to be a real limitation.

That's a nasty oversight.  Fixing with a new FUSE_COPY_FILE_RANGE_64
op, fallback to the legacy FUSE_COPY_FILE_RANGE.

There is another wrinkle: we'd need to check if the process runs in
32-bit compat mode, and reject size_t arguments larger than INT_MAX in
this case (with EOVERFLOW presumably).  But perhaps this should be
handled on the kernel side?  Currently, this doesn't seem to happen, and
we can get copy_file_range results in the in-band error range.
Applications have no way to disambiguate this.

That's not fuse specific, right?

Thanks,
Miklos