* Miklos Szeredi:

On Mon, 4 Aug 2025 at 11:42, Florian Weimer via fuse-devel
[off-list ref] wrote:

quoted

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.

Or adding a capability flag to switch from struct fuse_write_out to
something that uses an uint64_t value.  One complication: The struct
fuse_write_out layout is too close to a potential 64-bit version of it
on little-endian systems, so that proper testing might be difficult with
the obvious approach.

quoted

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?

In-kernel file systems can check if the request originated from a compat
process, using in_compat_syscall.  I don't think that's possible over
FUSE.

Thanks,
Florian