Re: [RFC PATCH v2 4/6] fuse: implementation of the FUSE_LOOKUP_HANDLE operation

[Luis Henriques <luis@igalia.com>]

On Fri, Jan 09 2026, Miklos Szeredi wrote:

> On Fri, 9 Jan 2026 at 12:57, Luis Henriques <luis@igalia.com> wrote:
>
>> I've been trying to wrap my head around all the suggested changes, and
>> experimenting with a few options.  Since there are some major things that
>> need to be modified, I'd like to confirm that I got them right:
>>
>> 1. In the old FUSE_LOOKUP, the args->in_args[0] will continue to use the
>>    struct fuse_entry_out, which won't be changed and will continue to have
>>    a static size.
>
> Yes.
>
>> 2. FUSE_LOOKUP_HANDLE will add a new out_arg, which will be dynamically
>>    allocated (using your suggestion: 'args->out_var_alloc').  This will be
>>    a new struct fuse_entry_handle_out, similar to fuse_entry_out, but
>>    replacing the struct fuse_attr by a struct fuse_statx, and adding the
>>    file handle struct.
>
> Another idea: let's simplify the interface by removing the attributes
> from the lookup reply entirely.  To get back the previous
> functionality, compound requests can be used: LOOKUP_HANDLE + STATX.

OK, interesting idea.  So, in that case we would have:

struct fuse_entry_handle_out {
	uint64_t nodeid;
	uint64_t generation;
	uint64_t entry_valid;
	struct fuse_file_handle fh;
}

I'll then need to have a look at the compound requests closely. (I had
previously skimmed through the patches that add open+getattr but didn't
gone too deep into it.)

>> 3. FUSE_LOOKUP_HANDLE will use the args->in_args[0] as an extension header
>
> No, extensions go after the regular request data: headers, payload,
> extension(s).
>
> We could think about changing that for uring, where it would make
> sense to put the extensions after the regular headers, but currently
> it doesn't work that way and goes into the payload section.
>
> In any case LOOKUP_HANDLE should follow the existing practice.

Hmm... I _think_ I had it right in my head, but totally messed up my text.
English is hard.  What I meant was that args->ext_idx would be set to the
in_args[] index that would describe the extension (for the lookup
operation, that would be '3', not '0' as I mentioned above).

And then the extension header would be created similarly to what's being
done for FUSE_EXT_GROUPS, using the same helper extend_arg().  That way, I
think we would have: headers - payload - extensions.

>>    (FUSE_EXT_HANDLE).  Note that other operations (e.g. those in function
>>    create_new_entry()) will actually need to *add* an extra extension
>>    header, as extension headers are already being used there.
>
> Right.
>
>>    This extension header will use the new struct fuse_entry_handle_out.
>
> Why _out?
>
> It should just be a struct fuse_ext_header followed by a struct
> fuse_file_handle.

Yes, of course.  My English was totally messed-up.  And I meant
'fuse_file_handle', not 'fuse_entry_handle_out'.

Cheer,
-- 
Luís