linux-kernel - Re: [RFC PATCH v2 4/6] fuse: implementation of the FUSE_LOOKUP

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Message-ID: <5d08a442-fd74-432f-a5da-4fa9db65e815@ddn.com>
Date: Wed, 17 Dec 2025 16:02:58 +0100
From: Bernd Schubert <bschubert@....com>
To: Luis Henriques <luis@...lia.com>
Cc: Miklos Szeredi <miklos@...redi.hu>, Amir Goldstein <amir73il@...il.com>,
 "Darrick J. Wong" <djwong@...nel.org>, Kevin Chen <kchen@....com>,
 Horst Birthelmer <hbirthelmer@....com>,
 "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 Matt Harvey <mharvey@...ptrading.com>,
 "kernel-dev@...lia.com" <kernel-dev@...lia.com>
Subject: Re: [RFC PATCH v2 4/6] fuse: implementation of the FUSE_LOOKUP_HANDLE
 operation



On 12/16/25 12:48, Luis Henriques wrote:
> On Mon, Dec 15 2025, Bernd Schubert wrote:
> 
>> On 12/12/25 19:12, Luis Henriques wrote:
>>> The implementation of LOOKUP_HANDLE modifies the LOOKUP operation to include
>>> an extra inarg: the file handle for the parent directory (if it is
>>> available).  Also, because fuse_entry_out now has a extra variable size
>>> struct (the actual handle), it also sets the out_argvar flag to true.
>>>
>>> Most of the other modifications in this patch are a fallout from these
>>> changes: because fuse_entry_out has been modified to include a variable size
>>> struct, every operation that receives such a parameter have to take this
>>> into account:
>>>
>>>    CREATE, LINK, LOOKUP, MKDIR, MKNOD, READDIRPLUS, SYMLINK, TMPFILE
>>>
>>> Signed-off-by: Luis Henriques <luis@...lia.com>
>>> ---
>>>   fs/fuse/dev.c             | 16 +++++++
>>>   fs/fuse/dir.c             | 87 ++++++++++++++++++++++++++++++---------
>>>   fs/fuse/fuse_i.h          | 34 +++++++++++++--
>>>   fs/fuse/inode.c           | 69 +++++++++++++++++++++++++++----
>>>   fs/fuse/readdir.c         | 10 ++---
>>>   include/uapi/linux/fuse.h |  8 ++++
>>>   6 files changed, 189 insertions(+), 35 deletions(-)
>>>
>>> diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
>>> index 629e8a043079..fc6acf45ae27 100644
>>> --- a/fs/fuse/dev.c
>>> +++ b/fs/fuse/dev.c
>>> @@ -606,6 +606,22 @@ static void fuse_adjust_compat(struct fuse_conn *fc, struct fuse_args *args)
>>>   	if (fc->minor < 4 && args->opcode == FUSE_STATFS)
>>>   		args->out_args[0].size = FUSE_COMPAT_STATFS_SIZE;
>>>   
>>> +	if (fc->minor < 45) {
>>
>> Could we use fc->lookup_handle here? Numbers are hard with backports
> 
> To be honest, I'm not sure this code is correct.  I just followed the
> pattern.  I'll need to dedicate some more time looking into this,
> specially because the READDIRPLUS op handling is still TBD.
> 
> <snip>
> 
>>> @@ -505,6 +535,30 @@ struct inode *fuse_iget(struct super_block *sb, u64 nodeid,
>>>   	if (!inode)
>>>   		return NULL;
>>>   
>>> +	fi = get_fuse_inode(inode);
>>> +	if (fc->lookup_handle) {
>>> +		if ((fh == NULL) && (nodeid != FUSE_ROOT_ID)) {
>>> +			pr_err("NULL file handle for nodeid %llu\n", nodeid);
>>> +			iput(inode);
>>> +			return NULL;
>>
>> Hmm, so there are conditions like "if (fi && fi->fh) {" in lookup and I
>> was thinking "nice, fuse-server can decide to skip the fh for some
>> inodes like FUSE_ROOT_ID. But now it gets forbidden here. In combination
>> with the other comment in fuse_inode_handle_alloc(), could be allocate
>> here to the needed size and allow fuse-server to not send the handle
>> for some files?
> 
> I'm not sure the code is consistent with this regard, but here I'm doing
> exactly that: allowing the fh to be NULL only for FUSE_ROOT_ID.  Or did I
> misunderstood your comment?

Sorry for late reply.

Yeah sorry, what I meant is that the file handle size might be different
for any of the inodes, in between 0 and max-size for any of the inodes?


Thanks,
Bernd