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

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Message-ID: <87ike6d4vx.fsf@wotan.olymp>
Date: Tue, 16 Dec 2025 11:48:18 +0000
From: Luis Henriques <luis@...lia.com>
To: Bernd Schubert <bschubert@....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 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?

Regarding the comment in fuse_inode_handle_alloc(), I believe I'll need to
rethink about it anyway, specially after some of the comments I've already
seen from Miklos (which I'm still going through).

Cheers,
-- 
Luís