linux-kernel - Re: [PATCH] fuse: add support for explicit export disabling

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Date: Wed, 24 Jan 2024 20:50:21 +0800
From: Jingbo Xu <jefflexu@...ux.alibaba.com>
To: Miklos Szeredi <miklos@...redi.hu>
Cc: linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
 amir73il@...il.com
Subject: Re: [PATCH] fuse: add support for explicit export disabling



On 1/24/24 8:16 PM, Miklos Szeredi wrote:
> On Wed, 24 Jan 2024 at 12:30, Jingbo Xu <jefflexu@...ux.alibaba.com> wrote:
>>
>> open_by_handle_at(2) can fail with -ESTALE with a valid handle returned
>> by a previous name_to_handle_at(2) for evicted fuse inodes, which is
>> especially common when entry_valid_timeout is 0, e.g. when the fuse
>> daemon is in "cache=none" mode.
>>
>> The time sequence is like:
>>
>>         name_to_handle_at(2)    # succeed
>>         evict fuse inode
>>         open_by_handle_at(2)    # fail
>>
>> The root cause is that, with 0 entry_valid_timeout, the dput() called in
>> name_to_handle_at(2) will trigger iput -> evict(), which will send
>> FUSE_FORGET to the daemon.  The following open_by_handle_at(2) will send
>> a new FUSE_LOOKUP request upon inode cache miss since the previous inode
>> eviction.  Then the fuse daemon may fail the FUSE_LOOKUP request with
>> -ENOENT as the cached metadata of the requested inode has already been
>> cleaned up during the previous FUSE_FORGET.  The returned -ENOENT is
>> treated as -ESTALE when open_by_handle_at(2) returns.
>>
>> This confuses the application somehow, as open_by_handle_at(2) fails
>> when the previous name_to_handle_at(2) succeeds.  The returned errno is
>> also confusing as the requested file is not deleted and already there.
>> It is reasonable to fail name_to_handle_at(2) early in this case, after
>> which the application can fallback to open(2) to access files.
>>
>> Since this issue typically appears when entry_valid_timeout is 0 which
>> is configured by the fuse daemon, the fuse daemon is the right person to
>> explicitly disable the export when required.
>>
>> Also considering FUSE_EXPORT_SUPPORT actually indicates the support for
>> lookups of "." and "..", and there are existing fuse daemons supporting
>> export without FUSE_EXPORT_SUPPORT set, for compatibility, we add a new
>> INIT flag for such purpose.
> 
> This looks good overall.
> 
>>
>> Signed-off-by: Jingbo Xu <jefflexu@...ux.alibaba.com>
>> ---
>> RFC: https://lore.kernel.org/all/20240123093701.94166-1-jefflexu@linux.alibaba.com/
>> ---
>>  fs/fuse/inode.c           | 11 ++++++++++-
>>  include/uapi/linux/fuse.h |  2 ++
>>  2 files changed, 12 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
>> index 2a6d44f91729..851940c0e930 100644
>> --- a/fs/fuse/inode.c
>> +++ b/fs/fuse/inode.c
>> @@ -1110,6 +1110,11 @@ static struct dentry *fuse_get_parent(struct dentry *child)
>>         return parent;
>>  }
>>
>> +/* only for fid encoding; no support for file handle */
>> +static const struct export_operations fuse_fid_operations = {
> 
> Nit: I'd call this fuse_no_export_operations (or something else that
> emphasizes the fact that this is only for encoding and not for full
> export support).

OK I will rename it to fuse_no_export_operations.

By the way do I need to bump and update the minor version of FUSE protocol?


-- 
Thanks,
Jingbo