[linux-cifs-client] [PATCH] cifs: add comments explaining cifs_new_fileinfo behavior

[Suresh Jayaraman]

On 05/11/2010 07:42 PM, Jeff Layton wrote:
> On Tue, 11 May 2010 18:18:08 +0530
> Suresh Jayaraman <sjayaraman at suse.de> wrote:
> 
>>>>> The comments help, but this code has bothered me for some time. Is it
>>>>> possible for the create to return success and for something else to
>>>>> happen such that the cifs_open is never called? I'd imagine that it is,
>>>>> and if so, then this this open file will be "leaked".
>>>>
>>>> I asked Shirish exactly the same question while discussing in the
>>>> #samba-technical irc channel. He does not see a leak, but thought you or
>>>> Steve will have a better idea..
>>>>
>>>> Steve:  is such a situation not possible at all?
>>>>
>>>
>>> I'm pretty sure it is possible and may even be somewhat likely.
>>> Consider this situation. vfs_create gets called from
>>> __open_namei_create. Something like this:
>>>
>>> do_last
>>>   __open_namei_create
>>>     vfs_create
>>>        inode create operation
>>>
>>> ...after this, __open_namei_create calls may_open to check permissions
>>> and can return an error. If that occurs, then I don't think the open op
>>> will ever be called.
>>>
>>> I think you can probably reproduce this by doing something like this:
>>>
>>> Have samba export a world-writable directory. Mount up the share with a
>>> user's credentials. Make sure that unix extensions are enabled. Have a
>>> different user do something like this into a file on the mount:
>>>
>>>     echo foo > /path/to/share/testfile
>>>
>>> It's probable that the file will be created, but the open-for-write
>>> permission check will fail and the open file will be left dangling.
>>>
>>
>> A quick test shows it is not leaking atleast in this case. What happens is:
>>   cifs_lookup() to the file returns NULL
>>   cifs_posix_open()
>>     CIFSPOSIXCreate() (file gets created)
>>     cifs_new_fileinfo() (updated the openFileList)
>>   lookup_instantiate_filp (gets the filep, calls cifs_open)
>>   followed by a cifs_close
>>
>> On the wire, I see SET_PATH_INFO with Level Of Interest set to Set File
>> Posix Open call followed by a Close.
>>
> 
> Hmm ok...sounds like the create on lookup stuff might be getting in the
> way of reproducing this (even though you said that cifs_lookup returned

I actually intended to mean cifs_lookup figured out file is not present.
Yes, create on lookup prevents this from occuring in this case.

> NULL). Maybe could do better reproducing this with a program that does
> an open(...O_EXCL|O_CREAT|O_WRONLY....) or something? The O_EXCL makes
> it fall through to cifs_create which doesn't do lookup_instantiate_filp.

I tried this.

open("f_creat", O_CREAT | O_WRONLY | O_TRUNC, 0644)

and

open("f_excl_creat", O_CREAT | O_WRONLY | O_EXCL, 0644)

In both the cases create on lookup comes into play.. I'm starting to
think whether cifs_create will never get called directly since it seemed
to be wrapped.

> In any case, I think the problem is valid. Clearly nothing will clean
> these up if cifs_open is never called after cifs_create is...right?
> 

Yes, absolutely that was my thinking too. But I'm yet to see a way to
crack this in practice..


Thanks,

-- 
Suresh Jayaraman