linux-kernel - Re: [patch 3/7] fs, notify: Add file handle entry into inotify_inode

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Wed, 14 Nov 2012 14:56:00 +0400
From:	Pavel Emelyanov <xemul@...allels.com>
To:	Tvrtko Ursulin <tvrtko.ursulin@...lan.co.uk>
CC:	Cyrill Gorcunov <gorcunov@...nvz.org>,
	David Rientjes <rientjes@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Al Viro <viro@...iv.linux.org.uk>,
	Alexey Dobriyan <adobriyan@...il.com>,
	James Bottomley <jbottomley@...allels.com>,
	Matthew Helsley <matt.helsley@...il.com>,
	aneesh.kumar@...ux.vnet.ibm.com, bfields@...ldses.org
Subject: Re: [patch 3/7] fs, notify: Add file handle entry into inotify_inode_mark

>>> How much space does a typical file system need to encode a handle? Am I
>>> right that for must it is just a few bytes? (I just glanced at the code
>>> so I might be wrong.) In which case, could the handle buffer be allocated
>>> dynamically depending on the underlying filesystem? Perhaps adding a
>>> facility to query a filesystem about its maximum handle buffer needs? Do
>>> you think the saving would justify this extra work?
>>
>> Well, the MAX_HANDLE_SZ is taken from NFSv4 and is 128 bytes which is quite
>> big for inotify extension indeed. The good news is that this amount of bytes
>> seem to be required for the most descriptive fhandle -- with info about
>> parent, etc. We don't need such, we can live with shorter handle, people
>> said that 40 bytes was enough for that.
>>
>> However, your idea about determining the handle size dynamically seems
>> promising. As far as I can see from the code we can call for encode_fh with
>> size equals zero and filesystem would report back the amount of bytes it
>> requires for a handle.
>>
>> We can try going this route, what do you think?
> 
> Sounds much better since that would only add one pointer to the watch 
> structure in the normal case.
> 
> Also at checkpoint time it will use only a few bytes (compared to 64) for the 
> encode buffer for most filesystems. This part is probably not that important 
> but still a win.

No, the thing is -- we need to know the handle _before_ we start checkpoint.
More exactly -- at the time the inotify_add_watch is called. So the memory save
would be not that big.

> Regards,
> 
> Tvrtko

Thanks,
Pavel

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/