linux-kernel - Re: Intercepting system calls

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Date:	Fri, 23 Dec 2011 22:33:45 +0530
From:	Gaurav Saxena <grvsaxena419@...il.com>
To:	John Stoffel <john@...ffel.org>
Cc:	richard -rw- weinberger <richard.weinberger@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: Intercepting system calls

Hello all , thanks for your reply.

On Fri, Dec 23, 2011 at 8:08 PM, John Stoffel <john@...ffel.org> wrote:
>>>>>> "Gaurav" == Gaurav Saxena <grvsaxena419@...il.com> writes:
>
> Gaurav> Hello John,
> Gaurav> On Thu, Dec 22, 2011 at 11:22 PM, John Stoffel <john@...ffel.org> wrote:
>>>>>>>> "Gaurav" == Gaurav Saxena <grvsaxena419@...il.com> writes:
>>>
>>>>> Intercepting system calls is very bad, don't do it.
>>>
> Gaurav> Oh. I will not do it then, could you suggest me something so
> Gaurav> that I could prevent unlink on some file say using kernel
> Gaurav> support. I want to save file on unlink how could I do that
> Gaurav> without kernel support, or how could I do that using a kernel
> Gaurav> module.
>>>
>>> Write a FUSE module to mount the filesystem through, inside your FUSE
>>> module you can intercept the unlink/truncate/write calls and decide
>>> what you want to have happen.
>
> Gaurav> I would like to write it differently for each type of filesystem ?
> Gaurav> Because what operations to call after my function depends on this.
> Gaurav> Also Can I mount "/" in this way ?
>
> Sounds like you really want to use inotify() then, but intercepting
> calls to write/truncate/unlink isn't going to be easy to handle across
> a large spectrum of filesystems without a bunch of hacking.
>
> Maybe if you can explain your requirements more clearly, with an
> example of what you're trying to accomplish, then people could help
> you more.
>
> Also, it might be more appropriate to talk to the 'fs-devel' mailing
> list.  I think it's on kernel.org, but not sure.
>
> John
>

I am trying to write an application which would create a backup for
the system so that it could be restored as it is. For example I create
a backup using my application. I just do nothing at time of backup so
it would be fast. Now whenever I see any deletion I would save that
file so that I could restore it. Also I would like to see for
modification/rename. I cannot do this using inotify as I would be
notified after actual deletion/write. I don't want to use SELinux
because I want to implement this on existing installed system. I was
earlier thinking of replacing system calls for open/unlink with my
custom calls which will call my functions before actual work and then
I would decide what to do I would also want to reject unlink request
for some of the files. But as I now know that its not working in
linux>3.0 . I had also seen dazuko which is not supporting linux>3.0
yet. Also there used to be a redirfs which used to work earlier but
the latest kernel is not supported yet. So I asked this question so
that I could get some solution/method of doing this.
I will try finding fs-devel list.
Thanks to you all for your help.

-- 
Thanks and Regards ,
Gaurav
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