Individual file access

Nobody III hungryninja101 at ...9...
Wed Jun 21 05:45:53 CEST 2017

I've started working on the implementation, and here's my plan:
The component tree is the same as what you suggested, but fs_filter
provides a "pipe" file to send requests (in XML format). The fs_filter
component responds by spawning the Qt file picker. When the user chooses a
file, the file is added to the filesystem provided by fs_filter, and
fs_filter acknowledges the packet. The app can then read another file/pipe
that contains the name of the selected file, then open the selected file.

While this approach is more complex, it allows for large sets of file
types/extensions, which is useful for many larger applications (e.g.
LibreOffice, Gimp). It also provides the name (but not the path) of the
file chosen, which is useful when a user is working with several files,
e.g. in a text editor. Hiding the name of the file may sound nice in
theory, but seems to provide little real security benefit.

Essentially, my modifications should greatly increase usability, at the
expense of a small amount of added complexity.

What are your thoughts? Any ideas for improvements?

Also, how do I spawn and use the file picker? Is there any existing example
of using a child component like a library?

On Wed, Jun 7, 2017 at 9:46 AM, Norman Feske <norman.feske at ...1...>

> Hi Ben,
> On 07.06.2017 05:05, Nobody III wrote:
> > To start off, I'm planning on building a component that handles file
> > open/save requests for other components, primarily graphical
> > applications. My plan is to use a qt5 QFileDialog for the user to select
> > one or more files, then modify the vfs server's configuration to attach
> > the selected file(s) to the component's filesystem.
> >
> > To attach individual files to a component's filesystem, it seems like I
> > should write a server-side link vfs plugin. The plugin would be like a
> > symlink, but would transparently resolve the link within the vfs server,
> > providing access to otherwise inaccessible files. Is this possible? If
> > so, can you give me some tips on how to implement it?
> I like your idea but would pick a slightly different direction.
> Let me illustrate the scenario that I would suggest:
>                        ------------------
>                       | File picker (Qt) |
>                        ------------------
>                               ^
>                               |
>       -----              -----------               -------------
>      | App | ---uses--> | fs filter | ---uses---> | file system |
>       -----              -----------               -------------
> The application (App) does not talk directly to a file-system server but
> has a file-system session to a filter component. To open a file, the app
> would generate an arbitrary file name 'a' and open the file 'a' at the
> filter. File 'a' does not actually exist though. The filter's job is to
> provide a virtual file 'a' such that read operations are redirected to a
> real file at the file system. In contrast to the app, the filter has
> access to the real file system. To determine the real file to be exposed
> to the app as 'a', the filter spawns a file-picker component as a child.
> When the file picker requests a file-system session from its parent (the
> filter), the filter hands out a read-only session to the real file
> system. Using this session, the file picker can present the file-system
> content via a QFileDialog. Once the user selects a file, the file picker
> informs the filter via a report submitted to a 'Report' session. Now,
> the filter knows that the virtual file 'a' corresponds to the selected
> real file and can forward read operations of the app to the real file
> system.
> The case for writing would be similar.
> This approach has the following interesting properties:
> * The app does not need to know any name of the real files stored in
>   the file system.
> * The filter is transparent to existing applications that open a file
>   given as argument. For example, mupdf would request always the same
>   pdf. But thanks to the filter, it would open the pdf selected by the
>   user. The real file name remains hidden.
> * By separating the filter from the file picker, the complex Qt code
>   does not need to be ultimately trusted. It has no write access to
>   the file system. Even though it could read data, it has no way to
>   leak it. In contrast, the filter has full read/write access to the
>   file system. So it must be trusted. But in contrast to the file
>   picker, its complexity is very low. Furthermore, the design in
>   principle supports alternative implementations of the file picker
>   without the need to modify the filter.
> * The scenario does not need to reconfigure any components. In
>   particular, there is no need for the file system to trust the filter.
>   From the file system's perspective, the filter is just a regular
>   client.
> * The file picker could be killed each time after picking a file.
> * If the app supports the selection of multiple files, it could open
>   a directory instead of a file.
> * The file name invented by the app may contain hints about the file
>   types the app supports. This could be taken into account by the file
>   picker (e.g., showing only PDF files if the app requested a file
>   called 'a.pdf').
> * The filter could remember the association of the virtual files with
>   real files even after the app has closed a file. If the app saves a
>   file with a name that was previously opened, it could open the already
>   known real file instead of presenting a file picker. (thereby
>   covering the distinction between the "save" vs. "save as" use cases)
> Cheers
> Norman
> --
> Dr.-Ing. Norman Feske
> Genode Labs
> ·
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