linux-kernel - Re: [PATCH 00/16 v3] tracing: Add new file system tracefs

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Date:	Mon, 26 Jan 2015 19:30:49 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 00/16 v3] tracing: Add new file system tracefs

On Mon, Jan 26, 2015 at 10:09:13AM -0500, Steven Rostedt wrote:
> There has been complaints that tracing is tied too much to debugfs,
> as there are systems that would like to perform tracing, but do
> not mount debugfs for security reasons. That is because any subsystem
> may use debugfs for debugging, and these interfaces are not always
> tested for security.
> 
> Creating a new tracefs that the tracing directory will now be attached
> to allows system admins the ability to access the tracing directory
> without the need to mount debugfs.
> 
> Another advantage is that debugfs does not support the system calls
> for mkdir and rmdir. Tracing uses these system calls to create new
> instances for sub buffers. This was done by a hack that hijacked the
> dentry ops from the "instances" debugfs dentry, and replacing it with
> one that could work.
> 
> Instead of using this hack, tracefs can provide a proper interface to
> allow the tracing system to have a mkdir and rmdir feature.
> 
> To maintain backward compatibility with older tools that expect that
> the tracing directory is mounted with debugfs, the tracing directory
> is still created under debugfs and tracefs is automatically mounted
> there.
> 
> Finally, a new directory is created when tracefs is enabled called
> /sys/kernel/tracing. This will be the new location that system admins
> may mount tracefs if they are not using debugfs.

You are still fighting an inconvenient API, but now it's not debugfs one -
it's your copy thereof.  Why not give your instances/ an inode_operations
of its own?  One with ->mkdir() and ->rmdir(), leaving all other directories
as-is.  That way you don't need the secondary methods at all.  And sure,
debugfs_create_dir() grabs ->i_mutex on parent, making you drop that in
your ->mkdir() if you want to call it.  But now you are not talking to it -
just to your own code, where you are free to change the calling conventions,
making it caller's responsibility to get that ->i_mutex.  The same goes for
the rmdir side...
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