linux-kernel - Re: [PATCH] debugfs: keep the old valid mode value when no explicity specify it

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Date:	Fri, 29 Aug 2014 11:24:33 -0700
From:	Greg KH <gregkh@...uxfoundation.org>
To:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
Cc:	Chen LinX <linx.z.chen@...el.com>, linux-kernel@...r.kernel.org,
	"He, Bo" <bo.he@...el.com>
Subject: Re: [PATCH] debugfs: keep the old valid mode value when no explicity
 specify it

On Fri, Aug 29, 2014 at 09:30:54AM +0800, Zhang, Yanmin wrote:
> On 2014/8/28 23:10, Greg KH wrote:
> 
> >On Thu, Aug 28, 2014 at 06:09:09PM +0800, Chen LinX wrote:
> >>From: "Chen, LinX" <linx.z.chen@...el.com>
> >>
> >>When mount debugfs with no mode specifed after it's mounted, the mount
> >>point mode will change to default mode(0700) even the mount operation was fail,
> >>this will cause some issues like can't get binder info in android.
> >I don't understand, what did you do to get into this state?
> 
> Greg,
> 
> Thanks for your kind quick comments. The patch description can be improved.
> 
> We hit the issue when debugging a UIWDT issue. Android framework has a good
> method to detect userspace hang and reports UIWDT issues. Android uses
> client/server model. Clients communicates with servers by binder. binder has
> debugfs interfaces. Some files show what threads are communicating with what
> other threads. If one thread is blocked for a long time, we can find the
> blocking chain from the binder info.
> 
> Since the error dumping process has no root access, booting process changes
> debugfs mount dir mode to 0755. When UIWDT happens, the error dumping
> process can read the info.
> 
> Unfortunately, some other scripts at booting try to mount debugfs for many times.
> No matter if the double mounting fails or succeeds, debugfs_parse_options changes
> the root inode's mode back to the default 0700. It means the effect of previous
> mode changing to 0755 is lost. At UIWDT, the dumping process can't save binder
> info to disk log files.

Then fix those other scripts, don't try to work around broken userspace
scripts by changing the kernel.

Sorry, I can't take this patch.

greg k-h
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