linux-kernel - Re: REGRESSION?: debugfs: inode: debugfs_create

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Date:   Tue, 12 Jun 2018 15:52:19 -0700
From:   Kees Cook <keescook@...omium.org>
To:     John Stultz <john.stultz@...aro.org>,
        Laura Abbott <labbott@...hat.com>
Cc:     Thomas Richter <tmricht@...ux.ibm.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: REGRESSION?: debugfs: inode: debugfs_create_dir uses mode
 permission from parent

On Tue, Jun 12, 2018 at 3:40 PM, John Stultz <john.stultz@...aro.org> wrote:
> Hey all,
>   I noticed recently that linus/master (plus patches) stopped booting
> to UI on HiKey960, and I bisected the issue down to:
> 92170b62f1c1 ("debugfs: inode: debugfs_create_dir uses mode permission
> from parent")
>
> On the HiKey960 board, we mount debugfs via:
>    mount debugfs /sys/kernel/debug /sys/kernel/debug mode=755
>
> But since the change, it seems most of the nodes in /sys/kernel/debug
> are: drwx------
>
> Which ends up breaking the egl library, keeping it from loading.
>
> This seems to be the expected behavior of the patch, but I'm surprised
> the mode override is now ignored. It seems like the potential fix here
> would be to set the /sys/kernel/debug dir to 755 prior to mounting? Or
> is there some other advised solution?

This change has already be reverted from other people reporting the
problem. Perhaps the root cause is the lack of propagation of the
mode= value?

-Kees

-- 
Kees Cook
Pixel Security