linux-kernel - mount_pseudo(), sget() and MS

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Date:   Tue, 27 Dec 2016 11:20:37 +0100
From:   Luis Ressel <aranea@...ah.de>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: mount_pseudo(), sget() and MS_KERNMOUNT

Hello,

With Linux 4.8, the sget() function in fs/super.c got a new permission
check: It now returns -EPERM if
(!(flags & MS_KERNMOUNT) && !ns_capable(user_ns, CAP_SYS_ADMIN)) .

I presume the first half is intented to detect in-kernel mounts? If so,
why doesn't mount_pseudo() (in fs/libfs.c) pass the MS_KERNMOUNT flag
to sget()?

This behaviour has caused a problem for me: During graphics driver
initalization, drm_fs_inode_new() (in drivers/gpu/drm/drm_drv.c) calls
simple_pin_fs(). The MS_KERNMOUNT flag is indeed passed down the
call chain from there, but it is lost when mount_pseudo() is called, as
that function doesn't take a 'flags' argument.

Hence, the first part of the above permission check fails. (The second
part also fails under some cicumstances due to a SELinux quirk, and
therefore the initalization of my graphics driver doesn't succeed.)

Regards,
Luis Ressel

(Apologies if I'm sending this inquiry to the wrong people. I don't
know much about the kernel development workflow, so I decided to send
it to whomever get_maintainer.pl spat out, plus the author of the
sget() change in 4.8.)