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From: Miklos Szeredi <miklos@szeredi.hu>
To: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Colin Walters <walters@verbum.org>,
	syzbot <syzbot+d6ec23007e951dadf3de@syzkaller.appspotmail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-mm <linux-mm@kvack.org>,
	Miklos Szeredi <mszeredi@redhat.com>,
	syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
	Al Viro <viro@zeniv.linux.org.uk>
Subject: Re: kernel BUG at mm/hugetlb.c:LINE!
Date: Wed, 20 May 2020 13:20:27 +0200	[thread overview]
Message-ID: <CAJfpegsNVB12MQ-Jgbb-f=+i3g0Xy52miT3TmUAYL951HVQS_w@mail.gmail.com> (raw)
In-Reply-To: <9a56a79a-88ed-9ff4-115e-ec169cba5c0b@oracle.com>

On Tue, May 19, 2020 at 2:35 AM Mike Kravetz <mike.kravetz@oracle.com> wrote:
>
> On 5/18/20 4:41 PM, Colin Walters wrote:
> >
> > On Tue, May 12, 2020, at 11:04 AM, Miklos Szeredi wrote:
> >
> >>> However, in this syzbot test case the 'file' is in an overlayfs filesystem
> >>> created as follows:
> >>>
> >>> mkdir("./file0", 000)                   = 0
> >>> mount(NULL, "./file0", "hugetlbfs", MS_MANDLOCK|MS_POSIXACL, NULL) = 0
> >>> chdir("./file0")                        = 0
> >>> mkdir("./file1", 000)                   = 0
> >>> mkdir("./bus", 000)                     = 0
> >>> mkdir("./file0", 000)                   = 0
> >>> mount("\177ELF\2\1\1", "./bus", "overlay", 0, "lowerdir=./bus,workdir=./file1,u"...) = 0
> >
> > Is there any actual valid use case for mounting an overlayfs on top of hugetlbfs?  I can't think of one.  Why isn't the response to this to instead only allow mounting overlayfs on top of basically a set of whitelisted filesystems?
> >
>
> I can not think of a use case.  I'll let Miklos comment on adding whitelist
> capability to overlayfs.

I've not heard of overlayfs being used over hugetlbfs.

Overlayfs on tmpfs is definitely used, I guess without hugepages.
But if we'd want to allow tmpfs _without_ hugepages but not tmpfs
_with_ hugepages, then we can't just whitelist based on filesystem
type, but need to look at mount options as well.  Which isn't really a
clean solution either.

> IMO - This BUG/report revealed two issues.  First is the BUG by mmap'ing
> a hugetlbfs file on overlayfs.  The other is that core mmap code will skip
> any filesystem specific get_unmapped area routine if on a union/overlay.
> My patch fixes both, but if we go with a whitelist approach and don't allow
> hugetlbfs I think we still need to address the filesystem specific
> get_unmapped area issue.  That is easy enough to do by adding a routine to
> overlayfs which calls the routine for the underlying fs.

I think the two are strongly related:  get_unmapped_area() adjusts the
address alignment, and the is_file_hugepages() call in
ksys_mmap_pgoff() adjusts the length alignment.

Is there any other purpose for which  f_op->get_unmapped_area() is
used by a filesystem?

Thanks,
Miklos

  reply	other threads:[~2020-05-20 11:20 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-04-06  3:06 syzbot
2020-04-06 22:05 ` Mike Kravetz
2020-05-12 15:04   ` Miklos Szeredi
2020-05-12 18:11     ` Mike Kravetz
2020-05-15 22:15       ` Mike Kravetz
2020-05-18 11:12         ` Miklos Szeredi
2020-05-18 23:22           ` Mike Kravetz
2020-05-18 23:41     ` Colin Walters
2020-05-19  0:35       ` Mike Kravetz
2020-05-20 11:20         ` Miklos Szeredi [this message]
2020-05-20 17:27           ` Mike Kravetz
2020-05-22 10:05             ` Miklos Szeredi
2020-05-28  0:01               ` Mike Kravetz
2020-05-28  8:37                 ` [PATCH v2] ovl: provide real_file() and overlayfs get_unmapped_area() kbuild test robot
2020-05-28 21:01                   ` Mike Kravetz
2020-06-04  9:16                     ` Miklos Szeredi
2020-06-11  0:13                       ` Mike Kravetz
2020-06-11  0:37                         ` Al Viro
2020-06-11  1:36                           ` Matthew Wilcox
2020-06-11  2:17                             ` Al Viro
2020-06-11  2:31                               ` Mike Kravetz

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