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* opening a file on a stacked overlayfs is broken.
@ 2016-11-25 14:56 Quentin Casasnovas
  2016-11-25 17:09 ` [PATCH] ovl: tentative fix for broken vfs_open() on stacked overlayfs Quentin Casasnovas
  0 siblings, 1 reply; 8+ messages in thread
From: Quentin Casasnovas @ 2016-11-25 14:56 UTC (permalink / raw)
  To: Miklos Szeredi; +Cc: lkml, Al Viro

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Hi,

Stacking an overlayfs on top of an overlayfs doens't work when it used to
(tested on v4.9-rc5):

  #!/bin/bash -xeu
  tmpdir=$(mktemp -d)
  pushd ${tmpdir}

  mkdir -p {upper,lower,work}
  echo 'foo' > lower/bar
  mount -t overlay level_zero upper -o lowerdir=lower,upperdir=upper,workdir=work
  cat upper/bar

  tmpdir2=$(mktemp -d)
  pushd ${tmpdir2}

  mkdir -p {upper,work}
  mount -t overlay level_one upper -o lowerdir=${tmpdir}/upper,upperdir=upper,workdir=work
  stat upper/bar  # Works fine
  cat upper/bar   # open() returns ENXIO

I _think_ (I haven't bisected it) the guilty commit is 2d902671ce1c ("vfs:
merge .d_select_inode() into .d_real()"), where vfs_open() -> uses d_real()
with a a NULL inode, which prevents any recursion from happening, when
before d_select_inode() would do the right thing and recurse.

It should be noted this has already been broken and fixed in the past year
in 1c8a47df36d7 ("ovl: fix open in stacked overlay").

Let me know if I can help,
Q

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^ permalink raw reply	[flat|nested] 8+ messages in thread
* Re: [PATCH] ovl: tentative fix for broken vfs_open() on stacked overlayfs.
@ 2016-11-29  9:32 Miklos Szeredi
  2016-11-29 10:03 ` Amir Goldstein
  2016-12-01 16:20 ` Quentin Casasnovas
  0 siblings, 2 replies; 8+ messages in thread
From: Miklos Szeredi @ 2016-11-29  9:32 UTC (permalink / raw)
  To: Quentin Casasnovas; +Cc: linux-kernel, linux-unionfs, Al Viro

On Mon, Nov 28, 2016 at 12:06:09PM +0100, Quentin Casasnovas wrote:

> > > > But it looks like it was re-introduced in:
> > > > 
> > > >   2d902671ce1c ("vfs: merge .d_select_inode() into .d_real()")

Here's a slightly different patch.  It should work exactly the same, but the
error handling is hopefully less broken.

Thanks,
Miklos
---

From: Miklos Szeredi <mszeredi@redhat.com>
Subject: ovl: fix d_real() for stacked fs

Handling of recursion in d_real() is completely broken.  Recursion is only
done in the 'inode != NULL' case.  But when opening the file we have
'inode == NULL' hence d_real() will return an overlay dentry.  This won't
work since overlayfs doesn't define its own file operations, so all file
ops will fail.

Fix by doing the recursion first and the check against the inode second.

Bash script to reproduce the issue written by Quentin:

 - 8< - - - - - 8< - - - - - 8< - - - - - 8< - - - -
tmpdir=$(mktemp -d)
pushd ${tmpdir}

mkdir -p {upper,lower,work}
echo -n 'rocks' > lower/ksplice
mount -t overlay level_zero upper -o lowerdir=lower,upperdir=upper,workdir=work
cat upper/ksplice

tmpdir2=$(mktemp -d)
pushd ${tmpdir2}

mkdir -p {upper,work}
mount -t overlay level_one upper -o lowerdir=${tmpdir}/upper,upperdir=upper,workdir=work
ls -l upper/ksplice
cat upper/ksplice
 - 8< - - - - - 8< - - - - - 8< - - - - - 8< - - - - 

Reported-by: Quentin Casasnovas <quentin.casasnovas@oracle.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Fixes: 2d902671ce1c ("vfs: merge .d_select_inode() into .d_real()")
Cc: <stable@vger.kernel.org> # v4.8+
---
 fs/overlayfs/super.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -328,11 +328,11 @@ static struct dentry *ovl_d_real(struct
 	if (!real)
 		goto bug;
 
+	/* Handle recursion */
+	real = d_real(real, inode, open_flags);
+
 	if (!inode || inode == d_inode(real))
 		return real;
-
-	/* Handle recursion */
-	return d_real(real, inode, open_flags);
 bug:
 	WARN(1, "ovl_d_real(%pd4, %s:%lu): real dentry not found\n", dentry,
 	     inode ? inode->i_sb->s_id : "NULL", inode ? inode->i_ino : 0);

^ permalink raw reply	[flat|nested] 8+ messages in thread
end of thread, other threads:[~2016-12-01 16:16 UTC | newest]

Thread overview: 8+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2016-11-25 14:56 opening a file on a stacked overlayfs is broken Quentin Casasnovas
2016-11-25 17:09 ` [PATCH] ovl: tentative fix for broken vfs_open() on stacked overlayfs Quentin Casasnovas
2016-11-25 19:28   ` Quentin Casasnovas
2016-11-28  9:45     ` Miklos Szeredi
2016-11-28 11:06       ` Quentin Casasnovas
2016-11-29  9:32 Miklos Szeredi
2016-11-29 10:03 ` Amir Goldstein
2016-12-01 16:20 ` Quentin Casasnovas

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