failed open: No data available

Fri Dec 18 22:55:03 UTC 2020

On Fri, Dec 18, 2020 at 3:50 PM Michael Labriola
<michael.d.labriola at gmail.com> wrote:
> On Fri, Dec 18, 2020 at 2:02 AM Amir Goldstein <amir73il at gmail.com> wrote:
> > On Fri, Dec 18, 2020 at 1:47 AM Michael Labriola
> > <michael.d.labriola at gmail.com> wrote:
> > >
> > > On Thu, Dec 17, 2020 at 4:56 PM Michael Labriola
> > > <michael.d.labriola at gmail.com> wrote:
> > > >
> > > > On Thu, Dec 17, 2020 at 3:25 PM Amir Goldstein <amir73il at gmail.com> wrote:
> > > > >
> > > > > On Thu, Dec 17, 2020 at 9:46 PM Michael Labriola
> > > > > <michael.d.labriola at gmail.com> wrote:
> > > > > >
> > > > > > On Thu, Dec 17, 2020 at 1:07 PM Amir Goldstein <amir73il at gmail.com> wrote:
> > > > > > >
> > > > > > > On Thu, Dec 17, 2020 at 6:22 PM Michael Labriola
> > > > > > *snip*
> > > > > > > > On Thu, Dec 17, 2020 at 7:00 AM Amir Goldstein <amir73il at gmail.com> wrote:
> > > > > > > > Thanks, Amir.  I didn't have CONFIG_DYNAMIC_DEBUG enabled, so
> > > > > > >
> > > > > > > I honestly don't expect to find much in the existing overlay debug prints
> > > > > > > but you never know..
> > > > > > > I suspect you will have to add debug prints to find the problem.
> > > > > >
> > > > > > Ok, here goes.  I had to setup a new virtual machine that doesn't use
> > > > > > overlayfs for its root filesystem because turning on dynamic debug
> > > > > > gave way too much output for a nice controlled test.  It's exhibiting
> > > > > > the same behavior as my previous tests (5.8 good, 5.9 bad).  The is
> > > > > > with a freshly compiled 5.9.15 w/ CONFIG_OVERLAY_FS_XINO_AUTO turned
> > > > > > off and CONFIG_DYNAMIC_DEBUG turned on.  Here's what we get:
> > > > > >
> > > > > >  echo "file fs/overlayfs/*  +p" > /sys/kernel/debug/dynamic_debug/control
> > > > > >  mount borky2.sqsh t
> > > > > >  mount -t tmpfs tmp tt
> > > > > >  mkdir -p tt/upper/{upper,work}
> > > > > >  mount -t overlay -o \
> > > > > >     lowerdir=t,upperdir=tt/upper/upper,workdir=tt/upper/work blarg ttt
> > > > > > [  164.505193] overlayfs: mkdir(work/work, 040000) = 0
> > > > > > [  164.505204] overlayfs: tmpfile(work/work, 0100000) = 0
> > > > > > [  164.505209] overlayfs: create(work/#3, 0100000) = 0
> > > > > > [  164.505210] overlayfs: rename(work/#3, work/#4, 0x4)
> > > > > > [  164.505216] overlayfs: unlink(work/#3) = 0
> > > > > > [  164.505217] overlayfs: unlink(work/#4) = 0
> > > > > > [  164.505221] overlayfs: setxattr(work/work,
> > > > > > "trusted.overlay.opaque", "0", 1, 0x0) = 0
> > > > > >
> > > > > >  touch ttt/FOO
> > > > > > touch: cannot touch 'ttt/FOO': No data available
> > > > > > [  191.919498] overlayfs: setxattr(upper/upper,
> > > > > > "trusted.overlay.impure", "y", 1, 0x0) = 0
> > > > > > [  191.919523] overlayfs: tmpfile(work/work, 0100644) = 0
> > > > > > [  191.919788] overlayfs: tmpfile(work/work, 0100644) = 0
> > > > > >
> > > > > > That give you any hints?  I'll start reading through the overlayfs
> > > > > > code.  I've never actually looked at it, so I'll be planting printk
> > > > > > calls at random.  ;-)
> > > > >
> > > > > We have seen that open("FOO", O_WRONLY) fails
> > > > > We know that FOO is lower at that time so that brings us to
> > > > >
> > > > > ovl_open
> > > > >   ovl_maybe_copy_up
> > > > >     ovl_copy_up_flags
> > > > >       ovl_copy_up_one
> > > > >         ovl_do_copy_up
> > > > >           ovl_set_impure
> > > > > [  191.919498] overlayfs: setxattr(upper/upper,
> > > > > "trusted.overlay.impure", "y", 1, 0x0) = 0
> > > > >           ovl_copy_up_tmpfile
> > > > >             ovl_do_tmpfile
> > > > > [  191.919523] overlayfs: tmpfile(work/work, 0100644) = 0
> > > > >             ovl_copy_up_inode
> > > > > This must be were we fail and likely in:
> > > > >               ovl_copy_xattr
> > > > >                  vfs_getxattr
> > > > > which can return -ENODATA, but it is not expected because the
> > > > > xattrs returned by vfs_listxattr should exist...
> > > > >
> > > > > So first guess would be to add a debug print for xattr 'name'
> > > > > and return value of vfs_getxattr().
> > > >
> > > > Ok, here we go.  I've added a bunch of printks all over the place.
> > > > Here's what we've got.  Things are unchanged during mount.  Trying to
> > > > touch FOO now gives me this:
> > > >
> > > > [  114.365444] ovl_open: start
> > > > [  114.365450] ovl_maybe_copy_up: start
> > > > [  114.365452] ovl_maybe_copy_up: need copy up
> > > > [  114.365454] ovl_maybe_copy_up: ovl_want_write succeeded
> > > > [  114.365459] ovl_copy_up_one: calling ovl_do_copy_up()
> > > > [  114.365460] ovl_do_copy_up: start
> > > > [  114.365462] ovl_do_copy_up: impure
> > > > [  114.365464] ovl_set_impure: start
> > > > [  114.365484] overlayfs: setxattr(upper/upper,
> > > > "trusted.overlay.impure", "y", 1, 0x0) = 0
> > > > [  114.365486] ovl_copy_up_tmpfile: start
> > > > [  114.365507] overlayfs: tmpfile(work/work, 0100644) = 0
> > > > [  114.365510] ovl_copy_up_inode: start
> > > > [  114.365511] ovl_copy_up_inode: ISREG && !metacopy
> > > > [  114.365625] ovl_copy_xattr: start
> > > > [  114.365630] ovl_copy_xattr: vfs_listxattr() returned 17
> > > > [  114.365632] ovl_copy_xattr: buf allocated good
> > > > [  114.365634] ovl_copy_xattr: vfs_listxattr() returned 17
> > > > [  114.365636] ovl_copy_xattr: slen=17
> > > > [  114.365638] ovl_copy_xattr: name='security.selinux'
> > >
> > > SELinux?  now that's not suspicious at all...
> > >
> > > > [  114.365643] ovl_copy_xattr: vfs_getxattr returned size=-61
> > > > [  114.365644] ovl_copy_xattr: cleaning up
> > > > [  114.365647] ovl_copy_up_inode: ovl_copy_xattr error=-61
> > > > [  114.365649] ovl_copy_up_one: error=-61
> > > > [  114.365651] ovl_copy_up_one: calling ovl_copy_up_end()
> > > > [  114.365653] ovl_copy_up_flags: ovl_copy_up_one error=-61
> > > > [  114.365655] ovl_maybe_copy_up: ovl_copy_up_flags error=-61
> > > > [  114.365658] ovl_open: ovl_maybe_copy_up error=-61
> > > > [  114.365728] ovl_copy_up_one: calling ovl_do_copy_up()
> > > > [  114.365730] ovl_do_copy_up: start
> > > > [  114.365731] ovl_do_copy_up: impure
> > > > [  114.365733] ovl_set_impure: start
> > > > [  114.365735] ovl_copy_up_tmpfile: start
> > > > [  114.365748] overlayfs: tmpfile(work/work, 0100644) = 0
> > > > [  114.365750] ovl_copy_up_inode: start
> > > > [  114.365752] ovl_copy_up_inode: ISREG && !metacopy
> > > > [  114.365770] ovl_copy_xattr: start
> > > > [  114.365773] ovl_copy_xattr: vfs_listxattr() returned 17
> > > > [  114.365774] ovl_copy_xattr: buf allocated good
> > > > [  114.365776] ovl_copy_xattr: vfs_listxattr() returned 17
> > > > [  114.365778] ovl_copy_xattr: slen=17
> > > > [  114.365780] ovl_copy_xattr: name='security.selinux'
> > > > [  114.365784] ovl_copy_xattr: vfs_getxattr returned size=-61
> > > > [  114.365785] ovl_copy_xattr: cleaning up
> > > > [  114.365787] ovl_copy_up_inode: ovl_copy_xattr error=-61
> > > > [  114.365789] ovl_copy_up_one: error=-61
> > > > [  114.365790] ovl_copy_up_one: calling ovl_copy_up_end()
> > > > [  114.365792] ovl_copy_up_flags: ovl_copy_up_one error=-61
> > > >
> > > *snip*
> > >
> > > So, the selinux stuff made me raise an eyebrow...  I've got selinux
> > > enabled in my kernel so that it's there if I boot up a RHEL box with
> > > this kernel.  But I'm using Ubuntu right now, and the rest of SELinux
> > > is not installed/enabled.  There shouldn't be any selinux labels in
> > > the files I slurped up into my squashfs image, so there shouldn't be
> > > any in the squashfs, so of course that won't work.
> > >
> > > I tried compiling CONFIG_SELINUX=n and guess what, it works now.  So
> > > that's at least a work-around for me.
> > >
> > > So, for whatever reason, between 5.8 and 5.9, having CONFIG_SELINUX=y
> > > but no security labels on the filesystem became a problem?  Is this
> > > something that needs to get fixed in overlayfs?  Or do you think it's
> > > a deeper problem that needs fixing elsewhere?
> > >
> >
> > It's both :)
> >
> > Attached two patches that should each fix the issue independently,
> > but we need to apply both. I only tested that they build.
> > Please verify that each applied individually solves the problem.
> >
> > The selinux- patch fixes an selinux regression introduced in kernel v5.9
> > the regression is manifested in your test case but goes beyond overlayfs.
> >
> > The ovl- patch is a workaround for the selinux regression, but it is also
> > a micro optimization that doesn't hurt, so worth applying it anyway.
>
> Ok, as expected, both patches independently fix the problem for me on
> my 5.9 kernel.  FYI, applying the ovl patch failed initially
> because ovl_is_private_xattr() grew an extra argument in 5.10.
>
> Woohoo!  Thanks, Amir!

Hi Amir, thanks for the patches but would you mind sending them to the
list not as attachments so we can review/comment/merge like we
normally do?

-- 
paul moore
www.paul-moore.com