Re: [PATCH] vfs: swap names of {do,vfs}_clone_file_range()

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On Sat, Nov 03, 2018 at 06:17:09PM +0200, Amir Goldstein wrote:
On Mon, Oct 22, 2018 at 8:56 PM Amir Goldstein <amir73il@xxxxxxxxx> wrote:

commit a725356b6659469d182d662f22d770d83d3bc7b5 upstream.

Commit 031a072a0b8a ("vfs: call vfs_clone_file_range() under freeze
protection") created a wrapper do_clone_file_range() around
vfs_clone_file_range() moving the freeze protection to former, so
overlayfs could call the latter.

The more common vfs practice is to call do_xxx helpers from vfs_xxx
helpers, where freeze protecction is taken in the vfs_xxx helper, so
this anomality could be a source of confusion.

It seems that commit 8ede205541ff ("ovl: add reflink/copyfile/dedup
support") may have fallen a victim to this confusion -
ovl_clone_file_range() calls the vfs_clone_file_range() helper in the
hope of getting freeze protection on upper fs, but in fact results in
overlayfs allowing to bypass upper fs freeze protection.

Swap the names of the two helpers to conform to common vfs practice
and call the correct helpers from overlayfs and nfsd.

Signed-off-by: Amir Goldstein <amir73il@xxxxxxxxx>
Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
Fixes: 031a072a0b8a ("vfs: call vfs_clone_file_range() under freeze...")
Signed-off-by: Amir Goldstein <amir73il@xxxxxxxxx>
---

Greg,

The upstream commit (-rc8) is not a bug fix, it's a vfs API fix.
If we do not apply it to stable kernels, we are going to have a
backporting landmine, should a future fix use vfs_clone_file_range()
it will not do the same thing upstream and in stable.

I backported the change to linux-4.18.y and added the Fixes label.
I verified that there are no pathces between the Fixes commit and
current master that use {vfs,do}_clone_file_range() and could be
considered for backporting to stable (all thoses that can be considered
are already in stable).

I verified that that with this backport applies to v4.18.16 there are no
regression of quick clone tests in xfstests with overlayfs and with xfs.
Backport patch also applies cleanly to v4.14.78.


Hi Greg,

I hope this email finds you well rested... I am going to assume that your
memory has been reset, so pinging to remind you on this with some new
information.

commit 452ce65951a2 ("vfs: plumb remap flags through the vfs clone
functions") is now in master as part of a patch series by Darrick to fix
several clone_file_range() issues. I am not sure if anyone is going to
backport that series, but just in case, its best to have the vfs API fix
in stable for this or any future commits that use the vfs helpers.
Specifically, the above commit will not apply to stable without $SUBJECT
patch, so there is no risk of silent stable regression, but the risk exists for
future patches.

I tested that my backport patch cleanly to v4.14.78 and that there are no
regression of quick clone tests in xfstests with overlayfs and with xfs.

Hi Amir,

It is quite an interesting issue. Any idea on how this might affect
<4.14 kernels?

Maybe we should consider a "landmine" coccinelle script to share between
folks who deal with backports...

Anyway, I've queued this for 4.18 and 4.14. Thank you.

--
Thanks,
Sasha



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