Re: [PATCH 0/7] overlay filesystem: request for inclusion

From: Miklos Szeredi
Date: Thu Jun 16 2011 - 12:08:37 EST

"J. R. Okajima" <hooanon05@xxxxxxxxxxx> writes:

>> over the years developing a standalone stackable file system based =
>> approach. These approaches were rejected largely due to their =
> :::
>> location for this functionality. There is some merit to a VFS based =
>> approach: unioning performs a fair amount of namespace manipulation =
>> (merging directories, eliminating duplications, whiteouts and opaques, =
>> etc.), and the VFS is often best suited for complex namespace =
>> operations.
> Exactly.
> I understand everybody likes simpler patch, and I have no objection to
> merge UnionMount into mainline. But this union-type-mount approach has
> some demerit which I have posted before. Those are inherited by
> overlayfs too, and Miklos called it "unPOSIXy behavior". I think the
> most part of the cause of these behaviour came from its design or
> architecture.

Yes, overlayfs shares some of the basic architecture of union-mounts.
The most important such property is that when a file is copied up, it's
like replacing the file with a new one:

cp /foo/bar /tmp/ttt
mv -f /tmp/ttt /foo/bar

Which is exactly the thing that some editors do when saving a modified
file, so most applications should handle this behavior fine. The truth
is a bit more complicated and the effect of the copy-up is more like

cp /foo/bar /tmp/ttt
mount --bind /tmp/ttt /foo/bar

> Additionally the number of members may be important too. Overlayfs
> supports only two members currently. When a user wants more layers,
> he has to mount another overlayfs over overlayfs. Since it is
> essentially equivalent to a recursive function call internally, and of
> course the stack size in kernel space is limited, I don't think it is
> good.

Good point about stack space.

Adding multiple read-only layers should be really easy, and could be one
of the first extensions after the merge.

> Also Miklos replied and said modifying the credentials internally does
> no harm to other threads. But I am still afraid it a security hole since
> the credentials is shared among threads. If I had time, I would test it
> by myself.

The credentials of the current task are not modified but replaced by
new, temporary credentials. This will only have an affect on a single

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