[Unionfs] unionfs has problems with file permissions

[Michael Tokarev]

[Quoting original message in full as it's quite old by now...]
Erez Zadok wrote at Wed, 4 Feb 2009 22:12:47 -0500:
> In message <498721B9.5040103 at msgid.tls.msk.ru>, Michael Tokarev writes:
>> Johannes Hirte wrote:
>>> Ok second try, since my first mail didn't reached the ML.
>>>
>>> I run into write problems with unionfs and nfs and found this thread on
>>> the ML:
>>>
>>> http://www.fsl.cs.sunysb.edu/pipermail/unionfs/2008-November/005949.html
>>>
>>> This describes pretty good my problem. I'm booting from network into an
>>> initrd, mounting a tmpfs (rw) and a nfs-export (ro) together with
>>> unionfs. The problem is, I can't modify a file from the nfs. Deleting
>>> or creating is no problem, only modifying ends in "Permission denied".
>> This is not unique to nfs - it also happens when the ro part is on
>> a read-only media like CD-rom for example, or squashfs, whatever.
> 
> Michael, please see my response to Johannes: any additional info you can
> give me will be helpful, including temp remote access, VMware images that
> reproduce the problem, live CD images I can try, etc.
> 
> My guess is that there's something different about your system and
> Johannes's system -- perhaps a difference that common to the two of you.
> Whatever is going on, it's not a common setup or issue: after all unionfs's
> main premise is to recognize readonly files and copy-up them automatically.

Ok.  I can't reproduce this problem with current kernels, no matter what
I try.  I studied code too -- there's no way current (or 2..3 release
before) unionfs will return EROFS or EACCESS here, it must be from some
upper layer (vfs?), but first I don't know kernel that good and second
I can't reproduce it anyway.  My theory was some caching somewhere in
vfs, but I can't back it up.

So I'd declare the problem as solved by now.  2.6.27 and 2.6.28 both works
just fine.  Maybe, just maybe, 2.6.27 itself and earlier (.10 and before)
2.6.27.y series also suffered from this prob, but it's definitely not present
in in 2.6.27.11+.  Or again, I just wasn't able to trigger it anymore, --
while previously it was trivial to trigger it by following steps I described
several times with any read-only media.

So... I can't add anything more to this at this time.  Bad it is or good --
time will tell ;)

Thank you!

/mjt