Re: union mounts, mfs, and newsyslog

[matthew sporleder <msporleder%gmail.com@localhost>]

On Sun, Jul 5, 2009 at 11:07 PM, David
Holland<dholland-current%netbsd.org@localhost> wrote:
> On Sun, Jul 05, 2009 at 09:39:04PM -0400, matthew sporleder wrote:
>  >                      if (rename(file1, file2))
>  >                              err(EXIT_FAILURE, "%s", file1);
>  > [...]
>  >
>  > /dev/wd0b       /var/log_mfs mfs rw,hidden,-s10M 0 0
>  > /var/log_mfs    /var/log union rw - -
>  >
>  > So is it a limitation of mfs, union mounting, or my ffs?
>
> Unionfs has all kinds of problems, not least with rename. The short
> answer is "it's not really expected to work". Although in this
> particular case, I would have thought it would... or if not, fail some
> other way, like by crashing, instead of apparently passing vnodes from
> mismatched layers to one layer's VOP_RENAME.
>
> If you want to make it work, the first step is probably to figure out
> what it's doing: create a simple test case that exhibits the same
> behavior (this is not necessarily so trivial...) and then poke into
> the kernel to figure out what union_rename thinks it's doing.
>
> However... there are a number of more or less insoluble problems with
> rename and unionfs (in more complicated cases) and you might prefer
> not to get involved.
>
/* ren.c */
#include <stdio.h>

int main (int argc, char *argv[])
{
        int ret;
        ret = rename(argv[1], argv[2]);
        ret == 0 ? : perror( "rename failed" );
        return(ret);
}

gcc -o /var/tmp/ren ren.c

mkdir /var/upper
mkdir /var/lower
touch /var/upper/willpass
touch /var/lower/willfail
mount -t union /var/upper /var/lower

(results so far)
<above>:/var/upper       832508     758010      51823  93% /var/lower
fester# ls -l /var/lower
total 0
-rw-r--r--  1 root  wheel  0 Jul  6 22:21 willfail
-rw-r--r--  1 root  wheel  0 Jul  6 22:21 willpass

... (back to the action)
/var/tmp/ren willpass pass
/var/tmp/ren willfail fail
rename failed: Cross-device link

(final results)
fester# ls -l /var/lower
total 0
-rw-r--r--  1 root  wheel  0 Jul  6 22:21 pass
-rw-r--r--  1 root  wheel  0 Jul  6 22:21 willfail

I guess when a rename is passed to the lower layer it should be turned
into a copy at the upper layer and then a rename.

Sorry this isn't in atf,
Matt