В линуксе mount --bind /dev /mnt/dev ЕМНИП

On Sat, Jun 30, 2018 at 1:46 PM, Slawa Olhovchenkov <slw@zxy.spb.ru> wrote:

> On Fri, Jun 29, 2018 at 05:59:00PM +0300, Gena Makhomed wrote:
>
> > > Т.е. директории должны быть как бы зеркалами друг друга.
> > >
> > > Это возможно сделать?
> >
> > Теоретически - наверное возможно, если написать свой модуль ядра,
> > который будет реализовывать эту логику на уровне файловой системы.
> >
>
> кажется в лялихе тоже такое было.
>
> MOUNT_UNIONFS(8) FreeBSD System Manager's Manual
> MOUNT_UNIONFS(8)
>
> NAME
> mount_unionfs — mount union file systems
>
> SYNOPSIS
> mount_unionfs [-b] [-o options] directory uniondir
>
> DESCRIPTION
> The mount_unionfs utility attaches directory above uniondir in such a
> way
> that the contents of both directory trees remain visible. By default,
> directory becomes the upper layer and uniondir becomes the lower
> layer.
>
> The options are as follows:
>
> -b Deprecated. Use -o below instead.
>
> -o Options are specified with the -o flag followed by an option..
> The following options are available:
>
> below Inverts the default position, so that directory
> becomes
> the lower layer and uniondir becomes the upper layer..
> However, uniondir remains the mount point.
>
> copymode = traditional | transparent | masquerade
> Specifies the way to create a file or a directory in
> the
> upper layer automatically when needed. The
> traditional
> mode uses the same way as the old unionfs for backward
> compatibility, and transparent duplicates the file and
> directory mode bits and the ownership in the lower
> layer
> to the created file in the upper layer. For behavior
> of
> the masquerade mode, see MASQUERADE MODE below.
>
> whiteout = always | whenneeded
> Specifies whether whiteouts should always be made in
> the
> upper layer when removing a file or directory or only
> when it already exists in the lower layer.
>
> udir=mode
> Specifies directory mode bits in octal for masquerade
> mode.
>
> ufile=mode
> Specifies file mode bits in octal for masquerade mode.
>
> gid=gid
> Specifies group for masquerade mode.
>
> uid=uid
> Specifies user for masquerade mode.
>
> To enforce file system security, the user mounting a file system must
> be
> superuser or else have write permission on the mounted-on directory.
> In
> addition, the vfs.usermount sysctl(8) variable must be set to 1 to
> permit
> file system mounting by ordinary users. However, note that
> transparent
> and masquerade modes require vfs.usermount to be set to 0 because this
> functionality can only be used by superusers.
>
> Filenames are looked up in the upper layer and then in the lower
> layer.
> If a directory is found in the lower layer, and there is no entry in
> the
> upper layer, then a shadow directory will be created in the upper
> layer.
> The ownership and the mode bits are set depending on the copymode
> option.
> In traditional mode, it will be owned by the user who originally did
> the
> union mount, with mode 0777 (“rwxrwxrwx”) modified by the umask in
> effect
> at that time.
>
> If a file exists in the upper layer then there is no way to access a
> file
> with the same name in the lower layer. If necessary, a combination of
> loopback and union mounts can be made which will still allow the lower
> files to be accessed by a different pathname.
>
> Except in the case of a directory, access to an object is granted via
> the
> normal file system access checks. For directories, the current user
> must
> have access to both the upper and lower directories (should they both
> exist).
>
> Requests to create or modify objects in uniondir are passed to the
> upper
> layer with the exception of a few special cases. An attempt to open
> for
> writing a file which exists in the lower layer causes a copy of the
> entire file to be made to the upper layer, and then for the upper
> layer
> copy to be opened. Similarly, an attempt to truncate a lower layer
> file
> to zero length causes an empty file to be created in the upper layer..
> Any other operation which would ultimately require modification to the
> lower layer fails with EROFS.
>
> The union file system manipulates the namespace, rather than
> individual
> file systems. The union operation applies recursively down the
> directory
> tree now rooted at uniondir. Thus any file systems which are mounted
> under uniondir will take part in the union operation. This differs
> from
> the union option to mount(8) which only applies the union operation to
> the mount point itself, and then only for lookups.
>
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