Documentation/filesystems/idmappings.rst
.. SPDX-License-Identifier: GPL-2.0
Most filesystem developers will have encountered idmappings. They are used when reading from or writing ownership to disk, reporting ownership to userspace, or for permission checking. This document is aimed at filesystem developers that want to know how idmappings work.
An idmapping is essentially a translation of a range of ids into another or the same range of ids. The notational convention for idmappings that is widely used in userspace is::
u:k:r
u indicates the first element in the upper idmapset U and k
indicates the first element in the lower idmapset K. The r parameter
indicates the range of the idmapping, i.e. how many ids are mapped. From now
on, we will always prefix ids with u or k to make it clear whether
we're talking about an id in the upper or lower idmapset.
To see what this looks like in practice, let's take the following idmapping::
u22:k10000:r3
and write down the mappings it will generate::
u22 -> k10000 u23 -> k10001 u24 -> k10002
From a mathematical viewpoint U and K are well-ordered sets and an
idmapping is an order isomorphism from U into K. So U and K are
order isomorphic. In fact, U and K are always well-ordered subsets of
the set of all possible ids usable on a given system.
Looking at this mathematically briefly will help us highlight some properties that make it easier to understand how we can translate between idmappings. For example, we know that the inverse idmapping is an order isomorphism as well::
k10000 -> u22 k10001 -> u23 k10002 -> u24
Given that we are dealing with order isomorphisms plus the fact that we're dealing with subsets we can embed idmappings into each other, i.e. we can sensibly translate between different idmappings. For example, assume we've been given the three idmappings::
and id k11000 which has been generated by the first idmapping by mapping
u1000 from the upper idmapset down to k11000 in the lower idmapset.
Because we're dealing with order isomorphic subsets it is meaningful to ask
what id k11000 corresponds to in the second or third idmapping. The
straightforward algorithm to use is to apply the inverse of the first idmapping,
mapping k11000 up to u1000. Afterwards, we can map u1000 down using
either the second idmapping mapping or third idmapping mapping. The second
idmapping would map u1000 down to k21000. The third idmapping would map
u1000 down to k31000.
If we were given the same task for the following three idmappings::
we would fail to translate as the sets aren't order isomorphic over the full
range of the first idmapping anymore (However they are order isomorphic over
the full range of the second idmapping.). Neither the second or third idmapping
contain u1000 in the upper idmapset U. This is equivalent to not having
an id mapped. We can simply say that u1000 is unmapped in the second and
third idmapping. The kernel will report unmapped ids as the overflowuid
(uid_t)-1 or overflowgid (gid_t)-1 to userspace.
The algorithm to calculate what a given id maps to is pretty simple. First, we
need to verify that the range can contain our target id. We will skip this step
for simplicity. After that if we want to know what id maps to we can do
simple calculations:
If we want to map from left to right::
u:k:r id - u + k = n
If we want to map from right to left::
u:k:r id - k + u = n
Instead of "left to right" we can also say "down" and instead of "right to left" we can also say "up". Obviously mapping down and up invert each other.
To see whether the simple formulas above work, consider the following two idmappings::
Assume we are given k21000 in the lower idmapset of the first idmapping. We
want to know what id this was mapped from in the upper idmapset of the first
idmapping. So we're mapping up in the first idmapping::
id - k + u = n k21000 - k20000 + u0 = u1000
Now assume we are given the id u1100 in the upper idmapset of the second
idmapping and we want to know what this id maps down to in the lower idmapset
of the second idmapping. This means we're mapping down in the second
idmapping::
id - u + k = n u1100 - u500 + k30000 = k30600
In the context of the kernel an idmapping can be interpreted as mapping a range of userspace ids into a range of kernel ids::
userspace-id:kernel-id:range
A userspace id is always an element in the upper idmapset of an idmapping of
type uid_t or gid_t and a kernel id is always an element in the lower
idmapset of an idmapping of type kuid_t or kgid_t. From now on
"userspace id" will be used to refer to the well known uid_t and gid_t
types and "kernel id" will be used to refer to kuid_t and kgid_t.
The kernel is mostly concerned with kernel ids. They are used when performing
permission checks and are stored in an inode's i_uid and i_gid field.
A userspace id on the other hand is an id that is reported to userspace by the
kernel, or is passed by userspace to the kernel, or a raw device id that is
written or read from disk.
Note that we are only concerned with idmappings as the kernel stores them not how userspace would specify them.
For the rest of this document we will prefix all userspace ids with u and
all kernel ids with k. Ranges of idmappings will be prefixed with r. So
an idmapping will be written as u0:k10000:r10000.
For example, within this idmapping, the id u1000 is an id in the upper
idmapset or "userspace idmapset" starting with u0. And it is mapped to
k11000 which is a kernel id in the lower idmapset or "kernel idmapset"
starting with k10000.
A kernel id is always created by an idmapping. Such idmappings are associated with user namespaces. Since we mainly care about how idmappings work we're not going to be concerned with how idmappings are created nor how they are used outside of the filesystem context. This is best left to an explanation of user namespaces.
The initial user namespace is special. It always has an idmapping of the following form::
u0:k0:r4294967295
which is an identity idmapping over the full range of ids available on this system.
Other user namespaces usually have non-identity idmappings such as::
u0:k10000:r10000
When a process creates or wants to change ownership of a file, or when the ownership of a file is read from disk by a filesystem, the userspace id is immediately translated into a kernel id according to the idmapping associated with the relevant user namespace.
For instance, consider a file that is stored on disk by a filesystem as being
owned by u1000:
If a filesystem were to be mounted in the initial user namespaces (as most
filesystems are) then the initial idmapping will be used. As we saw this is
simply the identity idmapping. This would mean id u1000 read from disk
would be mapped to id k1000. So an inode's i_uid and i_gid field
would contain k1000.
If a filesystem were to be mounted with an idmapping of u0:k10000:r10000
then u1000 read from disk would be mapped to k11000. So an inode's
i_uid and i_gid would contain k11000.
We've already seen briefly that it is possible to translate between different idmappings. We'll now take a closer look how that works.
Crossmapping
This translation algorithm is used by the kernel in quite a few places. For
example, it is used when reporting back the ownership of a file to userspace
via the ``stat()`` system call family.
If we've been given ``k11000`` from one idmapping we can map that id up in
another idmapping. In order for this to work both idmappings need to contain
the same kernel id in their kernel idmapsets. For example, consider the
following idmappings::
1. u0:k10000:r10000
2. u20000:k10000:r10000
and we are mapping ``u1000`` down to ``k11000`` in the first idmapping . We can
then translate ``k11000`` into a userspace id in the second idmapping using the
kernel idmapset of the second idmapping::
/* Map the kernel id up into a userspace id in the second idmapping. */
from_kuid(u20000:k10000:r10000, k11000) = u21000
Note, how we can get back to the kernel id in the first idmapping by inverting
the algorithm::
/* Map the userspace id down into a kernel id in the second idmapping. */
make_kuid(u20000:k10000:r10000, u21000) = k11000
/* Map the kernel id up into a userspace id in the first idmapping. */
from_kuid(u0:k10000:r10000, k11000) = u1000
This algorithm allows us to answer the question what userspace id a given
kernel id corresponds to in a given idmapping. In order to be able to answer
this question both idmappings need to contain the same kernel id in their
respective kernel idmapsets.
For example, when the kernel reads a raw userspace id from disk it maps it down
into a kernel id according to the idmapping associated with the filesystem.
Let's assume the filesystem was mounted with an idmapping of
``u0:k20000:r10000`` and it reads a file owned by ``u1000`` from disk. This
means ``u1000`` will be mapped to ``k21000`` which is what will be stored in
the inode's ``i_uid`` and ``i_gid`` field.
When someone in userspace calls ``stat()`` or a related function to get
ownership information about the file the kernel can't simply map the id back up
according to the filesystem's idmapping as this would give the wrong owner if
the caller is using an idmapping.
So the kernel will map the id back up in the idmapping of the caller. Let's
assume the caller has the somewhat unconventional idmapping
``u3000:k20000:r10000`` then ``k21000`` would map back up to ``u4000``.
Consequently the user would see that this file is owned by ``u4000``.
Remapping
~~~~~~~~~
It is possible to translate a kernel id from one idmapping to another one via
the userspace idmapset of the two idmappings. This is equivalent to remapping
a kernel id.
Let's look at an example. We are given the following two idmappings::
1. u0:k10000:r10000
2. u0:k20000:r10000
and we are given ``k11000`` in the first idmapping. In order to translate this
kernel id in the first idmapping into a kernel id in the second idmapping we
need to perform two steps:
1. Map the kernel id up into a userspace id in the first idmapping::
/* Map the kernel id up into a userspace id in the first idmapping. */
from_kuid(u0:k10000:r10000, k11000) = u1000
2. Map the userspace id down into a kernel id in the second idmapping::
/* Map the userspace id down into a kernel id in the second idmapping. */
make_kuid(u0:k20000:r10000, u1000) = k21000
As you can see we used the userspace idmapset in both idmappings to translate
the kernel id in one idmapping to a kernel id in another idmapping.
This allows us to answer the question what kernel id we would need to use to
get the same userspace id in another idmapping. In order to be able to answer
this question both idmappings need to contain the same userspace id in their
respective userspace idmapsets.
Note, how we can easily get back to the kernel id in the first idmapping by
inverting the algorithm:
1. Map the kernel id up into a userspace id in the second idmapping::
/* Map the kernel id up into a userspace id in the second idmapping. */
from_kuid(u0:k20000:r10000, k21000) = u1000
2. Map the userspace id down into a kernel id in the first idmapping::
/* Map the userspace id down into a kernel id in the first idmapping. */
make_kuid(u0:k10000:r10000, u1000) = k11000
Another way to look at this translation is to treat it as inverting one
idmapping and applying another idmapping if both idmappings have the relevant
userspace id mapped. This will come in handy when working with idmapped mounts.
Invalid translations
It is never valid to use an id in the kernel idmapset of one idmapping as the id in the userspace idmapset of another or the same idmapping. While the kernel idmapset always indicates an idmapset in the kernel id space the userspace idmapset indicates a userspace id. So the following translations are forbidden::
/* Map the userspace id down into a kernel id in the first idmapping. */ make_kuid(u0:k10000:r10000, u1000) = k11000
/* INVALID: Map the kernel id down into a kernel id in the second idmapping. */ make_kuid(u10000:k20000:r10000, k110000) = k21000 ~~~~~~~
and equally wrong::
/* Map the kernel id up into a userspace id in the first idmapping. */ from_kuid(u0:k10000:r10000, k11000) = u1000
/* INVALID: Map the userspace id up into a userspace id in the second idmapping. */ from_kuid(u20000:k0:r10000, u1000) = k21000 ~~~~~
Since userspace ids have type uid_t and gid_t and kernel ids have type
kuid_t and kgid_t the compiler will throw an error when they are
conflated. So the two examples above would cause a compilation failure.
The concepts of mapping an id down or mapping an id up are expressed in the two kernel functions filesystem developers are rather familiar with and which we've already used in this document::
/* Map the userspace id down into a kernel id. */ make_kuid(idmapping, uid)
/* Map the kernel id up into a userspace id. */ from_kuid(idmapping, kuid)
We will take an abbreviated look into how idmappings figure into creating
filesystem objects. For simplicity we will only look at what happens when the
VFS has already completed path lookup right before it calls into the filesystem
itself. So we're concerned with what happens when e.g. vfs_mkdir() is
called. We will also assume that the directory we're creating filesystem
objects in is readable and writable for everyone.
When creating a filesystem object the caller will look at the caller's
filesystem ids. These are just regular uid_t and gid_t userspace ids
but they are exclusively used when determining file ownership which is why they
are called "filesystem ids". They are usually identical to the uid and gid of
the caller but can differ. We will just assume they are always identical to not
get lost in too many details.
When the caller enters the kernel two things happen:
The second step is important as regular filesystem will ultimately need to map the kernel id back up into a userspace id when writing to disk. So with the second step the kernel guarantees that a valid userspace id can be written to disk. If it can't the kernel will refuse the creation request to not even remotely risk filesystem corruption.
The astute reader will have realized that this is simply a variation of the crossmapping algorithm we mentioned above in a previous section. First, the kernel maps the caller's userspace id down into a kernel id according to the caller's idmapping and then maps that kernel id up according to the filesystem's idmapping.
From the implementation point it's worth mentioning how idmappings are represented. All idmappings are taken from the corresponding user namespace.
- caller's idmapping (usually taken from ``current_user_ns()``)
- filesystem's idmapping (``sb->s_user_ns``)
- mount's idmapping (``mnt_idmap(vfsmnt)``)
Let's see some examples with caller/filesystem idmapping but without mount idmappings. This will exhibit some problems we can hit. After that we will revisit/reconsider these examples, this time using mount idmappings, to see how they can solve the problems we observed before.
Example 1
::
caller id: u1000
caller idmapping: u0:k0:r4294967295
filesystem idmapping: u0:k0:r4294967295
Both the caller and the filesystem use the identity idmapping:
1. Map the caller's userspace ids into kernel ids in the caller's idmapping::
make_kuid(u0:k0:r4294967295, u1000) = k1000
2. Verify that the caller's kernel ids can be mapped to userspace ids in the
filesystem's idmapping.
For this second step the kernel will call the function
``fsuidgid_has_mapping()`` which ultimately boils down to calling
``from_kuid()``::
from_kuid(u0:k0:r4294967295, k1000) = u1000
In this example both idmappings are the same so there's nothing exciting going
on. Ultimately the userspace id that lands on disk will be ``u1000``.
Example 2
::
caller id: u1000 caller idmapping: u0:k10000:r10000 filesystem idmapping: u0:k20000:r10000
Map the caller's userspace ids down into kernel ids in the caller's idmapping::
make_kuid(u0:k10000:r10000, u1000) = k11000
Verify that the caller's kernel ids can be mapped up to userspace ids in the filesystem's idmapping::
from_kuid(u0:k20000:r10000, k11000) = u-1
It's immediately clear that while the caller's userspace id could be successfully mapped down into kernel ids in the caller's idmapping the kernel ids could not be mapped up according to the filesystem's idmapping. So the kernel will deny this creation request.
Note that while this example is less common, because most filesystem can't be mounted with non-initial idmappings this is a general problem as we can see in the next examples.
Example 3
::
caller id: u1000
caller idmapping: u0:k10000:r10000
filesystem idmapping: u0:k0:r4294967295
1. Map the caller's userspace ids down into kernel ids in the caller's
idmapping::
make_kuid(u0:k10000:r10000, u1000) = k11000
2. Verify that the caller's kernel ids can be mapped up to userspace ids in the
filesystem's idmapping::
from_kuid(u0:k0:r4294967295, k11000) = u11000
We can see that the translation always succeeds. The userspace id that the
filesystem will ultimately put to disk will always be identical to the value of
the kernel id that was created in the caller's idmapping. This has mainly two
consequences.
First, that we can't allow a caller to ultimately write to disk with another
userspace id. We could only do this if we were to mount the whole filesystem
with the caller's or another idmapping. But that solution is limited to a few
filesystems and not very flexible. But this is a use-case that is pretty
important in containerized workloads.
Second, the caller will usually not be able to create any files or access
directories that have stricter permissions because none of the filesystem's
kernel ids map up into valid userspace ids in the caller's idmapping
1. Map raw userspace ids down to kernel ids in the filesystem's idmapping::
make_kuid(u0:k0:r4294967295, u1000) = k1000
2. Map kernel ids up to userspace ids in the caller's idmapping::
from_kuid(u0:k10000:r10000, k1000) = u-1
Example 4
::
file id: u1000 caller idmapping: u0:k10000:r10000 filesystem idmapping: u0:k0:r4294967295
In order to report ownership to userspace the kernel uses the crossmapping algorithm introduced in a previous section:
Map the userspace id on disk down into a kernel id in the filesystem's idmapping::
make_kuid(u0:k0:r4294967295, u1000) = k1000
Map the kernel id up into a userspace id in the caller's idmapping::
from_kuid(u0:k10000:r10000, k1000) = u-1
The crossmapping algorithm fails in this case because the kernel id in the filesystem idmapping cannot be mapped up to a userspace id in the caller's idmapping. Thus, the kernel will report the ownership of this file as the overflowid.
Example 5
::
file id: u1000
caller idmapping: u0:k10000:r10000
filesystem idmapping: u0:k20000:r10000
In order to report ownership to userspace the kernel uses the crossmapping
algorithm introduced in a previous section:
1. Map the userspace id on disk down into a kernel id in the filesystem's
idmapping::
make_kuid(u0:k20000:r10000, u1000) = k21000
2. Map the kernel id up into a userspace id in the caller's idmapping::
from_kuid(u0:k10000:r10000, k21000) = u-1
Again, the crossmapping algorithm fails in this case because the kernel id in
the filesystem idmapping cannot be mapped to a userspace id in the caller's
idmapping. Thus, the kernel will report the ownership of this file as the
overflowid.
Note how in the last two examples things would be simple if the caller would be
using the initial idmapping. For a filesystem mounted with the initial
idmapping it would be trivial. So we only consider a filesystem with an
idmapping of ``u0:k20000:r10000``:
1. Map the userspace id on disk down into a kernel id in the filesystem's
idmapping::
make_kuid(u0:k20000:r10000, u1000) = k21000
2. Map the kernel id up into a userspace id in the caller's idmapping::
from_kuid(u0:k0:r4294967295, k21000) = u21000
Idmappings on idmapped mounts
-----------------------------
The examples we've seen in the previous section where the caller's idmapping
and the filesystem's idmapping are incompatible causes various issues for
workloads. For a more complex but common example, consider two containers
started on the host. To completely prevent the two containers from affecting
each other, an administrator may often use different non-overlapping idmappings
for the two containers::
container1 idmapping: u0:k10000:r10000
container2 idmapping: u0:k20000:r10000
filesystem idmapping: u0:k30000:r10000
An administrator wanting to provide easy read-write access to the following set
of files::
dir id: u0
dir/file1 id: u1000
dir/file2 id: u2000
to both containers currently can't.
Of course the administrator has the option to recursively change ownership via
``chown()``. For example, they could change ownership so that ``dir`` and all
files below it can be crossmapped from the filesystem's into the container's
idmapping. Let's assume they change ownership so it is compatible with the
first container's idmapping::
dir id: u10000
dir/file1 id: u11000
dir/file2 id: u12000
This would still leave ``dir`` rather useless to the second container. In fact,
``dir`` and all files below it would continue to appear owned by the overflowid
for the second container.
Or consider another increasingly popular example. Some service managers such as
systemd implement a concept called "portable home directories". A user may want
to use their home directories on different machines where they are assigned
different login userspace ids. Most users will have ``u1000`` as the login id
on their machine at home and all files in their home directory will usually be
owned by ``u1000``. At uni or at work they may have another login id such as
``u1125``. This makes it rather difficult to interact with their home directory
on their work machine.
In both cases changing ownership recursively has grave implications. The most
obvious one is that ownership is changed globally and permanently. In the home
directory case this change in ownership would even need to happen every time the
user switches from their home to their work machine. For really large sets of
files this becomes increasingly costly.
If the user is lucky, they are dealing with a filesystem that is mountable
inside user namespaces. But this would also change ownership globally and the
change in ownership is tied to the lifetime of the filesystem mount, i.e. the
superblock. The only way to change ownership is to completely unmount the
filesystem and mount it again in another user namespace. This is usually
impossible because it would mean that all users currently accessing the
filesystem can't anymore. And it means that ``dir`` still can't be shared
between two containers with different idmappings.
But usually the user doesn't even have this option since most filesystems
aren't mountable inside containers. And not having them mountable might be
desirable as it doesn't require the filesystem to deal with malicious
filesystem images.
But the usecases mentioned above and more can be handled by idmapped mounts.
They allow to expose the same set of dentries with different ownership at
different mounts. This is achieved by marking the mounts with a user namespace
through the ``mount_setattr()`` system call. The idmapping associated with it
is then used to translate from the caller's idmapping to the filesystem's
idmapping and vica versa using the remapping algorithm we introduced above.
Idmapped mounts make it possible to change ownership in a temporary and
localized way. The ownership changes are restricted to a specific mount and the
ownership changes are tied to the lifetime of the mount. All other users and
locations where the filesystem is exposed are unaffected.
Filesystems that support idmapped mounts don't have any real reason to support
being mountable inside user namespaces. A filesystem could be exposed
completely under an idmapped mount to get the same effect. This has the
advantage that filesystems can leave the creation of the superblock to
privileged users in the initial user namespace.
However, it is perfectly possible to combine idmapped mounts with filesystems
mountable inside user namespaces. We will touch on this further below.
Filesystem types vs idmapped mount types
With the introduction of idmapped mounts we need to distinguish between filesystem ownership and mount ownership of a VFS object such as an inode. The owner of a inode might be different when looked at from a filesystem perspective than when looked at from an idmapped mount. Such fundamental conceptual distinctions should almost always be clearly expressed in the code. So, to distinguish idmapped mount ownership from filesystem ownership separate types have been introduced.
If a uid or gid has been generated using the filesystem or caller's idmapping
then we will use the kuid_t and kgid_t types. However, if a uid or gid
has been generated using a mount idmapping then we will be using the dedicated
vfsuid_t and vfsgid_t types.
All VFS helpers that generate or take uids and gids as arguments use the
vfsuid_t and vfsgid_t types and we will be able to rely on the compiler
to catch errors that originate from conflating filesystem and VFS uids and gids.
The vfsuid_t and vfsgid_t types are often mapped from and to kuid_t
and kgid_t types similar how kuid_t and kgid_t types are mapped
from and to uid_t and gid_t types::
uid_t <--> kuid_t <--> vfsuid_t gid_t <--> kgid_t <--> vfsgid_t
Whenever we report ownership based on a vfsuid_t or vfsgid_t type,
e.g., during stat(), or store ownership information in a shared VFS object
based on a vfsuid_t or vfsgid_t type, e.g., during chown() we can
use the vfsuid_into_kuid() and vfsgid_into_kgid() helpers.
To illustrate why this helper currently exists, consider what happens when we
change ownership of an inode from an idmapped mount. After we generated
a vfsuid_t or vfsgid_t based on the mount idmapping we later commit to
this vfsuid_t or vfsgid_t to become the new filesystem wide ownership.
Thus, we are turning the vfsuid_t or vfsgid_t into a global kuid_t
or kgid_t. And this can be done by using vfsuid_into_kuid() and
vfsgid_into_kgid().
Note, whenever a shared VFS object, e.g., a cached struct inode or a cached
struct posix_acl, stores ownership information a filesystem or "global"
kuid_t and kgid_t must be used. Ownership expressed via vfsuid_t
and vfsgid_t is specific to an idmapped mount.
We already noted that vfsuid_t and vfsgid_t types are generated based
on mount idmappings whereas kuid_t and kgid_t types are generated based
on filesystem idmappings. To prevent abusing filesystem idmappings to generate
vfsuid_t or vfsgid_t types or mount idmappings to generate kuid_t
or kgid_t types filesystem idmappings and mount idmappings are different
types as well.
All helpers that map to or from vfsuid_t and vfsgid_t types require
a mount idmapping to be passed which is of type struct mnt_idmap. Passing
a filesystem or caller idmapping will cause a compilation error.
Similar to how we prefix all userspace ids in this document with u and all
kernel ids with k we will prefix all VFS ids with v. So a mount
idmapping will be written as: u0:v10000:r10000.
Remapping helpers
Idmapping functions were added that translate between idmappings. They make use
of the remapping algorithm we've introduced earlier. We're going to look at:
- ``i_uid_into_vfsuid()`` and ``i_gid_into_vfsgid()``
The ``i_*id_into_vfs*id()`` functions translate filesystem's kernel ids into
VFS ids in the mount's idmapping::
/* Map the filesystem's kernel id up into a userspace id in the filesystem's idmapping. */
from_kuid(filesystem, kid) = uid
/* Map the filesystem's userspace id down ito a VFS id in the mount's idmapping. */
make_kuid(mount, uid) = kuid
- ``mapped_fsuid()`` and ``mapped_fsgid()``
The ``mapped_fs*id()`` functions translate the caller's kernel ids into
kernel ids in the filesystem's idmapping. This translation is achieved by
remapping the caller's VFS ids using the mount's idmapping::
/* Map the caller's VFS id up into a userspace id in the mount's idmapping. */
from_kuid(mount, kid) = uid
/* Map the mount's userspace id down into a kernel id in the filesystem's idmapping. */
make_kuid(filesystem, uid) = kuid
- ``vfsuid_into_kuid()`` and ``vfsgid_into_kgid()``
Whenever
Note that these two functions invert each other. Consider the following
idmappings::
caller idmapping: u0:k10000:r10000
filesystem idmapping: u0:k20000:r10000
mount idmapping: u0:v10000:r10000
Assume a file owned by ``u1000`` is read from disk. The filesystem maps this id
to ``k21000`` according to its idmapping. This is what is stored in the
inode's ``i_uid`` and ``i_gid`` fields.
When the caller queries the ownership of this file via ``stat()`` the kernel
would usually simply use the crossmapping algorithm and map the filesystem's
kernel id up to a userspace id in the caller's idmapping.
But when the caller is accessing the file on an idmapped mount the kernel will
first call ``i_uid_into_vfsuid()`` thereby translating the filesystem's kernel
id into a VFS id in the mount's idmapping::
i_uid_into_vfsuid(k21000):
/* Map the filesystem's kernel id up into a userspace id. */
from_kuid(u0:k20000:r10000, k21000) = u1000
/* Map the filesystem's userspace id down into a VFS id in the mount's idmapping. */
make_kuid(u0:v10000:r10000, u1000) = v11000
Finally, when the kernel reports the owner to the caller it will turn the
VFS id in the mount's idmapping into a userspace id in the caller's
idmapping::
k11000 = vfsuid_into_kuid(v11000)
from_kuid(u0:k10000:r10000, k11000) = u1000
We can test whether this algorithm really works by verifying what happens when
we create a new file. Let's say the user is creating a file with ``u1000``.
The kernel maps this to ``k11000`` in the caller's idmapping. Usually the
kernel would now apply the crossmapping, verifying that ``k11000`` can be
mapped to a userspace id in the filesystem's idmapping. Since ``k11000`` can't
be mapped up in the filesystem's idmapping directly this creation request
fails.
But when the caller is accessing the file on an idmapped mount the kernel will
first call ``mapped_fs*id()`` thereby translating the caller's kernel id into
a VFS id according to the mount's idmapping::
mapped_fsuid(k11000):
/* Map the caller's kernel id up into a userspace id in the mount's idmapping. */
from_kuid(u0:k10000:r10000, k11000) = u1000
/* Map the mount's userspace id down into a kernel id in the filesystem's idmapping. */
make_kuid(u0:v20000:r10000, u1000) = v21000
When finally writing to disk the kernel will then map ``v21000`` up into a
userspace id in the filesystem's idmapping::
k21000 = vfsuid_into_kuid(v21000)
from_kuid(u0:k20000:r10000, k21000) = u1000
As we can see, we end up with an invertible and therefore information
preserving algorithm. A file created from ``u1000`` on an idmapped mount will
also be reported as being owned by ``u1000`` and vica versa.
Let's now briefly reconsider the failing examples from earlier in the context
of idmapped mounts.
Example 2 reconsidered
::
caller id: u1000 caller idmapping: u0:k10000:r10000 filesystem idmapping: u0:k20000:r10000 mount idmapping: u0:v10000:r10000
When the caller is using a non-initial idmapping the common case is to attach the same idmapping to the mount. We now perform three steps:
Map the caller's userspace ids into kernel ids in the caller's idmapping::
make_kuid(u0:k10000:r10000, u1000) = k11000
Translate the caller's VFS id into a kernel id in the filesystem's idmapping::
mapped_fsuid(v11000): /* Map the VFS id up into a userspace id in the mount's idmapping. */ from_kuid(u0:v10000:r10000, v11000) = u1000
/* Map the userspace id down into a kernel id in the filesystem's idmapping. */ make_kuid(u0:k20000:r10000, u1000) = k21000
Verify that the caller's kernel ids can be mapped to userspace ids in the filesystem's idmapping::
from_kuid(u0:k20000:r10000, k21000) = u1000
So the ownership that lands on disk will be u1000.
Example 3 reconsidered
::
caller id: u1000
caller idmapping: u0:k10000:r10000
filesystem idmapping: u0:k0:r4294967295
mount idmapping: u0:v10000:r10000
The same translation algorithm works with the third example.
1. Map the caller's userspace ids into kernel ids in the caller's idmapping::
make_kuid(u0:k10000:r10000, u1000) = k11000
2. Translate the caller's VFS id into a kernel id in the filesystem's
idmapping::
mapped_fsuid(v11000):
/* Map the VFS id up into a userspace id in the mount's idmapping. */
from_kuid(u0:v10000:r10000, v11000) = u1000
/* Map the userspace id down into a kernel id in the filesystem's idmapping. */
make_kuid(u0:k0:r4294967295, u1000) = k1000
3. Verify that the caller's kernel ids can be mapped to userspace ids in the
filesystem's idmapping::
from_kuid(u0:k0:r4294967295, k1000) = u1000
So the ownership that lands on disk will be ``u1000``.
Example 4 reconsidered
::
file id: u1000 caller idmapping: u0:k10000:r10000 filesystem idmapping: u0:k0:r4294967295 mount idmapping: u0:v10000:r10000
In order to report ownership to userspace the kernel now does three steps using the translation algorithm we introduced earlier:
Map the userspace id on disk down into a kernel id in the filesystem's idmapping::
make_kuid(u0:k0:r4294967295, u1000) = k1000
Translate the kernel id into a VFS id in the mount's idmapping::
i_uid_into_vfsuid(k1000): /* Map the kernel id up into a userspace id in the filesystem's idmapping. */ from_kuid(u0:k0:r4294967295, k1000) = u1000
/* Map the userspace id down into a VFS id in the mounts's idmapping. */ make_kuid(u0:v10000:r10000, u1000) = v11000
Map the VFS id up into a userspace id in the caller's idmapping::
k11000 = vfsuid_into_kuid(v11000) from_kuid(u0:k10000:r10000, k11000) = u1000
Earlier, the caller's kernel id couldn't be crossmapped in the filesystems's
idmapping. With the idmapped mount in place it now can be crossmapped into the
filesystem's idmapping via the mount's idmapping. The file will now be created
with u1000 according to the mount's idmapping.
Example 5 reconsidered
::
file id: u1000
caller idmapping: u0:k10000:r10000
filesystem idmapping: u0:k20000:r10000
mount idmapping: u0:v10000:r10000
Again, in order to report ownership to userspace the kernel now does three
steps using the translation algorithm we introduced earlier:
1. Map the userspace id on disk down into a kernel id in the filesystem's
idmapping::
make_kuid(u0:k20000:r10000, u1000) = k21000
2. Translate the kernel id into a VFS id in the mount's idmapping::
i_uid_into_vfsuid(k21000):
/* Map the kernel id up into a userspace id in the filesystem's idmapping. */
from_kuid(u0:k20000:r10000, k21000) = u1000
/* Map the userspace id down into a VFS id in the mounts's idmapping. */
make_kuid(u0:v10000:r10000, u1000) = v11000
3. Map the VFS id up into a userspace id in the caller's idmapping::
k11000 = vfsuid_into_kuid(v11000)
from_kuid(u0:k10000:r10000, k11000) = u1000
Earlier, the file's kernel id couldn't be crossmapped in the filesystems's
idmapping. With the idmapped mount in place it now can be crossmapped into the
filesystem's idmapping via the mount's idmapping. The file is now owned by
``u1000`` according to the mount's idmapping.
Changing ownership on a home directory
We've seen above how idmapped mounts can be used to translate between idmappings when either the caller, the filesystem or both uses a non-initial idmapping. A wide range of usecases exist when the caller is using a non-initial idmapping. This mostly happens in the context of containerized workloads. The consequence is as we have seen that for both, filesystem's mounted with the initial idmapping and filesystems mounted with non-initial idmappings, access to the filesystem isn't working because the kernel ids can't be crossmapped between the caller's and the filesystem's idmapping.
As we've seen above idmapped mounts provide a solution to this by remapping the caller's or filesystem's idmapping according to the mount's idmapping.
Aside from containerized workloads, idmapped mounts have the advantage that they also work when both the caller and the filesystem use the initial idmapping which means users on the host can change the ownership of directories and files on a per-mount basis.
Consider our previous example where a user has their home directory on portable
storage. At home they have id u1000 and all files in their home directory
are owned by u1000 whereas at uni or work they have login id u1125.
Taking their home directory with them becomes problematic. They can't easily
access their files, they might not be able to write to disk without applying
lax permissions or ACLs and even if they can, they will end up with an annoying
mix of files and directories owned by u1000 and u1125.
Idmapped mounts allow to solve this problem. A user can create an idmapped mount for their home directory on their work computer or their computer at home depending on what ownership they would prefer to end up on the portable storage itself.
Let's assume they want all files on disk to belong to u1000. When the user
plugs in their portable storage at their work station they can setup a job that
creates an idmapped mount with the minimal idmapping u1000:k1125:r1. So now
when they create a file the kernel performs the following steps we already know
from above:::
caller id: u1125 caller idmapping: u0:k0:r4294967295 filesystem idmapping: u0:k0:r4294967295 mount idmapping: u1000:v1125:r1
Map the caller's userspace ids into kernel ids in the caller's idmapping::
make_kuid(u0:k0:r4294967295, u1125) = k1125
Translate the caller's VFS id into a kernel id in the filesystem's idmapping::
mapped_fsuid(v1125): /* Map the VFS id up into a userspace id in the mount's idmapping. */ from_kuid(u1000:v1125:r1, v1125) = u1000
/* Map the userspace id down into a kernel id in the filesystem's idmapping. */ make_kuid(u0:k0:r4294967295, u1000) = k1000
Verify that the caller's filesystem ids can be mapped to userspace ids in the filesystem's idmapping::
from_kuid(u0:k0:r4294967295, k1000) = u1000
So ultimately the file will be created with u1000 on disk.
Now let's briefly look at what ownership the caller with id u1125 will see
on their work computer:
::
file id: u1000 caller idmapping: u0:k0:r4294967295 filesystem idmapping: u0:k0:r4294967295 mount idmapping: u1000:v1125:r1
Map the userspace id on disk down into a kernel id in the filesystem's idmapping::
make_kuid(u0:k0:r4294967295, u1000) = k1000
Translate the kernel id into a VFS id in the mount's idmapping::
i_uid_into_vfsuid(k1000): /* Map the kernel id up into a userspace id in the filesystem's idmapping. */ from_kuid(u0:k0:r4294967295, k1000) = u1000
/* Map the userspace id down into a VFS id in the mounts's idmapping. */ make_kuid(u1000:v1125:r1, u1000) = v1125
Map the VFS id up into a userspace id in the caller's idmapping::
k1125 = vfsuid_into_kuid(v1125) from_kuid(u0:k0:r4294967295, k1125) = u1125
So ultimately the caller will be reported that the file belongs to u1125
which is the caller's userspace id on their workstation in our example.
The raw userspace id that is put on disk is u1000 so when the user takes
their home directory back to their home computer where they are assigned
u1000 using the initial idmapping and mount the filesystem with the initial
idmapping they will see all those files owned by u1000.