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Documentation: update idmappings.rst

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Quite a lot has changed over the last few kernel releases with the
introduction of vfs{g,u}id_t and struct mnt_idmap. Update the
documentation accordingly.

Cc: Seth Forshee <sforshee@kernel.org>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>
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Christian Brauner 2023-03-06 15:11:42 +01:00 committed by Christian Brauner (Microsoft)
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Documentation/filesystems/idmappings.rst
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@ -241,7 +241,7 @@ 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 slighly unconventional idmapping
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``.
@ -320,6 +320,10 @@ and equally wrong::
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.
Idmappings when creating filesystem objects
-------------------------------------------
@ -623,42 +627,105 @@ 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 filesytem 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
two:
of the remapping algorithm we've introduced earlier. We're going to look at:
- ``i_uid_into_mnt()`` and ``i_gid_into_mnt()``
- ``i_uid_into_vfsuid()`` and ``i_gid_into_vfsgid()``
The ``i_*id_into_mnt()`` functions translate filesystem's kernel ids into
kernel ids in the mount's idmapping::
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 kernel id in the mount's idmapping. */
/* 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 kernel ids using the mount's idmapping::
remapping the caller's VFS ids using the mount's idmapping::
/* Map the caller's kernel id up into a userspace id in 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:k10000: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
@ -669,20 +736,21 @@ 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_mnt()`` thereby translating the filesystem's kernel id
into a kernel id in the mount's idmapping::
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_mnt(k21000):
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 ito a kernel id in the mount's idmapping. */
make_kuid(u0:k10000:r10000, u1000) = k11000
/* 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
kernel id in the mount's idmapping into a userspace id in the caller's
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
@ -696,18 +764,19 @@ 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 kernel id according to the mount's idmapping::
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:k20000:r10000, u1000) = k21000
make_kuid(u0:v20000:r10000, u1000) = v21000
When finally writing to disk the kernel will then map ``k21000`` up into a
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
@ -725,7 +794,7 @@ Example 2 reconsidered
caller id: u1000
caller idmapping: u0:k10000:r10000
filesystem idmapping: u0:k20000:r10000
mount idmapping: u0:k10000: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:
@ -734,12 +803,12 @@ the same idmapping to the mount. We now perform three steps:
make_kuid(u0:k10000:r10000, u1000) = k11000
2. Translate the caller's kernel id into a kernel id in the filesystem's
2. Translate the caller's VFS id into a kernel id in the filesystem's
idmapping::
mapped_fsuid(k11000):
/* Map the kernel id up into a userspace id in the mount's idmapping. */
from_kuid(u0:k10000:r10000, k11000) = u1000
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
@ -759,7 +828,7 @@ Example 3 reconsidered
caller id: u1000
caller idmapping: u0:k10000:r10000
filesystem idmapping: u0:k0:r4294967295
mount idmapping: u0:k10000:r10000
mount idmapping: u0:v10000:r10000
The same translation algorithm works with the third example.
@ -767,12 +836,12 @@ The same translation algorithm works with the third example.
make_kuid(u0:k10000:r10000, u1000) = k11000
2. Translate the caller's kernel id into a kernel id in the filesystem's
2. Translate the caller's VFS id into a kernel id in the filesystem's
idmapping::
mapped_fsuid(k11000):
/* Map the kernel id up into a userspace id in the mount's idmapping. */
from_kuid(u0:k10000:r10000, k11000) = u1000
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
@ -792,7 +861,7 @@ Example 4 reconsidered
file id: u1000
caller idmapping: u0:k10000:r10000
filesystem idmapping: u0:k0:r4294967295
mount idmapping: u0:k10000:r10000
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:
@ -802,17 +871,18 @@ the translation algorithm we introduced earlier:
make_kuid(u0:k0:r4294967295, u1000) = k1000
2. Translate the kernel id into a kernel id in the mount's idmapping::
2. Translate the kernel id into a VFS id in the mount's idmapping::
i_uid_into_mnt(k1000):
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 kernel id in the mounts's idmapping. */
make_kuid(u0:k10000:r10000, u1000) = k11000
/* Map the userspace id down into a VFS id in the mounts's idmapping. */
make_kuid(u0:v10000:r10000, u1000) = v11000
3. Map the kernel id up into a userspace id in the caller's idmapping::
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 caller's kernel id couldn't be crossmapped in the filesystems's
@ -828,7 +898,7 @@ Example 5 reconsidered
file id: u1000
caller idmapping: u0:k10000:r10000
filesystem idmapping: u0:k20000:r10000
mount idmapping: u0:k10000: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:
@ -838,17 +908,18 @@ steps using the translation algorithm we introduced earlier:
make_kuid(u0:k20000:r10000, u1000) = k21000
2. Translate the kernel id into a kernel id in the mount's idmapping::
2. Translate the kernel id into a VFS id in the mount's idmapping::
i_uid_into_mnt(k21000):
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 kernel id in the mounts's idmapping. */
make_kuid(u0:k10000:r10000, u1000) = k11000
/* Map the userspace id down into a VFS id in the mounts's idmapping. */
make_kuid(u0:v10000:r10000, u1000) = v11000
3. Map the kernel id up into a userspace id in the caller's idmapping::
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
@ -899,23 +970,23 @@ from above:::
caller id: u1125
caller idmapping: u0:k0:r4294967295
filesystem idmapping: u0:k0:r4294967295
mount idmapping: u1000:k1125:r1
mount idmapping: u1000:v1125:r1
1. Map the caller's userspace ids into kernel ids in the caller's idmapping::
make_kuid(u0:k0:r4294967295, u1125) = k1125
2. Translate the caller's kernel id into a kernel id in the filesystem's
2. Translate the caller's VFS id into a kernel id in the filesystem's
idmapping::
mapped_fsuid(k1125):
/* Map the kernel id up into a userspace id in the mount's idmapping. */
from_kuid(u1000:k1125:r1, k1125) = u1000
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
2. Verify that the caller's kernel ids can be mapped to userspace ids in the
2. 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
@ -930,24 +1001,25 @@ on their work computer:
file id: u1000
caller idmapping: u0:k0:r4294967295
filesystem idmapping: u0:k0:r4294967295
mount idmapping: u1000:k1125:r1
mount idmapping: u1000:v1125:r1
1. Map the userspace id on disk down into a kernel id in the filesystem's
idmapping::
make_kuid(u0:k0:r4294967295, u1000) = k1000
2. Translate the kernel id into a kernel id in the mount's idmapping::
2. Translate the kernel id into a VFS id in the mount's idmapping::
i_uid_into_mnt(k1000):
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 kernel id in the mounts's idmapping. */
make_kuid(u1000:k1125:r1, u1000) = k1125
/* Map the userspace id down into a VFS id in the mounts's idmapping. */
make_kuid(u1000:v1125:r1, u1000) = v1125
3. Map the kernel id up into a userspace id in the caller's idmapping::
3. 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``