[PATCH v3 00/14] ceph: support idmapped mounts

[PATCH v3 00/14] ceph: support idmapped mounts

[Alexander Mikhalitsyn]

Dear friends,

This patchset was originally developed by Christian Brauner but I'll continue
to push it forward. Christian allowed me to do that :)

This feature is already actively used/tested with LXD/LXC project.

Git tree (based on https://github.com/ceph/ceph-client.git master):
https://github.com/mihalicyn/linux/tree/fs.idmapped.ceph

In the version 3 I've changed only two commits:
- fs: export mnt_idmap_get/mnt_idmap_put
- ceph: allow idmapped setattr inode op
and added a new one:
- ceph: pass idmap to __ceph_setattr

I can confirm that version 3 passes xfstests.

Kind regards,
Alex

Original description from Christian:
========================================================================
This patch series enables cephfs to support idmapped mounts, i.e. the
ability to alter ownership information on a per-mount basis.

Container managers such as LXD support sharaing data via cephfs between
the host and unprivileged containers and between unprivileged containers.
They may all use different idmappings. Idmapped mounts can be used to
create mounts with the idmapping used for the container (or a different
one specific to the use-case).

There are in fact more use-cases such as remapping ownership for
mountpoints on the host itself to grant or restrict access to different
users or to make it possible to enforce that programs running as root
will write with a non-zero {g,u}id to disk.

The patch series is simple overall and few changes are needed to cephfs.
There is one cephfs specific issue that I would like to discuss and
solve which I explain in detail in:

[PATCH 02/12] ceph: handle idmapped mounts in create_request_message()

It has to do with how to handle mds serves which have id-based access
restrictions configured. I would ask you to please take a look at the
explanation in the aforementioned patch.

The patch series passes the vfs and idmapped mount testsuite as part of
xfstests. To run it you will need a config like:

[ceph]
export FSTYP=ceph
export TEST_DIR=/mnt/test
export TEST_DEV=10.103.182.10:6789:/
export TEST_FS_MOUNT_OPTS="-o name=admin,secret=$password

and then simply call

sudo ./check -g idmapped

========================================================================

Alexander Mikhalitsyn (2):
  fs: export mnt_idmap_get/mnt_idmap_put
  ceph: pass idmap to __ceph_setattr

Christian Brauner (12):
  ceph: stash idmapping in mdsc request
  ceph: handle idmapped mounts in create_request_message()
  ceph: allow idmapped mknod inode op
  ceph: allow idmapped symlink inode op
  ceph: allow idmapped mkdir inode op
  ceph: allow idmapped rename inode op
  ceph: allow idmapped getattr inode op
  ceph: allow idmapped permission inode op
  ceph: allow idmapped setattr inode op
  ceph/acl: allow idmapped set_acl inode op
  ceph/file: allow idmapped atomic_open inode op
  ceph: allow idmapped mounts

 fs/ceph/acl.c                 |  6 +++---
 fs/ceph/dir.c                 |  4 ++++
 fs/ceph/file.c                | 10 +++++++--
 fs/ceph/inode.c               | 38 ++++++++++++++++++++++++-----------
 fs/ceph/mds_client.c          | 29 ++++++++++++++++++++++----
 fs/ceph/mds_client.h          |  1 +
 fs/ceph/super.c               |  2 +-
 fs/ceph/super.h               |  3 ++-
 fs/mnt_idmapping.c            |  2 ++
 include/linux/mnt_idmapping.h |  3 +++
 10 files changed, 75 insertions(+), 23 deletions(-)

-- 
2.34.1

[PATCH v3 01/14] fs: export mnt_idmap_get/mnt_idmap_put

[Alexander Mikhalitsyn]

These helpers are required to support idmapped mounts in the Cephfs.

Cc: Christian Brauner <brauner@kernel.org>
Cc: Xiubo Li <xiubli@redhat.com>
Cc: Jeff Layton <jlayton@kernel.org>
Cc: Ilya Dryomov <idryomov@gmail.com>
Cc: ceph-devel@vger.kernel.org
Cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
---
v3:
	- EXPORT_SYMBOL -> EXPORT_SYMBOL_GPL as Christoph Hellwig suggested
---
 fs/mnt_idmapping.c            | 2 ++
 include/linux/mnt_idmapping.h | 3 +++
 2 files changed, 5 insertions(+)

diff --git a/fs/mnt_idmapping.c b/fs/mnt_idmapping.c
index 4905665c47d0..57d1dedf3f8f 100644
--- a/fs/mnt_idmapping.c
+++ b/fs/mnt_idmapping.c
@@ -256,6 +256,7 @@ struct mnt_idmap *mnt_idmap_get(struct mnt_idmap *idmap)
 
 	return idmap;
 }
+EXPORT_SYMBOL_GPL(mnt_idmap_get);
 
 /**
  * mnt_idmap_put - put a reference to an idmapping
@@ -271,3 +272,4 @@ void mnt_idmap_put(struct mnt_idmap *idmap)
 		kfree(idmap);
 	}
 }
+EXPORT_SYMBOL_GPL(mnt_idmap_put);
diff --git a/include/linux/mnt_idmapping.h b/include/linux/mnt_idmapping.h
index 057c89867aa2..b8da2db4ecd2 100644
--- a/include/linux/mnt_idmapping.h
+++ b/include/linux/mnt_idmapping.h
@@ -115,6 +115,9 @@ static inline bool vfsgid_eq_kgid(vfsgid_t vfsgid, kgid_t kgid)
 
 int vfsgid_in_group_p(vfsgid_t vfsgid);
 
+struct mnt_idmap *mnt_idmap_get(struct mnt_idmap *idmap);
+void mnt_idmap_put(struct mnt_idmap *idmap);
+
 vfsuid_t make_vfsuid(struct mnt_idmap *idmap,
 		     struct user_namespace *fs_userns, kuid_t kuid);
 
-- 
2.34.1

[PATCH v3 02/14] ceph: stash idmapping in mdsc request

[Alexander Mikhalitsyn]

From: Christian Brauner <christian.brauner@ubuntu.com>

When sending a mds request cephfs will send relevant data for the
requested operation. For creation requests the caller's fs{g,u}id is
used to set the ownership of the newly created filesystem object. For
setattr requests the caller can pass in arbitrary {g,u}id values to
which the relevant filesystem object is supposed to be changed.

If the caller is performing the relevant operation via an idmapped mount
cephfs simply needs to take the idmapping into account when it sends the
relevant mds request.

In order to support idmapped mounts for cephfs we stash the idmapping
whenever they are relevant for the operation for the duration of the
request. Since mds requests can be queued and performed asynchronously
we make sure to keep the idmapping around and release it once the
request has finished.

In follow-up patches we will use this to send correct ownership
information over the wire. This patch just adds the basic infrastructure
to keep the idmapping around. The actual conversion patches are all
fairly minimal.

Cc: Xiubo Li <xiubli@redhat.com>
Cc: Jeff Layton <jlayton@kernel.org>
Cc: Ilya Dryomov <idryomov@gmail.com>
Cc: ceph-devel@vger.kernel.org
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
---
 fs/ceph/mds_client.c | 7 +++++++
 fs/ceph/mds_client.h | 1 +
 2 files changed, 8 insertions(+)

diff --git a/fs/ceph/mds_client.c b/fs/ceph/mds_client.c
index 4c0f22acf53d..810c3db2e369 100644
--- a/fs/ceph/mds_client.c
+++ b/fs/ceph/mds_client.c
@@ -12,6 +12,7 @@
 #include <linux/bits.h>
 #include <linux/ktime.h>
 #include <linux/bitmap.h>
+#include <linux/mnt_idmapping.h>
 
 #include "super.h"
 #include "mds_client.h"
@@ -962,6 +963,8 @@ void ceph_mdsc_release_request(struct kref *kref)
 	kfree(req->r_path1);
 	kfree(req->r_path2);
 	put_cred(req->r_cred);
+	if (req->r_mnt_idmap != &nop_mnt_idmap)
+		mnt_idmap_put(req->r_mnt_idmap);
 	if (req->r_pagelist)
 		ceph_pagelist_release(req->r_pagelist);
 	put_request_session(req);
@@ -1018,6 +1021,10 @@ static void __register_request(struct ceph_mds_client *mdsc,
 	insert_request(&mdsc->request_tree, req);
 
 	req->r_cred = get_current_cred();
+	if (!req->r_mnt_idmap)
+		req->r_mnt_idmap = &nop_mnt_idmap;
+	else
+		mnt_idmap_get(req->r_mnt_idmap);
 
 	if (mdsc->oldest_tid == 0 && req->r_op != CEPH_MDS_OP_SETFILELOCK)
 		mdsc->oldest_tid = req->r_tid;
diff --git a/fs/ceph/mds_client.h b/fs/ceph/mds_client.h
index 724307ff89cd..32001ade1ea7 100644
--- a/fs/ceph/mds_client.h
+++ b/fs/ceph/mds_client.h
@@ -280,6 +280,7 @@ struct ceph_mds_request {
 	int r_fmode;        /* file mode, if expecting cap */
 	int r_request_release_offset;
 	const struct cred *r_cred;
+	struct mnt_idmap *r_mnt_idmap;
 	struct timespec64 r_stamp;
 
 	/* for choosing which mds to send this request to */
-- 
2.34.1

[PATCH v3 03/14] ceph: handle idmapped mounts in create_request_message()

[Alexander Mikhalitsyn]

From: Christian Brauner <christian.brauner@ubuntu.com>

Inode operations that create a new filesystem object such as ->mknod,
->create, ->mkdir() and others don't take a {g,u}id argument explicitly.
Instead the caller's fs{g,u}id is used for the {g,u}id of the new
filesystem object.

Cephfs mds creation request argument structures mirror this filesystem
behavior. They don't encode a {g,u}id explicitly. Instead the caller's
fs{g,u}id that is always sent as part of any mds request is used by the
servers to set the {g,u}id of the new filesystem object.

In order to ensure that the correct {g,u}id is used map the caller's
fs{g,u}id for creation requests. This doesn't require complex changes.
It suffices to pass in the relevant idmapping recorded in the request
message. If this request message was triggered from an inode operation
that creates filesystem objects it will have passed down the relevant
idmaping. If this is a request message that was triggered from an inode
operation that doens't need to take idmappings into account the initial
idmapping is passed down which is an identity mapping and thus is
guaranteed to leave the caller's fs{g,u}id unchanged.,u}id is sent.

The last few weeks before Christmas 2021 I have spent time not just
reading and poking the cephfs kernel code but also took a look at the
ceph mds server userspace to ensure I didn't miss some subtlety.

This made me aware of one complication to solve. All requests send the
caller's fs{g,u}id over the wire. The caller's fs{g,u}id matters for the
server in exactly two cases:

1. to set the ownership for creation requests
2. to determine whether this client is allowed access on this server

Case 1. we already covered and explained. Case 2. is only relevant for
servers where an explicit uid access restriction has been set. That is
to say the mds server restricts access to requests coming from a
specific uid. Servers without uid restrictions will grant access to
requests from any uid by setting MDS_AUTH_UID_ANY.

Case 2. introduces the complication because the caller's fs{g,u}id is
not just used to record ownership but also serves as the {g,u}id used
when checking access to the server.

Consider a user mounting a cephfs client and creating an idmapped mount
from it that maps files owned by uid 1000 to be owned uid 0:

mount -t cephfs -o [...] /unmapped
mount-idmapped --map-mount 1000:0:1 /idmapped

That is to say if the mounted cephfs filesystem contains a file "file1"
which is owned by uid 1000:

- looking at it via /unmapped/file1 will report it as owned by uid 1000
  (One can think of this as the on-disk value.)
- looking at it via /idmapped/file1 will report it as owned by uid 0

Now, consider creating new files via the idmapped mount at /idmapped.
When a caller with fs{g,u}id 1000 creates a file "file2" by going
through the idmapped mount mounted at /idmapped it will create a file
that is owned by uid 1000 on-disk, i.e.:

- looking at it via /unmapped/file2 will report it as owned by uid 1000
- looking at it via /idmapped/file2 will report it as owned by uid 0

Now consider an mds server that has a uid access restriction set and
only grants access to requests from uid 0.

If the client sends a creation request for a file e.g. /idmapped/file2
it will send the caller's fs{g,u}id idmapped according to the idmapped
mount. So if the caller has fs{g,u}id 1000 it will be mapped to {g,u}id
0 in the idmapped mount and will be sent over the wire allowing the
caller access to the mds server.

However, if the caller is not issuing a creation request the caller's
fs{g,u}id will be send without the mount's idmapping applied. So if the
caller that just successfully created a new file on the restricted mds
server sends a request as fs{g,u}id 1000 access will be refused. This
however is inconsistent.

From my perspective the root of the problem lies in the fact that
creation requests implicitly infer the ownership from the {g,u}id that
gets sent along with every mds request.

I have thought of multiple ways of addressing this problem but the one I
prefer is to give all mds requests that create a filesystem object a
proper, separate {g,u}id field entry in the argument struct. This is,
for example how ->setattr mds requests work.

This way the caller's fs{g,u}id can be used consistenly for server
access checks and is separated from the ownership for new filesystem
objects.

Servers could then be updated to refuse creation requests whenever the
{g,u}id used for access checking doesn't match the {g,u}id used for
creating the filesystem object just as is done for setattr requests on a
uid restricted server. But I am, of course, open to other suggestions.

Cc: Xiubo Li <xiubli@redhat.com>
Cc: Jeff Layton <jlayton@kernel.org>
Cc: Ilya Dryomov <idryomov@gmail.com>
Cc: ceph-devel@vger.kernel.org
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
---
 fs/ceph/mds_client.c | 22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/fs/ceph/mds_client.c b/fs/ceph/mds_client.c
index 810c3db2e369..e4265843b838 100644
--- a/fs/ceph/mds_client.c
+++ b/fs/ceph/mds_client.c
@@ -2583,6 +2583,8 @@ static struct ceph_msg *create_request_message(struct ceph_mds_session *session,
 	void *p, *end;
 	int ret;
 	bool legacy = !(session->s_con.peer_features & CEPH_FEATURE_FS_BTIME);
+	kuid_t caller_fsuid;
+	kgid_t caller_fsgid;
 
 	ret = set_request_path_attr(req->r_inode, req->r_dentry,
 			      req->r_parent, req->r_path1, req->r_ino1.ino,
@@ -2651,10 +2653,22 @@ static struct ceph_msg *create_request_message(struct ceph_mds_session *session,
 
 	head->mdsmap_epoch = cpu_to_le32(mdsc->mdsmap->m_epoch);
 	head->op = cpu_to_le32(req->r_op);
-	head->caller_uid = cpu_to_le32(from_kuid(&init_user_ns,
-						 req->r_cred->fsuid));
-	head->caller_gid = cpu_to_le32(from_kgid(&init_user_ns,
-						 req->r_cred->fsgid));
+	/*
+	 * Inode operations that create filesystem objects based on the
+	 * caller's fs{g,u}id like ->mknod(), ->create(), ->mkdir() etc. don't
+	 * have separate {g,u}id fields in their respective structs in the
+	 * ceph_mds_request_args union. Instead the caller_{g,u}id field is
+	 * used to set ownership of the newly created inode by the mds server.
+	 * For these inode operations we need to send the mapped fs{g,u}id over
+	 * the wire. For other cases we simple set req->r_mnt_idmap to the
+	 * initial idmapping meaning the unmapped fs{g,u}id is sent.
+	 */
+	caller_fsuid = from_vfsuid(req->r_mnt_idmap, &init_user_ns,
+					VFSUIDT_INIT(req->r_cred->fsuid));
+	caller_fsgid = from_vfsgid(req->r_mnt_idmap, &init_user_ns,
+					VFSGIDT_INIT(req->r_cred->fsgid));
+	head->caller_uid = cpu_to_le32(from_kuid(&init_user_ns, caller_fsuid));
+	head->caller_gid = cpu_to_le32(from_kgid(&init_user_ns, caller_fsgid));
 	head->ino = cpu_to_le64(req->r_deleg_ino);
 	head->args = req->r_args;
 
-- 
2.34.1

[PATCH v3 04/14] ceph: allow idmapped mknod inode op

[Alexander Mikhalitsyn]

From: Christian Brauner <christian.brauner@ubuntu.com>

Enable ceph_mknod() to handle idmapped mounts. This is just a matter of
passing down the mount's idmapping.

Cc: Xiubo Li <xiubli@redhat.com>
Cc: Jeff Layton <jlayton@kernel.org>
Cc: Ilya Dryomov <idryomov@gmail.com>
Cc: ceph-devel@vger.kernel.org
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
---
 fs/ceph/dir.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/ceph/dir.c b/fs/ceph/dir.c
index cb67ac821f0e..8d3fedb3629b 100644
--- a/fs/ceph/dir.c
+++ b/fs/ceph/dir.c
@@ -884,6 +884,7 @@ static int ceph_mknod(struct mnt_idmap *idmap, struct inode *dir,
 	req->r_parent = dir;
 	ihold(dir);
 	set_bit(CEPH_MDS_R_PARENT_LOCKED, &req->r_req_flags);
+	req->r_mnt_idmap = idmap;
 	req->r_args.mknod.mode = cpu_to_le32(mode);
 	req->r_args.mknod.rdev = cpu_to_le32(rdev);
 	req->r_dentry_drop = CEPH_CAP_FILE_SHARED | CEPH_CAP_AUTH_EXCL;
-- 
2.34.1

[PATCH v3 05/14] ceph: allow idmapped symlink inode op

[Alexander Mikhalitsyn]

From: Christian Brauner <christian.brauner@ubuntu.com>

Enable ceph_symlink() to handle idmapped mounts. This is just a matter
of passing down the mount's idmapping.

Cc: Xiubo Li <xiubli@redhat.com>
Cc: Jeff Layton <jlayton@kernel.org>
Cc: Ilya Dryomov <idryomov@gmail.com>
Cc: ceph-devel@vger.kernel.org
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
---
 fs/ceph/dir.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/ceph/dir.c b/fs/ceph/dir.c
index 8d3fedb3629b..3996572060da 100644
--- a/fs/ceph/dir.c
+++ b/fs/ceph/dir.c
@@ -956,6 +956,7 @@ static int ceph_symlink(struct mnt_idmap *idmap, struct inode *dir,
 	req->r_num_caps = 2;
 	req->r_dentry_drop = CEPH_CAP_FILE_SHARED | CEPH_CAP_AUTH_EXCL;
 	req->r_dentry_unless = CEPH_CAP_FILE_EXCL;
+	req->r_mnt_idmap = idmap;
 	if (as_ctx.pagelist) {
 		req->r_pagelist = as_ctx.pagelist;
 		as_ctx.pagelist = NULL;
-- 
2.34.1

[PATCH v3 06/14] ceph: allow idmapped mkdir inode op

[Alexander Mikhalitsyn]

From: Christian Brauner <christian.brauner@ubuntu.com>

Enable ceph_mkdir() to handle idmapped mounts. This is just a matter of
passing down the mount's idmapping.

Cc: Xiubo Li <xiubli@redhat.com>
Cc: Jeff Layton <jlayton@kernel.org>
Cc: Ilya Dryomov <idryomov@gmail.com>
Cc: ceph-devel@vger.kernel.org
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
---
 fs/ceph/dir.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/ceph/dir.c b/fs/ceph/dir.c
index 3996572060da..a4b1ee5ce6b6 100644
--- a/fs/ceph/dir.c
+++ b/fs/ceph/dir.c
@@ -1024,6 +1024,7 @@ static int ceph_mkdir(struct mnt_idmap *idmap, struct inode *dir,
 	ihold(dir);
 	set_bit(CEPH_MDS_R_PARENT_LOCKED, &req->r_req_flags);
 	req->r_args.mkdir.mode = cpu_to_le32(mode);
+	req->r_mnt_idmap = idmap;
 	req->r_dentry_drop = CEPH_CAP_FILE_SHARED | CEPH_CAP_AUTH_EXCL;
 	req->r_dentry_unless = CEPH_CAP_FILE_EXCL;
 	if (as_ctx.pagelist) {
-- 
2.34.1

[PATCH v3 07/14] ceph: allow idmapped rename inode op

[Alexander Mikhalitsyn]

From: Christian Brauner <christian.brauner@ubuntu.com>

Enable ceph_rename() to handle idmapped mounts. This is just a matter of
passing down the mount's idmapping.

Cc: Xiubo Li <xiubli@redhat.com>
Cc: Jeff Layton <jlayton@kernel.org>
Cc: Ilya Dryomov <idryomov@gmail.com>
Cc: ceph-devel@vger.kernel.org
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
---
 fs/ceph/dir.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/ceph/dir.c b/fs/ceph/dir.c
index a4b1ee5ce6b6..7ae02a690464 100644
--- a/fs/ceph/dir.c
+++ b/fs/ceph/dir.c
@@ -1327,6 +1327,7 @@ static int ceph_rename(struct mnt_idmap *idmap, struct inode *old_dir,
 	req->r_old_dentry_unless = CEPH_CAP_FILE_EXCL;
 	req->r_dentry_drop = CEPH_CAP_FILE_SHARED;
 	req->r_dentry_unless = CEPH_CAP_FILE_EXCL;
+	req->r_mnt_idmap = idmap;
 	/* release LINK_RDCACHE on source inode (mds will lock it) */
 	req->r_old_inode_drop = CEPH_CAP_LINK_SHARED | CEPH_CAP_LINK_EXCL;
 	if (d_really_is_positive(new_dentry)) {
-- 
2.34.1

[PATCH v3 08/14] ceph: allow idmapped getattr inode op

[Alexander Mikhalitsyn]

From: Christian Brauner <christian.brauner@ubuntu.com>

Enable ceph_getattr() to handle idmapped mounts. This is just a matter
of passing down the mount's idmapping.

Cc: Xiubo Li <xiubli@redhat.com>
Cc: Jeff Layton <jlayton@kernel.org>
Cc: Ilya Dryomov <idryomov@gmail.com>
Cc: ceph-devel@vger.kernel.org
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
---
 fs/ceph/inode.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/ceph/inode.c b/fs/ceph/inode.c
index 8e5f41d45283..2e988612ed6c 100644
--- a/fs/ceph/inode.c
+++ b/fs/ceph/inode.c
@@ -2465,7 +2465,7 @@ int ceph_getattr(struct mnt_idmap *idmap, const struct path *path,
 			return err;
 	}
 
-	generic_fillattr(&nop_mnt_idmap, inode, stat);
+	generic_fillattr(idmap, inode, stat);
 	stat->ino = ceph_present_inode(inode);
 
 	/*
-- 
2.34.1

[PATCH v3 09/14] ceph: allow idmapped permission inode op

[Alexander Mikhalitsyn]

From: Christian Brauner <christian.brauner@ubuntu.com>

Enable ceph_permission() to handle idmapped mounts. This is just a
matter of passing down the mount's idmapping.

Cc: Xiubo Li <xiubli@redhat.com>
Cc: Jeff Layton <jlayton@kernel.org>
Cc: Ilya Dryomov <idryomov@gmail.com>
Cc: ceph-devel@vger.kernel.org
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
---
 fs/ceph/inode.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/ceph/inode.c b/fs/ceph/inode.c
index 2e988612ed6c..37e1cbfc7c89 100644
--- a/fs/ceph/inode.c
+++ b/fs/ceph/inode.c
@@ -2408,7 +2408,7 @@ int ceph_permission(struct mnt_idmap *idmap, struct inode *inode,
 	err = ceph_do_getattr(inode, CEPH_CAP_AUTH_SHARED, false);
 
 	if (!err)
-		err = generic_permission(&nop_mnt_idmap, inode, mask);
+		err = generic_permission(idmap, inode, mask);
 	return err;
 }
 
-- 
2.34.1

[PATCH v3 10/14] ceph: pass idmap to __ceph_setattr

[Alexander Mikhalitsyn]

Just pass down the mount's idmapping to __ceph_setattr,
because we will need it later.

Cc: Xiubo Li <xiubli@redhat.com>
Cc: Jeff Layton <jlayton@kernel.org>
Cc: Ilya Dryomov <idryomov@gmail.com>
Cc: brauner@kernel.org
Cc: ceph-devel@vger.kernel.org
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
---
 fs/ceph/acl.c   | 4 ++--
 fs/ceph/inode.c | 5 +++--
 fs/ceph/super.h | 3 ++-
 3 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/fs/ceph/acl.c b/fs/ceph/acl.c
index 6945a938d396..51ffef848429 100644
--- a/fs/ceph/acl.c
+++ b/fs/ceph/acl.c
@@ -140,7 +140,7 @@ int ceph_set_acl(struct mnt_idmap *idmap, struct dentry *dentry,
 		newattrs.ia_ctime = current_time(inode);
 		newattrs.ia_mode = new_mode;
 		newattrs.ia_valid = ATTR_MODE | ATTR_CTIME;
-		ret = __ceph_setattr(inode, &newattrs);
+		ret = __ceph_setattr(idmap, inode, &newattrs);
 		if (ret)
 			goto out_free;
 	}
@@ -151,7 +151,7 @@ int ceph_set_acl(struct mnt_idmap *idmap, struct dentry *dentry,
 			newattrs.ia_ctime = old_ctime;
 			newattrs.ia_mode = old_mode;
 			newattrs.ia_valid = ATTR_MODE | ATTR_CTIME;
-			__ceph_setattr(inode, &newattrs);
+			__ceph_setattr(idmap, inode, &newattrs);
 		}
 		goto out_free;
 	}
diff --git a/fs/ceph/inode.c b/fs/ceph/inode.c
index 37e1cbfc7c89..bcd9b506ec3b 100644
--- a/fs/ceph/inode.c
+++ b/fs/ceph/inode.c
@@ -2009,7 +2009,8 @@ static const struct inode_operations ceph_symlink_iops = {
 	.listxattr = ceph_listxattr,
 };
 
-int __ceph_setattr(struct inode *inode, struct iattr *attr)
+int __ceph_setattr(struct mnt_idmap *idmap, struct inode *inode,
+		   struct iattr *attr)
 {
 	struct ceph_inode_info *ci = ceph_inode(inode);
 	unsigned int ia_valid = attr->ia_valid;
@@ -2252,7 +2253,7 @@ int ceph_setattr(struct mnt_idmap *idmap, struct dentry *dentry,
 	    ceph_quota_is_max_bytes_exceeded(inode, attr->ia_size))
 		return -EDQUOT;
 
-	err = __ceph_setattr(inode, attr);
+	err = __ceph_setattr(idmap, inode, attr);
 
 	if (err >= 0 && (attr->ia_valid & ATTR_MODE))
 		err = posix_acl_chmod(&nop_mnt_idmap, dentry, attr->ia_mode);
diff --git a/fs/ceph/super.h b/fs/ceph/super.h
index d24bf0db5234..d9cc27307cb7 100644
--- a/fs/ceph/super.h
+++ b/fs/ceph/super.h
@@ -1052,7 +1052,8 @@ static inline int ceph_do_getattr(struct inode *inode, int mask, bool force)
 }
 extern int ceph_permission(struct mnt_idmap *idmap,
 			   struct inode *inode, int mask);
-extern int __ceph_setattr(struct inode *inode, struct iattr *attr);
+extern int __ceph_setattr(struct mnt_idmap *idmap, struct inode *inode,
+			  struct iattr *attr);
 extern int ceph_setattr(struct mnt_idmap *idmap,
 			struct dentry *dentry, struct iattr *attr);
 extern int ceph_getattr(struct mnt_idmap *idmap,
-- 
2.34.1

[PATCH v3 11/14] ceph: allow idmapped setattr inode op

[Alexander Mikhalitsyn]

From: Christian Brauner <christian.brauner@ubuntu.com>

Enable __ceph_setattr() to handle idmapped mounts. This is just a matter
of passing down the mount's idmapping.

Cc: Xiubo Li <xiubli@redhat.com>
Cc: Jeff Layton <jlayton@kernel.org>
Cc: Ilya Dryomov <idryomov@gmail.com>
Cc: ceph-devel@vger.kernel.org
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
[ adapted to b27c82e12965 ("attr: port attribute changes to new types") ]
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
---
v3:
	- reworked as Christian suggested here:
	https://lore.kernel.org/lkml/20230602-vorzeichen-praktikum-f17931692301@brauner/
---
 fs/ceph/inode.c | 29 +++++++++++++++++++++--------
 1 file changed, 21 insertions(+), 8 deletions(-)

diff --git a/fs/ceph/inode.c b/fs/ceph/inode.c
index bcd9b506ec3b..09c78a67738b 100644
--- a/fs/ceph/inode.c
+++ b/fs/ceph/inode.c
@@ -2051,17 +2051,27 @@ int __ceph_setattr(struct mnt_idmap *idmap, struct inode *inode,
 
 	dout("setattr %p issued %s\n", inode, ceph_cap_string(issued));
 
+	/*
+	 * The attr->ia_{g,u}id members contain the target {g,u}id we're
+	 * sending over the wire. The mount idmapping only matters when we
+	 * create new filesystem objects based on the caller's mapped
+	 * fs{g,u}id.
+	 */
+	req->r_mnt_idmap = &nop_mnt_idmap;
 	if (ia_valid & ATTR_UID) {
 		dout("setattr %p uid %d -> %d\n", inode,
 		     from_kuid(&init_user_ns, inode->i_uid),
 		     from_kuid(&init_user_ns, attr->ia_uid));
 		if (issued & CEPH_CAP_AUTH_EXCL) {
-			inode->i_uid = attr->ia_uid;
+			inode->i_uid = from_vfsuid(idmap, i_user_ns(inode),
+						   attr->ia_vfsuid);
 			dirtied |= CEPH_CAP_AUTH_EXCL;
 		} else if ((issued & CEPH_CAP_AUTH_SHARED) == 0 ||
-			   !uid_eq(attr->ia_uid, inode->i_uid)) {
+			   !uid_eq(from_vfsuid(idmap, i_user_ns(inode), attr->ia_vfsuid),
+				   inode->i_uid)) {
+			kuid_t fsuid = from_vfsuid(idmap, i_user_ns(inode), attr->ia_vfsuid);
 			req->r_args.setattr.uid = cpu_to_le32(
-				from_kuid(&init_user_ns, attr->ia_uid));
+				from_kuid(&init_user_ns, fsuid));
 			mask |= CEPH_SETATTR_UID;
 			release |= CEPH_CAP_AUTH_SHARED;
 		}
@@ -2071,12 +2081,15 @@ int __ceph_setattr(struct mnt_idmap *idmap, struct inode *inode,
 		     from_kgid(&init_user_ns, inode->i_gid),
 		     from_kgid(&init_user_ns, attr->ia_gid));
 		if (issued & CEPH_CAP_AUTH_EXCL) {
-			inode->i_gid = attr->ia_gid;
+			inode->i_gid = from_vfsgid(idmap, i_user_ns(inode),
+						   attr->ia_vfsgid);
 			dirtied |= CEPH_CAP_AUTH_EXCL;
 		} else if ((issued & CEPH_CAP_AUTH_SHARED) == 0 ||
-			   !gid_eq(attr->ia_gid, inode->i_gid)) {
+			   !gid_eq(from_vfsgid(idmap, i_user_ns(inode), attr->ia_vfsgid),
+				   inode->i_gid)) {
+			kgid_t fsgid = from_vfsgid(idmap, i_user_ns(inode), attr->ia_vfsgid);
 			req->r_args.setattr.gid = cpu_to_le32(
-				from_kgid(&init_user_ns, attr->ia_gid));
+				from_kgid(&init_user_ns, fsgid));
 			mask |= CEPH_SETATTR_GID;
 			release |= CEPH_CAP_AUTH_SHARED;
 		}
@@ -2241,7 +2254,7 @@ int ceph_setattr(struct mnt_idmap *idmap, struct dentry *dentry,
 	if (ceph_inode_is_shutdown(inode))
 		return -ESTALE;
 
-	err = setattr_prepare(&nop_mnt_idmap, dentry, attr);
+	err = setattr_prepare(idmap, dentry, attr);
 	if (err != 0)
 		return err;
 
@@ -2256,7 +2269,7 @@ int ceph_setattr(struct mnt_idmap *idmap, struct dentry *dentry,
 	err = __ceph_setattr(idmap, inode, attr);
 
 	if (err >= 0 && (attr->ia_valid & ATTR_MODE))
-		err = posix_acl_chmod(&nop_mnt_idmap, dentry, attr->ia_mode);
+		err = posix_acl_chmod(idmap, dentry, attr->ia_mode);
 
 	return err;
 }
-- 
2.34.1

[PATCH v3 12/14] ceph/acl: allow idmapped set_acl inode op

[Alexander Mikhalitsyn]

From: Christian Brauner <christian.brauner@ubuntu.com>

Enable ceph_set_acl() to handle idmapped mounts. This is just a matter
of passing down the mount's idmapping.

Cc: Xiubo Li <xiubli@redhat.com>
Cc: Jeff Layton <jlayton@kernel.org>
Cc: Ilya Dryomov <idryomov@gmail.com>
Cc: ceph-devel@vger.kernel.org
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
---
 fs/ceph/acl.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/ceph/acl.c b/fs/ceph/acl.c
index 51ffef848429..d0ca5a0060d8 100644
--- a/fs/ceph/acl.c
+++ b/fs/ceph/acl.c
@@ -105,7 +105,7 @@ int ceph_set_acl(struct mnt_idmap *idmap, struct dentry *dentry,
 	case ACL_TYPE_ACCESS:
 		name = XATTR_NAME_POSIX_ACL_ACCESS;
 		if (acl) {
-			ret = posix_acl_update_mode(&nop_mnt_idmap, inode,
+			ret = posix_acl_update_mode(idmap, inode,
 						    &new_mode, &acl);
 			if (ret)
 				goto out;
-- 
2.34.1

[PATCH v3 13/14] ceph/file: allow idmapped atomic_open inode op

[Alexander Mikhalitsyn]

From: Christian Brauner <christian.brauner@ubuntu.com>

Enable ceph_atomic_open() to handle idmapped mounts. This is just a
matter of passing down the mount's idmapping.

Cc: Xiubo Li <xiubli@redhat.com>
Cc: Jeff Layton <jlayton@kernel.org>
Cc: Ilya Dryomov <idryomov@gmail.com>
Cc: ceph-devel@vger.kernel.org
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
[ adapted to 5fadbd9929 ("ceph: rely on vfs for setgid stripping") ]
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
---
 fs/ceph/file.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/fs/ceph/file.c b/fs/ceph/file.c
index f4d8bf7dec88..f00bfda4b1d2 100644
--- a/fs/ceph/file.c
+++ b/fs/ceph/file.c
@@ -654,7 +654,9 @@ static int ceph_finish_async_create(struct inode *dir, struct dentry *dentry,
 	in.truncate_seq = cpu_to_le32(1);
 	in.truncate_size = cpu_to_le64(-1ULL);
 	in.xattr_version = cpu_to_le64(1);
-	in.uid = cpu_to_le32(from_kuid(&init_user_ns, current_fsuid()));
+	in.uid = cpu_to_le32(from_kuid(&init_user_ns,
+				       mapped_fsuid(req->r_mnt_idmap,
+						    &init_user_ns)));
 	if (dir->i_mode & S_ISGID) {
 		in.gid = cpu_to_le32(from_kgid(&init_user_ns, dir->i_gid));
 
@@ -662,7 +664,9 @@ static int ceph_finish_async_create(struct inode *dir, struct dentry *dentry,
 		if (S_ISDIR(mode))
 			mode |= S_ISGID;
 	} else {
-		in.gid = cpu_to_le32(from_kgid(&init_user_ns, current_fsgid()));
+		in.gid = cpu_to_le32(from_kgid(&init_user_ns,
+				     mapped_fsgid(req->r_mnt_idmap,
+						  &init_user_ns)));
 	}
 	in.mode = cpu_to_le32((u32)mode);
 
@@ -731,6 +735,7 @@ int ceph_atomic_open(struct inode *dir, struct dentry *dentry,
 		     struct file *file, unsigned flags, umode_t mode)
 {
 	struct ceph_fs_client *fsc = ceph_sb_to_client(dir->i_sb);
+	struct mnt_idmap *idmap = file_mnt_idmap(file);
 	struct ceph_mds_client *mdsc = fsc->mdsc;
 	struct ceph_mds_request *req;
 	struct dentry *dn;
@@ -786,6 +791,7 @@ int ceph_atomic_open(struct inode *dir, struct dentry *dentry,
 		mask |= CEPH_CAP_XATTR_SHARED;
 	req->r_args.open.mask = cpu_to_le32(mask);
 	req->r_parent = dir;
+	req->r_mnt_idmap = idmap;
 	ihold(dir);
 
 	if (flags & O_CREAT) {
-- 
2.34.1

[PATCH v3 14/14] ceph: allow idmapped mounts

[Alexander Mikhalitsyn]

From: Christian Brauner <christian.brauner@ubuntu.com>

Now that we converted cephfs internally to account for idmapped mounts
allow the creation of idmapped mounts on by setting the FS_ALLOW_IDMAP
flag.

Cc: Xiubo Li <xiubli@redhat.com>
Cc: Jeff Layton <jlayton@kernel.org>
Cc: Ilya Dryomov <idryomov@gmail.com>
Cc: ceph-devel@vger.kernel.org
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
---
 fs/ceph/super.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/ceph/super.c b/fs/ceph/super.c
index 3fc48b43cab0..4f6e6d57f3f1 100644
--- a/fs/ceph/super.c
+++ b/fs/ceph/super.c
@@ -1389,7 +1389,7 @@ static struct file_system_type ceph_fs_type = {
 	.name		= "ceph",
 	.init_fs_context = ceph_init_fs_context,
 	.kill_sb	= ceph_kill_sb,
-	.fs_flags	= FS_RENAME_DOES_D_MOVE,
+	.fs_flags	= FS_RENAME_DOES_D_MOVE | FS_ALLOW_IDMAP,
 };
 MODULE_ALIAS_FS("ceph");
 
-- 
2.34.1

Re: [PATCH v3 03/14] ceph: handle idmapped mounts in create_request_message()

[Aleksandr Mikhalitsyn]

On Wed, Jun 7, 2023 at 5:21 PM Alexander Mikhalitsyn
<aleksandr.mikhalitsyn@canonical.com> wrote:
>
> From: Christian Brauner <christian.brauner@ubuntu.com>
>
> Inode operations that create a new filesystem object such as ->mknod,
> ->create, ->mkdir() and others don't take a {g,u}id argument explicitly.
> Instead the caller's fs{g,u}id is used for the {g,u}id of the new
> filesystem object.
>
> Cephfs mds creation request argument structures mirror this filesystem
> behavior. They don't encode a {g,u}id explicitly. Instead the caller's
> fs{g,u}id that is always sent as part of any mds request is used by the
> servers to set the {g,u}id of the new filesystem object.
>
> In order to ensure that the correct {g,u}id is used map the caller's
> fs{g,u}id for creation requests. This doesn't require complex changes.
> It suffices to pass in the relevant idmapping recorded in the request
> message. If this request message was triggered from an inode operation
> that creates filesystem objects it will have passed down the relevant
> idmaping. If this is a request message that was triggered from an inode
> operation that doens't need to take idmappings into account the initial
> idmapping is passed down which is an identity mapping and thus is
> guaranteed to leave the caller's fs{g,u}id unchanged.,u}id is sent.
>
> The last few weeks before Christmas 2021 I have spent time not just
> reading and poking the cephfs kernel code but also took a look at the
> ceph mds server userspace to ensure I didn't miss some subtlety.
>
> This made me aware of one complication to solve. All requests send the
> caller's fs{g,u}id over the wire. The caller's fs{g,u}id matters for the
> server in exactly two cases:
>
> 1. to set the ownership for creation requests
> 2. to determine whether this client is allowed access on this server
>
> Case 1. we already covered and explained. Case 2. is only relevant for
> servers where an explicit uid access restriction has been set. That is
> to say the mds server restricts access to requests coming from a
> specific uid. Servers without uid restrictions will grant access to
> requests from any uid by setting MDS_AUTH_UID_ANY.
>
> Case 2. introduces the complication because the caller's fs{g,u}id is
> not just used to record ownership but also serves as the {g,u}id used
> when checking access to the server.
>
> Consider a user mounting a cephfs client and creating an idmapped mount
> from it that maps files owned by uid 1000 to be owned uid 0:
>
> mount -t cephfs -o [...] /unmapped
> mount-idmapped --map-mount 1000:0:1 /idmapped
>
> That is to say if the mounted cephfs filesystem contains a file "file1"
> which is owned by uid 1000:
>
> - looking at it via /unmapped/file1 will report it as owned by uid 1000
>   (One can think of this as the on-disk value.)
> - looking at it via /idmapped/file1 will report it as owned by uid 0
>
> Now, consider creating new files via the idmapped mount at /idmapped.
> When a caller with fs{g,u}id 1000 creates a file "file2" by going
> through the idmapped mount mounted at /idmapped it will create a file
> that is owned by uid 1000 on-disk, i.e.:
>
> - looking at it via /unmapped/file2 will report it as owned by uid 1000
> - looking at it via /idmapped/file2 will report it as owned by uid 0
>
> Now consider an mds server that has a uid access restriction set and
> only grants access to requests from uid 0.
>
> If the client sends a creation request for a file e.g. /idmapped/file2
> it will send the caller's fs{g,u}id idmapped according to the idmapped
> mount. So if the caller has fs{g,u}id 1000 it will be mapped to {g,u}id
> 0 in the idmapped mount and will be sent over the wire allowing the
> caller access to the mds server.
>
> However, if the caller is not issuing a creation request the caller's
> fs{g,u}id will be send without the mount's idmapping applied. So if the
> caller that just successfully created a new file on the restricted mds
> server sends a request as fs{g,u}id 1000 access will be refused. This
> however is inconsistent.
>
> From my perspective the root of the problem lies in the fact that
> creation requests implicitly infer the ownership from the {g,u}id that
> gets sent along with every mds request.
>
> I have thought of multiple ways of addressing this problem but the one I
> prefer is to give all mds requests that create a filesystem object a
> proper, separate {g,u}id field entry in the argument struct. This is,
> for example how ->setattr mds requests work.
>
> This way the caller's fs{g,u}id can be used consistenly for server
> access checks and is separated from the ownership for new filesystem
> objects.
>
> Servers could then be updated to refuse creation requests whenever the
> {g,u}id used for access checking doesn't match the {g,u}id used for
> creating the filesystem object just as is done for setattr requests on a
> uid restricted server. But I am, of course, open to other suggestions.
>
> Cc: Xiubo Li <xiubli@redhat.com>
> Cc: Jeff Layton <jlayton@kernel.org>
> Cc: Ilya Dryomov <idryomov@gmail.com>
> Cc: ceph-devel@vger.kernel.org
> Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
> Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
> ---
>  fs/ceph/mds_client.c | 22 ++++++++++++++++++----
>  1 file changed, 18 insertions(+), 4 deletions(-)
>
> diff --git a/fs/ceph/mds_client.c b/fs/ceph/mds_client.c
> index 810c3db2e369..e4265843b838 100644
> --- a/fs/ceph/mds_client.c
> +++ b/fs/ceph/mds_client.c
> @@ -2583,6 +2583,8 @@ static struct ceph_msg *create_request_message(struct ceph_mds_session *session,
>         void *p, *end;
>         int ret;
>         bool legacy = !(session->s_con.peer_features & CEPH_FEATURE_FS_BTIME);
> +       kuid_t caller_fsuid;
> +       kgid_t caller_fsgid;
>
>         ret = set_request_path_attr(req->r_inode, req->r_dentry,
>                               req->r_parent, req->r_path1, req->r_ino1.ino,
> @@ -2651,10 +2653,22 @@ static struct ceph_msg *create_request_message(struct ceph_mds_session *session,
>
>         head->mdsmap_epoch = cpu_to_le32(mdsc->mdsmap->m_epoch);
>         head->op = cpu_to_le32(req->r_op);
> -       head->caller_uid = cpu_to_le32(from_kuid(&init_user_ns,
> -                                                req->r_cred->fsuid));
> -       head->caller_gid = cpu_to_le32(from_kgid(&init_user_ns,
> -                                                req->r_cred->fsgid));
> +       /*
> +        * Inode operations that create filesystem objects based on the
> +        * caller's fs{g,u}id like ->mknod(), ->create(), ->mkdir() etc. don't
> +        * have separate {g,u}id fields in their respective structs in the
> +        * ceph_mds_request_args union. Instead the caller_{g,u}id field is
> +        * used to set ownership of the newly created inode by the mds server.
> +        * For these inode operations we need to send the mapped fs{g,u}id over
> +        * the wire. For other cases we simple set req->r_mnt_idmap to the
> +        * initial idmapping meaning the unmapped fs{g,u}id is sent.
> +        */
> +       caller_fsuid = from_vfsuid(req->r_mnt_idmap, &init_user_ns,
> +                                       VFSUIDT_INIT(req->r_cred->fsuid));
> +       caller_fsgid = from_vfsgid(req->r_mnt_idmap, &init_user_ns,
> +                                       VFSGIDT_INIT(req->r_cred->fsgid));
> +       head->caller_uid = cpu_to_le32(from_kuid(&init_user_ns, caller_fsuid));
> +       head->caller_gid = cpu_to_le32(from_kgid(&init_user_ns, caller_fsgid));
>         head->ino = cpu_to_le64(req->r_deleg_ino);
>         head->args = req->r_args;
>
> --
> 2.34.1
>

Probably it's worth adding to a commit message or cover letter, but
let it be there for now.

Explanation/demonstration from this thread:
https://lore.kernel.org/lkml/CAEivzxefBRPozUPQxYgVh0gOpjsovtBuJ3w9BoqSizpST_YxTA@mail.gmail.com/#t

1. Mount cephfs

mount.ceph admin@XYZ.cephfs=/ /mnt/ceph -o
mon_addr=127.0.0.1:6789,secret=very_secret_key

2. Make 1000:1000 a root dentry owner (it will be convenient because
we want to use mapping 1000:0:1 for simplicity)

chown 1000:1000 /mnt/ceph

3. create an idmapped mount based on a regular /mnt/ceph mount using a
mount-idmapped tool that was written by Christian.
[ taken from https://raw.githubusercontent.com/brauner/mount-idmapped/master/mount-idmapped.c
]

./mount-idmapped --map-mount b:1000:0:1 /mnt/ceph /mnt/ceph_idmapped

"b" stands for "both", so we are creating a mapping of length 1 for
both UID and GID.
1000 is a UID/GID "on-disk", 0 is a mapped UID/GID.

4. Just to be precise, let's look at which UID/GID we have now.

root@ubuntu:/home/ubuntu# ls -lan /mnt/ceph
total 4
drwxrwxrwx 2 1000 1000    0 Jun  1 17:51 .
drwxr-xr-x 4    0    0 4096 Jun  1 16:55 ..

root@ubuntu:/home/ubuntu# ls -lan /mnt/ceph_idmapped
total 4
drwxrwxrwx 2 0 0    0 Jun  1 17:51 .
drwxr-xr-x 4 0 0 4096 Jun  1 16:55 ..

5. Now let's create a bunch of files with different owners and through
different mounts (idmapped/non-idmapped).

5.1. Create a file from 0:0 through the idmapped mount (it should
appear as 1000:1000 on disk)
root@ubuntu:/home/ubuntu# sudo -u#0 -g#0 touch
/mnt/ceph_idmapped/created_through_idmapped_mnt_with_uid0

5.2. Create a file from 1000:1000 through the idmapped mount (should
fail because 1000:1000 is not a valid UID/GID as it can't be mapped
back to the "on-disk" UID/GID set).
root@ubuntu:/home/ubuntu# sudo -u#1000 -g#1000 touch
/mnt/ceph_idmapped/created_through_idmapped_mnt_with_uid1000
touch: cannot touch
'/mnt/ceph_idmapped/created_through_idmapped_mnt_with_uid1000': Value
too large for defined data type

... and we've got EOVERFLOW. That's correct!

5.3. Create a file from 0:0 but through the regular mount. (it should
appear as overflowuid(=65534) in idmapped mount, because 0:0 on-disk
is not mapped to the UID/GID set).

root@ubuntu:/home/ubuntu# sudo -u#0 -g#0 touch
/mnt/ceph/created_directly_with_uid0

5.4. Create a file from 1000:1000 but through the regular mount. (it
should appear as 0:0 in idmapped mount, because 1000 (on-disk) mapped
to 0).

root@ubuntu:/home/ubuntu# sudo -u#1000 -g#1000 touch
/mnt/ceph/created_directly_with_uid1000

6. Now let's look on the result:

root@ubuntu:/home/ubuntu# ls -lan /mnt/ceph
total 4
drwxrwxrwx 2 1000 1000    3 Jun  1 17:54 .
drwxr-xr-x 4    0    0 4096 Jun  1 16:55 ..
-rw-r--r-- 1    0    0    0 Jun  1 17:54 created_directly_with_uid0
-rw-rw-r-- 1 1000 1000    0 Jun  1 17:54 created_directly_with_uid1000
-rw-r--r-- 1 1000 1000    0 Jun  1 17:53 created_through_idmapped_mnt_with_uid0

root@ubuntu:/home/ubuntu# ls -lan /mnt/ceph_idmapped
total 4
drwxrwxrwx 2     0     0    3 Jun  1 17:54 .
drwxr-xr-x 4     0     0 4096 Jun  1 16:55 ..
-rw-r--r-- 1 65534 65534    0 Jun  1 17:54 created_directly_with_uid0
-rw-rw-r-- 1     0     0    0 Jun  1 17:54 created_directly_with_uid1000
-rw-r--r-- 1     0     0    0 Jun  1 17:53
created_through_idmapped_mnt_with_uid0