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clone_mnt(): simplify the propagation-related logics

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The underlying rules are simple:
	* MNT_SHARED should be set iff ->mnt_group_id of new mount ends up
non-zero.
	* mounts should be on the same ->mnt_share cyclic list iff they have
the same non-zero ->mnt_group_id value.
	* CL_PRIVATE is mutually exclusive with MNT_SHARED, MNT_SLAVE,
MNT_SHARED_TO_SLAVE and MNT_EXPIRE; the whole point of that thing is to
get a clone of old mount that would *not* be on any namespace-related
lists.

The above allows to make the logics more straightforward; what's more,
it makes the proof that invariants are maintained much simpler.
The variant in mainline is safe (aside of a very narrow race with
unsafe modification of mnt_flags right after we had the mount exposed
in superblock's ->s_mounts; theoretically it can race with ro remount
of the original, but it's not easy to hit), but proof of its correctness
is really unpleasant.

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
Al Viro
2025-05-07 14:05:50 -04:00
parent d08fa7f44a
commit 49acacdc7c
1 changed files with 13 additions and 13 deletions
Download Patch File Download Diff File Expand all files Collapse all files

26
fs/namespace.c
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@@ -1301,6 +1301,9 @@ static struct mount *clone_mnt(struct mount *old, struct dentry *root,
if (!mnt)
return ERR_PTR(-ENOMEM);
mnt->mnt.mnt_flags = READ_ONCE(old->mnt.mnt_flags) &
~MNT_INTERNAL_FLAGS;
if (flag & (CL_SLAVE | CL_PRIVATE | CL_SHARED_TO_SLAVE))
mnt->mnt_group_id = 0; /* not a peer of original */
else
@@ -1312,8 +1315,8 @@ static struct mount *clone_mnt(struct mount *old, struct dentry *root,
goto out_free;
}
mnt->mnt.mnt_flags = old->mnt.mnt_flags;
mnt->mnt.mnt_flags &= ~(MNT_WRITE_HOLD|MNT_MARKED|MNT_INTERNAL|MNT_LOCKED);
if (mnt->mnt_group_id)
set_mnt_shared(mnt);
atomic_inc(&sb->s_active);
mnt->mnt.mnt_idmap = mnt_idmap_get(mnt_idmap(&old->mnt));
@@ -1326,22 +1329,20 @@ static struct mount *clone_mnt(struct mount *old, struct dentry *root,
list_add_tail(&mnt->mnt_instance, &sb->s_mounts);
unlock_mount_hash();
if (flag & CL_PRIVATE) // we are done with it
return mnt;
if (peers(mnt, old))
list_add(&mnt->mnt_share, &old->mnt_share);
if ((flag & CL_SLAVE) ||
((flag & CL_SHARED_TO_SLAVE) && IS_MNT_SHARED(old))) {
list_add(&mnt->mnt_slave, &old->mnt_slave_list);
mnt->mnt_master = old;
CLEAR_MNT_SHARED(mnt);
} else if (!(flag & CL_PRIVATE)) {
if ((flag & CL_MAKE_SHARED) || IS_MNT_SHARED(old))
list_add(&mnt->mnt_share, &old->mnt_share);
if (IS_MNT_SLAVE(old))
list_add(&mnt->mnt_slave, &old->mnt_slave);
} else if (IS_MNT_SLAVE(old)) {
list_add(&mnt->mnt_slave, &old->mnt_slave);
mnt->mnt_master = old->mnt_master;
} else {
CLEAR_MNT_SHARED(mnt);
}
if (flag & CL_MAKE_SHARED)
set_mnt_shared(mnt);
/* stick the duplicate mount on the same expiry list
* as the original if that was on one */
@@ -1349,7 +1350,6 @@ static struct mount *clone_mnt(struct mount *old, struct dentry *root,
if (!list_empty(&old->mnt_expire))
list_add(&mnt->mnt_expire, &old->mnt_expire);
}
return mnt;
out_free: