From 49acacdc7cd3c1dcf5190a80ea9e6ca2ffa06cec Mon Sep 17 00:00:00 2001 From: Al Viro Date: Wed, 7 May 2025 14:05:50 -0400 Subject: [PATCH] clone_mnt(): simplify the propagation-related logics 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 Signed-off-by: Al Viro --- fs/namespace.c | 26 +++++++++++++------------- 1 file changed, 13 insertions(+), 13 deletions(-) diff --git a/fs/namespace.c b/fs/namespace.c index 791904128f1e9..12cf69da43209 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -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: -- 2.47.2