On 2/6/26 2:16 PM, Waiman Long wrote:
On 2/6/26 12:22 AM, Al Viro wrote:
On Thu, Feb 05, 2026 at 11:11:51PM -0500, Waiman Long wrote:
__latent_entropy
struct mnt_namespace *copy_mnt_ns(u64 flags, struct mnt_namespace *ns,
struct user_namespace *user_ns, struct fs_struct
*new_fs)
{
:
if (new_fs) {
if (&p->mnt == new_fs->root.mnt) {
new_fs->root.mnt = mntget(&q->mnt);
rootmnt = &p->mnt;
}
if (&p->mnt == new_fs->pwd.mnt) {
new_fs->pwd.mnt = mntget(&q->mnt);
pwdmnt = &p->mnt;
}
}
It is replacing the fs->pwd.mnt with a new one while pwd_refs is 1.
I can
make this work with the new fs_struct field. I do have one question
though.
Do we need to acquire write_seqlock(&new_fs->seq) if we are changing
root or
pwd here or if the new_fs are in such a state that it will never
change when
this copying operation is in progress?
In all cases when we get to that point, new_fs is always a freshly
created private copy of current->fs, not reachable from anywhere
other than stack frames of the callers, but the proof is not pretty.
copy_mnt_ns() is called only by create_new_namespaces() and it gets to
copying anything if and only if CLONE_NEWNS is in the flags. So far,
so good. The call in create_new_namespaces() is
new_nsp->mnt_ns = copy_mnt_ns(flags, tsk->nsproxy->mnt_ns,
user_ns, new_fs);
Thanks for the detailed explanation. After further investigation as to
while the pwd_refs is set, I found out the code path leading to this
situation is the unshare syscall.
__x64_sys_unshare()
=> ksys_unshare()
=> unshare_fs(unshare_flags, &new_fs)
=> unshare_nsproxy_namespaces(unshare_flags, &new_nsproxy,
new_cred, new_fs);
=> create_new_namespaces(unshare_flags, current, user_ns,
new_fs ? new_fs : current->fs);
Here, CLONE_FS isn't set in unshare_flags. So new_fs is NULL and
current->fs is passed down to create_new_namespaces(). That is why
pwd_refs can be set in this case. So it looks like the comment in
copy_mnt_ns() saying that the fs_struct is private is no longer true,
at least in this case. So changing fs_struct without taking the lock
can lead to unexpected result.
Should we add locking to make it safe?
I guess if private means fs->users == 1, the condition could still be true.
Cheers,
Longman
