[RFC PATCH 3/6] fs/namespace: Add vfs_open_detached_mount()

[sashiko-bot at kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] fs: mount reference leak in vfs_open_detached_mount() error path
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commit 7bbd41a97d183f30490f0856f394037fd1846a6f
Author: Pratyush Yadav (Google) <pratyush at kernel.org>

fs/namespace: Add vfs_open_detached_mount()

This commit factors out the creation of an anonymous mount namespace and an
O_PATH file for a detached mount into a new helper, vfs_open_detached_mount.
This allows in-kernel mount creators to hand a detached mount file to
userspace, similar to what fsmount does.

> diff --git a/fs/namespace.c b/fs/namespace.c
> index 1ecd96c918b33..5e7ba2b61dd9a 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
[ ... ]
> +/**
> + * vfs_open_detached_mount - Publish a new mount as a detached mount file.
> + * @mnt: The mount to publish. Must not be attached to a mount namespace. The
> + *       caller's reference is consumed on success.
[ ... ]
> + * Return: the new file, or an ERR_PTR. On failure the caller's reference to
> + *         @mnt is dropped, as the mount cannot be published.
> + */
> +struct file *vfs_open_detached_mount(struct vfsmount *mnt)
> +{
> +	struct path path __free(path_put) = {};
> +	struct mnt_namespace *ns;
> +	struct file *file;
> +
> +	if (WARN_ON_ONCE(real_mount(mnt)->mnt_ns))
> +		return ERR_PTR(-EINVAL);

[Severity: Medium]
Does this error path leak the vfsmount reference?

The function documentation explicitly guarantees that on failure, the
caller's reference to mnt is dropped. If a caller inadvertently passes a
mount already attached to a mount namespace, this WARN_ON_ONCE condition
evaluates to true and the function returns an error without dropping the
reference.

For comparison, the error path immediately below for alloc_mnt_ns
failure correctly calls mntput(mnt) before returning.

> +
> +	ns = alloc_mnt_ns(current->nsproxy->mnt_ns->user_ns, true);
> +	if (IS_ERR(ns)) {
> +		mntput(mnt);
> +		return ERR_CAST(ns);
> +	}
[ ... ]

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Sashiko AI review · https://sashiko.dev/#/patchset/20260923224408.3745689-1-pratyush@kernel.org?part=3