Bug 218336

Summary:	mount(2) doesn't document ENOSPC when /proc/sys/fs/mount-max is exceeded
Product:	Documentation	Reporter:	Simon McVittie (smcv)
Component:	man-pages	Assignee:	documentation_man-pages (documentation_man-pages)
Status:	NEW ---
Severity:	normal	CC:	alx
Priority:	P3
Hardware:	All
OS:	Linux
Kernel Version:		Subsystem:
Regression:	No	Bisected commit-id:

Description Simon McVittie 2024-01-03 15:32:05 UTC

When setting up mount points in a new mount namespace, if /proc/sys/fs/mount-max is exceeded, mount(2) will fail with ENOSPC. This isn't documented in mount(2) as a possible error result.

In this situation, the error message "No space left on device" from strerror() is misleading: usually all of the user's filesystem devices will have plenty of space. The actual problem is that an essentially arbitrary anti-denial-of-service limit, orthogonal to how much space exists on any particular device, has been exceeded.

For example, this could easily happen when running Flatpak, which creates new user and mount namespaces using the bubblewrap tool:

$ echo 50 | sudo tee /proc/sys/fs/mount-max
$ flatpak run org.gnome.Platform//45       
bwrap: Failed to mount tmpfs: No space left on device
error: ldconfig failed, exit status 256

I'm going to send a PR to bubblewrap to make it special-case ENOSPC and display a clearer error message in this case. It would be helpful if mount(2) indicated ENOSPC as a possible error here, so that maintainers of projects like bubblewrap could make sure to allow for it.

https://github.com/ValveSoftware/steam-runtime/issues/637 is a real-world report of this happening in Steam's Steam Linux Runtime container framework, which is very similar to Flatpak.

Comment 1 Simon McVittie 2024-01-03 16:04:08 UTC

The bubblewrap PR is https://github.com/containers/bubblewrap/pull/615.

The text I would hope to see in the man page could perhaps be something like:

**ENOSPC**
    The mount operation would cause the current mount namespace to
    have more mount points than allowed by _/proc/sys/fs/mount-max_.

...

**SEE ALSO**
    ..., https://www.kernel.org/doc/html/latest/admin-guide/sysctl/fs.html#mount-max

Or if the man-pages maintainers would prefer to have text that anticipates potential future kernel changes, perhaps more generally:

**ENOSPC**
    The mount operation would cause a resource limit to be exceeded,
    for example more mount points than allowed by _/proc/sys/fs/mount-max_
    in the current mount namespace.

Comment 2 Alejandro Colomar 2024-01-03 16:29:15 UTC

Would you mind sending a patch?  Please check this: <https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING>.

Thanks!