bug#46292: ‘guix environment -C’ fails with Linux 4.19 (Debian)

["Ludovic Courtès" <ludovic.courtes@inria.fr>]

I’m observing this:

--8<---------------cut here---------------start------------->8---
$ guix environment --ad-hoc coreutils -C
guix environment: error: mount: mount "/gnu/store/mmhimfwmmidf09jw1plw3aw1g1zn2nkh-bash-static-5.0.16" on "/tmp/guix-directory.Nagh8Y//gnu/store/mmhimfwmmidf09jw1plw3aw1g1zn2nkh-bash-static-5.0.16": Operation not permitted
$ uname -rv
4.19.0-13-amd64 #1 SMP Debian 4.19.160-2 (2020-11-28)
$ cat /proc/sys/kernel/unprivileged_userns_clone
1
--8<---------------cut here---------------end--------------->8---

Excerpt of the strace log:

--8<---------------cut here---------------start------------->8---
7605  mkdir("/tmp/guix-directory.EtXAVT/dev/mqueue", 0777) = 0
7605  mount("mqueue", "/tmp/guix-directory.EtXAVT//dev/mqueue", "mqueue", MS_NOSUID|MS_NODEV|MS_NOEXEC, NULL) = 0
7605  stat("/home/lcourtes", {st_mode=S_IFDIR|0710, st_size=4096, ...}) = 0
7605  mkdir("/tmp", 0777)               = -1 EEXIST (File exists)
7605  mkdir("/tmp/guix-directory.EtXAVT", 0777) = -1 EEXIST (File exists)
7605  mkdir("/tmp/guix-directory.EtXAVT/home", 0777) = 0
7605  mkdir("/tmp/guix-directory.EtXAVT/home/lcourtes", 0777) = 0
7605  mount("/home/lcourtes", "/tmp/guix-directory.EtXAVT//home/lcourtes", 0xeea390, MS_BIND, NULL) = 0
7605  stat("/gnu/store/mmhimfwmmidf09jw1plw3aw1g1zn2nkh-bash-static-5.0.16", {st_mode=S_IFDIR|0555, st_size=4096, ...}) = 0
7605  mkdir("/tmp", 0777)               = -1 EEXIST (File exists)
7605  mkdir("/tmp/guix-directory.EtXAVT", 0777) = -1 EEXIST (File exists)
7605  mkdir("/tmp/guix-directory.EtXAVT/gnu", 0777) = 0
7605  mkdir("/tmp/guix-directory.EtXAVT/gnu/store", 0777) = 0
7605  mkdir("/tmp/guix-directory.EtXAVT/gnu/store/mmhimfwmmidf09jw1plw3aw1g1zn2nkh-bash-static-5.0.16", 0777) = 0
7605  mount("/gnu/store/mmhimfwmmidf09jw1plw3aw1g1zn2nkh-bash-static-5.0.16", "/tmp/guix-directory.EtXAVT//gnu/store/mmhimfwmmidf09jw1plw3aw1g1zn2nkh-bash-static-5.0.16", 0xeea3b0, MS_RDONLY|MS_BIND, NULL) = 0
7605  mount("/gnu/store/mmhimfwmmidf09jw1plw3aw1g1zn2nkh-bash-static-5.0.16", "/tmp/guix-directory.EtXAVT//gnu/store/mmhimfwmmidf09jw1plw3aw1g1zn2nkh-bash-static-5.0.16", 0xeea3d0, MS_RDONLY|MS_REMOUNT|MS_BIND, NULL) = -1 EPERM (Operation not permitted)
--8<---------------cut here---------------end--------------->8---

The read-only remount comes from ‘mount-file-system’ in (gnu build
file-systems):

    ;; For read-only bind mounts, an extra remount is needed, as per
    ;; <http://lwn.net/Articles/281157/>, which still applies to Linux
    ;; 4.0.
    (when (and (= MS_BIND (logand flags MS_BIND))
               (= MS_RDONLY (logand flags MS_RDONLY)))
      (let ((flags (logior MS_BIND MS_REMOUNT MS_RDONLY)))
        (mount source mount-point type flags #f)))

This recipe has been working well “forever”, although it’s probably
unnecessary with recent kernels (the LWN article is from 2008).

The problem may have to do with the fact that /gnu/store is an NFS
mount.  Indeed, similar commands fail on $HOME (also an NFS mount):

--8<---------------cut here---------------start------------->8---
$ mkdir t m
$ unshare -mrf
# mount --bind ./t ./m
# mount --bind -r -o remount ./t ./m
mount: /home/lcourtes/m: permission denied.
--8<---------------cut here---------------end--------------->8---

… but they succeed on /tmp (not an NFS mount):

--8<---------------cut here---------------start------------->8---
$ mkdir /tmp/t
$ mkdir /tmp/m
$ unshare -mrf
# mount --bind /tmp/{t,m}
# mount --bind -r -o remount /tmp/{t,m}
--8<---------------cut here---------------end--------------->8---

To be continued…

Ludo’.