bug#71226: ‘guix shell -C’ doesn’t work on Ubuntu 24.04

[Ludovic Courtès]

On Ubuntu 24.04, ‘guix shell -C’ has its child process (in a separate
mount namespace) fail to mount a tmpfs:

--8<---------------cut here---------------start------------->8---
294642 clone(child_stack=NULL, 
flags=CLONE_NEWNS|CLONE_NEWCGROUP|CLONE_NEWUTS|CLONE_NEWIPC|CLONE_NEWUSER|CLONE_NEWPID|CLONE_NEWNET|SIGCHLD)
 = 294653
294642 close(15)                        = 0
294642 getuid()                         = 1000
294642 getgid()                         = 1000
294653 close(16)                        = 0
294642 openat(AT_FDCWD, "/proc/294653/setgroups", O_WRONLY|O_CREAT|O_TRUNC, 
0666 <unfinished ...>
294653 read(15,  <unfinished ...>
294642 <... openat resumed>)            = 6
294642 newfstatat(6, "", {st_mode=S_IFREG|0644, st_size=0, ...}, AT_EMPTY_PATH) 
= 0
294642 lseek(6, 0, SEEK_CUR)            = 0
294642 write(6, "deny", 4)              = 4
294642 close(6)                         = 0
294642 openat(AT_FDCWD, "/proc/294653/uid_map", O_WRONLY|O_CREAT|O_TRUNC, 0666) 
= 6
294642 newfstatat(6, "", {st_mode=S_IFREG|0644, st_size=0, ...}, AT_EMPTY_PATH) 
= 0
294642 lseek(6, 0, SEEK_CUR)            = 0
294642 write(6, "1000 1000 1", 11)      = 11
294642 close(6)                         = 0
294642 openat(AT_FDCWD, "/proc/294653/gid_map", O_WRONLY|O_CREAT|O_TRUNC, 0666) 
= 6
294642 newfstatat(6, "", {st_mode=S_IFREG|0644, st_size=0, ...}, AT_EMPTY_PATH) 
= 0
294642 lseek(6, 0, SEEK_CUR)            = 0
294642 write(6, "1000 1000 1", 11)      = 11
294642 close(6)                         = 0
294642 write(16, "ready", 5)            = 5
294653 <... read resumed>"r", 1)        = 1
294642 write(16, "\n", 1)               = 1
294653 read(15, "e", 1)                 = 1
294642 read(16,  <unfinished ...>
294653 read(15, "a", 1)                 = 1
294653 read(15, "d", 1)                 = 1
294653 read(15, "y", 1)                 = 1
294653 read(15, "\n", 1)                = 1
294653 mount("none", "/tmp/guix-directory.3DaoGp", "tmpfs", 0, NULL) = -1 
EACCES (Permission denied)
294653 write(15, "(", 1)                = 1
294642 <... read resumed>"(", 1)        = 1
294653 write(15, "system-error", 12 <unfinished ...>
--8<---------------cut here---------------end--------------->8---

(It used to work on Ubuntu 22.)

Ludo’.