bug#70165: D-Bus system service breaks reconfiguration when /var/run/dbus is present + /run and /var/run are on separate file systems.

[Hilton Chain]

On Fri, 05 Apr 2024 02:52:09 +0800,
Liliana Marie Prikler wrote:
>
> Am Donnerstag, dem 04.04.2024 um 00:36 +0800 schrieb Hilton Chain:
> > Hi,
> >
> > I have /var/run and /run on separate file systems, recently I noticed
> > system reconfiguration stopped with "guix system: error: rename-file:
> > Invalid cross-device link":
> >
> > --8<---------------cut here---------------start------------->8---
> > newfstatat(AT_FDCWD, "/run", {st_mode=S_IFDIR|0755, st_size=440,
> > ...}, AT_SYMLINK_NOFOLLOW) = 0
> > newfstatat(AT_FDCWD, "/run/dbus", {st_mode=S_IFDIR|0700, st_size=40,
> > ...}, AT_SYMLINK_NOFOLLOW) = 0
> > mkdir("/run", 0777)                     = -1 EEXIST (File exists)
> > mkdir("/run/dbus", 0777)                = -1 EEXIST (File exists)
> > chown("/run/dbus", 988, 983)            = 0
> > chmod("/run/dbus", 0755)                = 0
> > symlink("/run/dbus", "/var/run/dbus")   = -1 EEXIST (File exists)
> > readlink("/var/run/dbus", 0x1634730, 100) = -1 EINVAL (Invalid
> > argument)
> > openat(AT_FDCWD, "/var/run/dbus",
> > O_RDONLY|O_NONBLOCK|O_CLOEXEC|O_DIRECTORY) = 17
> > newfstatat(17, "", {st_mode=S_IFDIR|0755, st_size=80, ...},
> > AT_EMPTY_PATH) = 0
> > getdents64(17, 0x16dfe10 /* 4 entries */, 32768) = 112
> > rename("/var/run/dbus/system_bus_socket",
> > "/run/dbus/system_bus_socket") = -1 EXDEV (Invalid cross-device link)
> > close(13)                               = 0
> > write(2, "\33[1m\33[0mguix system: error: rena"..., 67guix
> > system: error: rename-file: Invalid cross-device link
> > ) = 67
> > exit_group(1)                           = ?
> > +++ exited with 1 +++
> > --8<---------------cut here---------------end--------------->8---
> >
> > It's because /var/run/dbus was used for dbus service before, and now
> > migration to /run/dbus is done with ‘rename-file’:
> >
> > --8<---------------cut here---------------start------------->8---
> > (rename-file (string-append "/var/run/dbus/" next)
> >              (string-append "/run/dbus/" next))
> > --8<---------------cut here---------------end--------------->8---
> >
> > I think the logic can be improved for this case, but not sure how at
> > the moment.
> > What do you think?
> Is there a *good* reason to have those be separate file systems? :D
> What does the FHS say?
>
> Anyhow, cross-fs symlinks and rename-file ought to work assuming that
> both file-systems are already mounted.  Is one of them not needed for
> boot?

Specifically, I'm using tmpfs for them:

--8<---------------cut here---------------start------------->8---
(file-system
  (type "tmpfs")
  (mount-point "/run")
  (device "tmpfs")
  (flags '(no-dev strict-atime))
  (options "mode=0755,nr_inodes=800k,size=20%")
  (needed-for-boot? #t)
  (check? #f))

(file-system
  (type "tmpfs")
  (mount-point "/var/run")
  (device "tmpfs")
  (flags '(no-suid no-dev strict-atime))
  (options "mode=0755,nr_inodes=800k,size=20%")
  (needed-for-boot? #t)
  (check? #f))
--8<---------------cut here---------------end--------------->8---


And here's a reproducer:

--8<---------------cut here---------------start------------->8---
test_dir="$(mktemp --directory)"

sudo mount --type tmpfs none "$test_dir"
touch "$test_dir/test"

guile -c "(rename-file \"$test_dir/test\" \"/tmp/test\")"

sudo umount "$test_dir"
rmdir "$test_dir"
--8<---------------cut here---------------end--------------->8---

--8<---------------cut here---------------start------------->8---
Backtrace:
In ice-9/boot-9.scm:
  1752:10  6 (with-exception-handler _ _ #:unwind? _ # _)
In unknown file:
           5 (apply-smob/0 #<thunk 7061ede8c300>)
In ice-9/boot-9.scm:
    724:2  4 (call-with-prompt ("prompt") #<procedure 7061ede99320 …> …)
In ice-9/eval.scm:
    619:8  3 (_ #(#(#<directory (guile-user) 7061ede8fc80>)))
In ice-9/command-line.scm:
   185:19  2 (_ #<input: string 7061ede89850>)
In unknown file:
           1 (eval (rename-file "/tmp/tmp.9wyzRfQ28l/test" "/tmp/…") #)
           0 (rename-file "/tmp/tmp.9wyzRfQ28l/test" "/tmp/test")

ERROR: In procedure rename-file:
In procedure rename-file: Invalid cross-device link
--8<---------------cut here---------------end--------------->8---