Re: [PATCH RFC] util: pick a better runtime directory when XDG_RUNTIME_DIR isn't set

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On Tue, Feb 18, 2025 at 09:33:43AM -0500, Laine Stump wrote:
> On 2/18/25 4:26 AM, Daniel P. Berrangé wrote:
> > On Mon, Feb 17, 2025 at 03:11:56PM -0500, Laine Stump wrote:
> > > On 2/17/25 5:28 AM, Daniel P. Berrangé wrote:
> > > > On Mon, Feb 17, 2025 at 02:14:49AM -0500, Laine Stump wrote:
> > > > > But sometimes XDG_RUNTIME_DIR isn't set in the user's environment.
> > > > 
> > > > Do you have examples of scenarios in which this happens, and
> > > > yet the /run/user/NNNN directory is still being created, as
> > > > that rather sounds like something is broken outside of libvirt.
> > > 
> > > After seeing the bug report, I replicated the situation by ssh'ing in as a
> > > user that hadn't previously logged in, and then unsetting XDG_RUNTIME_DIR. I
> > > hadn't thought there might be some other case where the user could be logged
> > > in but XDG_RUNTIME_DIR had never been set.
> > > 
> > > But after seeing your question I tried running
> > > 
> > >    sudo $someuser virsh list
> > 
> > NB, that is the classic sudo usage trapdoor, because they didn't
> > make "-i" (aka --login) the default, so your environment is not
> > populated correctly.
> > 
> > I'd hope that when passing sudo -i ... it will do the right
> > thing
> 
> It seems not. If I login as $someuser, start a guest, then in a separate
> terminal window from root run:
> 
>       sudo -u $someuser -i virsh list
> 
> It returns an empty list (the same as if I omit the -i). By running the same
> command without "virsh list", I'm given a shell instance, and within that
> shell I can see that $UID, $USER, and $EUID are all set, but
> $XDG_RUNTIME_DIR is not.

Hmm, this appears to be caused by systemd_pam

When using "su -" (similar seen with sudo)

su[5870]: pam_systemd(su-l:session): pam-systemd initializing
su[5870]: pam_systemd(su-l:session): New sd-bus connection (system-bus-pam-systemd-5870) opened.
su[5870]: pam_systemd(su-l:session): Asking logind to create session: uid=1001 pid=5870 service=su-l type=tty class=user desktop= seat= vtnr=0 tty=pts/3 display= remote=no remote_user=root remote_host=
su[5870]: pam_systemd(su-l:session): Session limits: memory_max=n/a tasks_max=n/a cpu_weight=n/a io_weight=n/a runtime_max_sec=n/a
su[5870]: pam_systemd(su-l:session): Not creating session: Already running in a session or user slice
su[5870]: pam_systemd(su-l:session): pam-systemd shutting down

vs used with ssh:

sshd-session[5937]: pam_systemd(sshd:session): pam-systemd initializing
sshd-session[5937]: pam_systemd(sshd:session): New sd-bus connection (system-bus-pam-systemd-5937) opened.
sshd-session[5937]: pam_systemd(sshd:session): Asking logind to create session: uid=0 pid=5937 service=sshd type=tty class=user desktop= seat= vtnr=0 tty= display= remote=yes remote_user= remote_host=10.42.28.158
sshd-session[5937]: pam_systemd(sshd:session): Session limits: memory_max=n/a tasks_max=n/a cpu_weight=n/a io_weight=n/a runtime_max_sec=n/a
sshd-session[5937]: pam_systemd(sshd:session): Reply from logind: id=12 object_path=/org/freedesktop/login1/session/_312 runtime_path=/run/user/0 session_fd=9 seat= vtnr=0 original_uid=0


So if the current user is already in a login sesssion, it'll refuse to
start a new session.

I struggle to understand the rationale for this behaviour. It seems
guaranteed to break stuff...

With regards,
Daniel
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