[systemd-devel] Question about the killing spree during the transition from the initrd to the root file system.

[Demi Marie Obenour]

On Thu, Jul 04, 2024 at 03:18:04PM +0200, Lennart Poettering wrote:
> On Do, 04.07.24 11:24, chenruyi (A) (chenruyi2 at huawei.com) wrote:
> 
> > Hi,
> >
> > I have some processes in my initrd needed to be excluded from the killing spree
> > during switch-root and needed to continue to run in the root file system. I read
> > the ROOT_STORAGE_DAEMONS.md and the source code of killall.c, and I've learned
> > that there are methods to exclude the processes from the killing spree, such as
> > setting `@` to `argv[0][0]`.
> >
> > However, I'm not sure if this is without potential consequences. For example, could
> > it be that even though my processes survive, some resources that the processes
> > depends on are discarded after switch-root, such as file
> > descriptors?
> 
> No, these belong to your process, systemd couldn't really reach into
> your processes to close them, even if it wanted to.
> 
> But do note that any files you keep open or mapped at the moment of transition
> will remain pinned in memory, and cannot be released by the
> kernel. this means that even though during the tmpfs→host transition
> we generally destory the initrd's tmpfs' contents, the stuff you keep
> pinned will stick around.
> 
> Generally, only special purpose software should be left around that
> way, if it is carefully written to handle this. For example it is not
> allowed to dlopen() anything (and hence no NSS either! No
> gethostbyname() or getpwnam() or so), because you'd otherwise end up
> with a weird mix of match of shared libs from the initrd and the host.

If one does need to e.g. do DNS lookups in such a process, what is the
best way to do it?
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab
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