Re: [PATCH 11/20] Add /dev/fd and /dev/std*

Alyssa Ross
2025-09-08T09:18:33Z

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Demi Marie Obenour <demiobenour@gmail.com> writes:

> This is the same as 14483e1a690c (img/app: add /dev/fd and /dev/std*),
> but for the host and for vm/sys/net. While only Spectrum-provided code
> should run in these VMs, third-party dependencies of Spectrum might
> assume these links exist, and them being missing could cause severe
> bugs. For instance, code writing to /dev/stdout could create a file in
> /dev rather than actually writing to stdout.
>
> In the host, the links are added in the initramfs. Since /dev is
> created by the kernel and moved (via mount --move) from the initramfs to
> the main system, adding the links in the main system is not necessary
> and in fact would fail.
>
> Also reorder the moving of /sys, /proc, and /dev from the initramfs to
> the root filesystem to minimize the time that /dev and /proc are not
> mounted. /proc is considered more important than /dev.
>
> Signed-off-by: Demi Marie Obenour <demiobenour@gmail.com>

This is really two independent patches in one as well.

> ---
> host/initramfs/etc/init | 7 ++++++-
> vm/sys/net/etc/s6-linux-init/scripts/rc.init | 5 +++++
> 2 files changed, 11 insertions(+), 1 deletion(-)
>
> diff --git a/host/initramfs/etc/init b/host/initramfs/etc/init
> index 719488741b6d31564c2c17c0e41f15d16b1c0a08..b72108ab96630e2a846063551772b0b29ca27bdf 100755
> --- a/host/initramfs/etc/init
> +++ b/host/initramfs/etc/init
> @@ -6,6 +6,11 @@ export PATH /bin
>
> if { mount -a }
>
> +if { ln -s /proc/self/fd /dev }
> +if { ln -s /proc/self/fd/0 /dev/stdin }
> +if { ln -s /proc/self/fd/1 /dev/stdout }
> +if { ln -s /proc/self/fd/2 /dev/stderr }
> +

Would prefer to do this in host/rootfs, in the interest of not creating
more implicit requirements on initramfs in that system. initramfs is
deterministic enough that it's vanishingly unlikely we'll find that it
requires these only sometimes.

> piperw 3 4
> if { fdmove 1 4 /etc/getuuids }
> fdclose 4
> @@ -45,9 +50,9 @@ background { rm /dev/rootfs /dev/verity }
> if { mount /dev/mapper/root-verity /mnt/root }
> wait { $mdevd_pid }
>
> -if { mount --move /proc /mnt/root/proc }
> if { mount --move /sys /mnt/root/sys }
> if { mount --move /dev /mnt/root/dev }
> +if { mount --move /proc /mnt/root/proc }

I don't understand this. There are no other processes running, so how
could the order possibly matter? There's nothing to race against.

>
> switch_root /mnt/root
> /etc/init
> diff --git a/vm/sys/net/etc/s6-linux-init/scripts/rc.init b/vm/sys/net/etc/s6-linux-init/scripts/rc.init
> index 1016d0c62bc6103bc9e865a389f5d482ef6c2b76..eaf037ec123afcaeafced93096c4f35c2388f385 100755
> --- a/vm/sys/net/etc/s6-linux-init/scripts/rc.init
> +++ b/vm/sys/net/etc/s6-linux-init/scripts/rc.init
> @@ -2,6 +2,11 @@
> # SPDX-License-Identifier: EUPL-1.2+
> # SPDX-FileCopyrightText: 2020-2022 Alyssa Ross <hi@alyssa.is>
>
> +if { ln -s /proc/self/fd /dev }
> +if { ln -s /proc/self/fd/0 /dev/stdin }
> +if { ln -s /proc/self/fd/1 /dev/stdout }
> +if { ln -s /proc/self/fd/2 /dev/stderr }
> +
> if { s6-rc-init -c /etc/s6-rc /run/service }
>
> if { mkdir -p /dev/pts /dev/shm }
>
> --
> 2.51.0

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