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

Demi Marie Obenour
2025-09-08T18:12:17Z

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On 9/8/25 05:18, Alyssa Ross wrote:
> 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.

Will fix.

>> ---
>> 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.

Is there a plan to run Spectrum OS without an initramfs?
If not, then adding these can only help and cannot hurt.

>> 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.

Does Busybox switch_root already move these mounts?
I know util-linux switch_root does.

>> 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

--
Sincerely,
Demi Marie Obenour (she/her/hers)

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