Login without asking for the username

We have wanted to do this since forever, but the last straw was gem5 aarch64,
which runs too fast, and makes it very hard to write "root" withing 60 seconds!

Another possibility to solve that would have been to find an answer for:
https://unix.stackexchange.com/questions/340333/how-can-i-get-bin-login-to-not-timeout
which we didn't, but not typing root at all is even better.

Take this opportunity to document how to login into user0.

sshd: automate and document further

gdbserver: automatically startup internet on /gdbserver.sh

README.adoc

@@ -30,7 +30,6 @@ If you don't want to wait, you could also try to compile the examples and run th
 After QEMU opens up, you can start playing with the kernel modules:
 
 ....
-root
 insmod /hello.ko
 insmod /hello2.ko
 rmmod hello
@@ -268,7 +267,6 @@ But on the other hand maybe you are interested in observing the interrupts gener
 When debugging a module, it becomes tedious to wait for build and re-type:
 
 ....
-root
 /modulename.sh
 ....
 
@@ -851,8 +849,6 @@ But TODO I don't think you can see where you are in the kernel source code and l
 
 Step debug userland processes to understand how they are talking to the kernel.
 
-First ensure that <<networking>> is enabled.
-
 Then in guest:
 
 ....
@@ -1919,13 +1915,23 @@ We us this exact procedure to connect to <<gdbserver>>.
 
 https://unix.stackexchange.com/questions/124681/how-to-ssh-from-host-to-guest-using-qemu/307557#307557
 
-Uses OpenSSH's `sshd`, enable with the option:
+Not enabled by default due to the build / runtime overhead. To enable, build with:
 
 ....
-BR2_PACKAGE_OPENSSH=y
+./build -B 'BR2_PACKAGE_OPENSSH=y'
 ....
 
-Not enabled by default due to the build / runtime overhead, but it was tested and worked at the time of this commit.
+Then inside the guest turn on sshd:
+
+....
+/sshd.sh
+....
+
+and finally on host:
+
+....
+ssh root@localhost -p 45456
+....
 
 ===== gem5 host to guest networking
 
@@ -3178,6 +3184,34 @@ diff -u .config.olg .config
 
 Then copy and paste the diff additions to link:br2[] to make them permanent.
 
+=== Change user
+
+At startup, we login automatically as the `root` user as explained at: https://unix.stackexchange.com/questions/299408/how-to-login-automatically-without-typing-root-in-buildroot-x86-64-qemu/300152#300152
+
+If you want to switch to another user to test some permissions, we have already created an `user0` user through the link:user_table[] file, and you can just login as that user with:
+
+....
+login user0
+....
+
+and password:
+
+....
+a
+....
+
+Then test that the user changed with:
+
+....
+id
+....
+
+which gives:
+
+....
+uid=1000(user0) gid=1000(user0) groups=1000(user0)
+....
+
 === ccache
 
 We have link:https://buildroot.org/downloads/manual/manual.html#ccache[enabled ccached] builds by default.

build-usage.adoc

@@ -13,7 +13,7 @@
 |`-b` |`BR2_FILE`           | Also use the given Buildroot configuration fragment file.
                               Pass multiple times to use multiple fragment files.
 |`-B` |`BR2_CONFIG`         | Add a single Buildroot option to the current build.
-                              Example: `-B 'BR2_TARGET_ROOTFS_EXT2_SIZE="500M"'`
+                              Example: `-B 'BR2_TARGET_ROOTFS_EXT2_SIZE="512M"'`
 |`-c` |`BR2_FILE`           | Also use the given Buildroot configuration fragment file.
                               Pass multiple times to use multiple fragment files.
 |`-C` |                     | Skip the Buildroot configuration. Saves a few seconds,

rootfs_overlay/etc/inittab

@@ -8,7 +8,7 @@
 ::sysinit:/etc/init.d/rcS
 # https://unix.stackexchange.com/questions/299408/how-to-login-automatically-without-typing-root-in-buildroot-x86-64-qemu
 #console::respawn:/bin/sh
-console::respawn:/sbin/getty -n -L console 0 vt100
+console::respawn:/sbin/getty -n -L -l /loginroot.sh console 0 vt100
 ::ctrlaltdel:/sbin/reboot
 ::shutdown:/etc/init.d/rcK
 ::shutdown:/sbin/swapoff -a

rootfs_overlay/etc/ssh/sshd_config

@@ -0,0 +1,4 @@
+AuthorizedKeysFile	.ssh/authorized_keys
+Subsystem	sftp	/usr/libexec/sftp-server
+PermitRootLogin yes
+PermitEmptyPasswords yes

rootfs_overlay/gdbserver.sh

@@ -1,2 +1,3 @@
 #!/bin/sh
+/sbin/ifup -a
 gdbserver :45455 "$@"

rootfs_overlay/loginroot.sh

@@ -0,0 +1,2 @@
+#!/bin/sh
+exec /bin/login root

rootfs_overlay/sshd.sh

@@ -0,0 +1,4 @@
+#!/bin/sh
+/sbin/ifup -a
+/usr/bin/ssh-keygen -A
+/usr/sbin/sshd

rootfs_post_build_script

@@ -15,4 +15,5 @@ rm -rf \
   "${target_dir}/etc/init.d/S01logging" \
   "${target_dir}/etc/init.d/S20urandom" \
   "${target_dir}/etc/init.d/S40network" \
+  "${target_dir}/etc/init.d/S50sshd" \
 ;

run

@@ -165,7 +165,7 @@ ${debug_vm} \
 -gdb tcp::1234 \
 -m ${memory} \
 -monitor telnet::45454,server,nowait \
--netdev user,hostfwd=tcp::45455-:45455,id=net0 \
+-netdev user,hostfwd=tcp::45455-:45455,hostfwd=tcp::45456-:22,id=net0 \
 -smp ${cpus} \
 -virtfs local,path=9p,mount_tag=host_scratch,security_model=mapped,id=host_scratch \
 -virtfs local,path=${out_dir}/build,mount_tag=host_out,security_model=mapped,id=host_out \