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rcutorture: Add initrd support for systems lacking dracut

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The support for creating initrd directories using dracut is a great
improvement over having to always hand-create them, it is a bit annoying
to have to install some otherwise irrelevant package just to be able to
run rcutorture.  This commit therefore adds support for creating initrd
directories on systems innocent of dracut.  You do need gcc, but then
again you need that to build the kernel (or to build llvm) in any case.

The idea is to create an initrd directory containing nothing but a
statically linked binary having a for-loop over a long-term sleep().
The result is a Linux kernel with almost no userspace: even the
time-honored /dev, /lib, /tmp, and /usr directories are gone.  In fact,
the only directory present is "/", but only because I don't know how to
get rid of it, at least short of not having an initrd in the first place.
Although statically linked binaries are much maligned, and rightly so,
their disadvantages seem to be irrelevant for this particular use case.
From https://www.akkadia.org/drepper/no_static_linking.html:

1.	Fixes are difficult to apply to hordes of widely scattered
	statically linked binaries.  But in this case, there is only one
	binary, but there would otherwise be no fewer than four libraries.

2.	Security measures like local address randomization cannot be used.
	Prudence prevents me from asserting that it is impossible to
	base a remote attack on a networking-free rcutorture instance.
	Nevertheless, bonus points to the first person who comes up with
	such an attack!

3.	More efficient use of physical memory.  Not in this case, given
	that libc is 1.8MB and the statically linked binary "only" 800K.

4.	Features such as locales, name service switch (NSS),
	internationalized domain names (IDN) tool, and so on require
	dynamic linking.  Bonus points to the first person coming up
	with a valid rcutorture use case requiring these features in
	its initrd.

5.	Accidental violations of (L)GPL.  Actually, this change actually
	helps -avoid- such violations by reducing the temptation to
	pass around tarballs of rcutorture-ready initrd directories.
	After all, the rcutorture scripts automatically create an initrd
	directory for you, so why bother with the tarballs?

6.	Tools and hacks like ltrace, LD_PRELOAD, LD_PROFILE, and LD_AUDIT
	don't work.  Again, bonus points to the first person coming up
	with a valid rcutorture use case requiring these features in
	its initrd.

Nevertheless, the script will use dracut if available, and will create the
statically linked binary only when dracut are missing.  Those preferring
the smaller initrd directory resulting from the statically linked binary
(like me) are free to hand-edit mkinitrd.sh to remove the code using
dracut.  ;-)

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit is contained in:
Paul E. McKenney 2018-08-23 10:48:18 -07:00 committed by Paul E. McKenney
parent 8f15c682ac
commit 38e630424b
2 changed files with 45 additions and 94 deletions
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40
tools/testing/selftests/rcutorture/bin/mkinitrd.sh
View File

@ -46,15 +46,41 @@ done
__EOF___
# Try using dracut to create initrd
command -v dracut >/dev/null 2>&1 || { echo >&2 "Dracut not installed"; exit 1; }
echo Creating $D/initrd using dracut.
if command -v dracut >/dev/null 2>&1
then
echo Creating $D/initrd using dracut.
# Filesystem creation
dracut --force --no-hostonly --no-hostonly-cmdline --module "base" $T/initramfs.img
cd $D
mkdir initrd
cd initrd
zcat $T/initramfs.img | cpio -id
cp $T/init init
chmod +x init
echo Done creating $D/initrd using dracut
exit 0
fi
# Filesystem creation
dracut --force --no-hostonly --no-hostonly-cmdline --module "base" $T/initramfs.img
# No dracut, so create a C-language initrd/init program and statically
# link it. This results in a very small initrd, but might be a bit less
# future-proof than dracut.
echo "Could not find dracut, attempting C initrd"
cd $D
mkdir initrd
cd initrd
zcat $T/initramfs.img | cpio -id
cp $T/init init
echo Done creating $D/initrd using dracut
cat > init.c << '___EOF___'
#include <unistd.h>
int main(int argc, int argv[])
{
for (;;)
sleep(1000*1000*1000); /* One gigasecond is ~30 years. */
return 0;
}
___EOF___
gcc -static -Os -o init init.c
strip init
rm init.c
echo "Done creating a statically linked C-language initrd"
exit 0

99
tools/testing/selftests/rcutorture/doc/initrd.txt
View File

@ -1,9 +1,12 @@
This document describes one way to create the initrd directory hierarchy
in order to allow an initrd to be built into your kernel. The trick
here is to steal the initrd file used on your Linux laptop, Ubuntu in
this case. There are probably much better ways of doing this.
The rcutorture scripting tools automatically create the needed initrd
directory using dracut. Failing that, this tool will create an initrd
containing a single statically linked binary named "init" that loops
over a very long sleep() call. In both cases, this creation is done
by tools/testing/selftests/rcutorture/bin/mkinitrd.sh.
That said, here are the commands:
However, if you are attempting to run rcutorture on a system that does
not have dracut installed, and if you don't like the notion of static
linking, you might wish to press an existing initrd into service:
------------------------------------------------------------------------
cd tools/testing/selftests/rcutorture
@ -11,22 +14,7 @@ zcat /initrd.img > /tmp/initrd.img.zcat
mkdir initrd
cd initrd
cpio -id < /tmp/initrd.img.zcat
------------------------------------------------------------------------
Another way to create an initramfs image is using "dracut"[1], which is
available on many distros, however the initramfs dracut generates is a cpio
archive with another cpio archive in it, so an extra step is needed to create
the initrd directory hierarchy.
Here are the commands to create a initrd directory for rcutorture using
dracut:
------------------------------------------------------------------------
dracut --no-hostonly --no-hostonly-cmdline --module "base bash shutdown" /tmp/initramfs.img
cd tools/testing/selftests/rcutorture
mkdir initrd
cd initrd
/usr/lib/dracut/skipcpio /tmp/initramfs.img | zcat | cpio -id < /tmp/initramfs.img
# Manually verify that initrd contains needed binaries and libraries.
------------------------------------------------------------------------
Interestingly enough, if you are running rcutorture, you don't really
@ -39,75 +27,12 @@ with 0755 mode.
------------------------------------------------------------------------
#!/bin/sh
[ -d /dev ] || mkdir -m 0755 /dev
[ -d /root ] || mkdir -m 0700 /root
[ -d /sys ] || mkdir /sys
[ -d /proc ] || mkdir /proc
[ -d /tmp ] || mkdir /tmp
mkdir -p /var/lock
mount -t sysfs -o nodev,noexec,nosuid sysfs /sys
mount -t proc -o nodev,noexec,nosuid proc /proc
# Some things don't work properly without /etc/mtab.
ln -sf /proc/mounts /etc/mtab
# Note that this only becomes /dev on the real filesystem if udev's scripts
# are used; which they will be, but it's worth pointing out
if ! mount -t devtmpfs -o mode=0755 udev /dev; then
echo "W: devtmpfs not available, falling back to tmpfs for /dev"
mount -t tmpfs -o mode=0755 udev /dev
[ -e /dev/console ] || mknod --mode=600 /dev/console c 5 1
[ -e /dev/kmsg ] || mknod --mode=644 /dev/kmsg c 1 11
[ -e /dev/null ] || mknod --mode=666 /dev/null c 1 3
fi
mkdir /dev/pts
mount -t devpts -o noexec,nosuid,gid=5,mode=0620 devpts /dev/pts || true
mount -t tmpfs -o "nosuid,size=20%,mode=0755" tmpfs /run
mkdir /run/initramfs
# compatibility symlink for the pre-oneiric locations
ln -s /run/initramfs /dev/.initramfs
# Export relevant variables
export ROOT=
export ROOTDELAY=
export ROOTFLAGS=
export ROOTFSTYPE=
export IP=
export BOOT=
export BOOTIF=
export UBIMTD=
export break=
export init=/sbin/init
export quiet=n
export readonly=y
export rootmnt=/root
export debug=
export panic=
export blacklist=
export resume=
export resume_offset=
export recovery=
for i in /sys/devices/system/cpu/cpu*/online
do
case $i in
'/sys/devices/system/cpu/cpu0/online')
;;
'/sys/devices/system/cpu/cpu*/online')
;;
*)
echo 1 > $i
;;
esac
done
while :
do
sleep 10
done
------------------------------------------------------------------------
References:
[1]: https://dracut.wiki.kernel.org/index.php/Main_Page
[2]: http://blog.elastocloud.org/2015/06/rapid-linux-kernel-devtest-with-qemu.html
[3]: https://www.centos.org/forums/viewtopic.php?t=51621
This approach also allows most of the binaries and libraries in the
initrd filesystem to be dispensed with, which can save significant
space in rcutorture's "res" directory.