On Thu, 2024-09-12 at 10:53 +0800, lonial con wrote:
> I have never written a selftest before. I wrote a simple POC to
> demonstrate this bug. This POC can crash the Linux kernel 6.6.50. I
> think the ebpf code in the POC will be helpful for writing a
> selftest.
Well all depends on how familiar you want to get with selftests
infrastructure :) Here is a promised intro. If you don't want to
bother, please let me know, I can write the selftest.
If you do want to bother, feel free to ask any questions.
***
Please find a minimal recipe allowing to compile and run selftests in
a chroot at the bottom of this email. You would probably want to
adjust it, e.g. setup a user matching your local user inside chroot
and do a bind mount for sources directory etc.
After setting up the environment you will have to write the test.
BPF selftests reside in the following directory:
- tools/testing/selftests/bpf/
Nowadays we mostly add selftests to test_progs executable and use
bpftool skeletons / libbpf to simplify maps and programs creation.
The files located under prog_tests/ directory are compiled as host
programs, the files located under progs/ are compiled as BPF programs
(and libbpf skeletons are generated for these programs).
Skeletons generated for files from progs/ are used in tests declared
in prog_tests/.
Your POC structure:
- sets up a few maps
- sets up data for ringbuf
- loads and runs BPF program
You can look at a selftests that have similar structure, e.g.:
- tools/testing/selftests/bpf/prog_tests/ringbuf.c
- tools/testing/selftests/bpf/progs/test_ringbuf.c
Interesting parts of the 'prog_tests/ringbuf.c':
// this includes skeleton generated by bpftool
#include "test_ringbuf.lskel.h"
static void ringbuf_subtest(void)
{
...
// use generated methods to setup maps and programs
skel = test_ringbuf_lskel__open();
...
err = test_ringbuf_lskel__load(skel);
// you can do bpf_prog_run here as well
}
// build system automatically wires up functions
// void test_*(void) as entry points for tests
// executed by test_progs binary
void test_ringbuf(void)
{
// needed for tests filtering, e.g. -t option for test_progs
if (test__start_subtest("ringbuf"))
ringbuf_subtest();
...
}
*** chroot selftests build/run recipe follows ***
# First, setup the bullseye chroot
sudo /usr/sbin/debootstrap --variant=buildd --arch=amd64 bullseye bullseye-chroot/ http://deb.debian.org/debian
# provide {dev,proc} for chroot
sudo mount --rbind /dev ./bullseye-chroot/dev/
sudo mount --make-rslave ./bullseye-chroot/dev/
sudo mount -t proc proc ./bullseye-chroot/proc/
# enter chroot
sudo chroot ./bullseye-chroot
# Install build tools
apt install build-essential bc flex bison git libelf-dev libssl-dev \
docutils-common rsync curl zstd qemu-system-x86 sudo cmake \
libdw-dev lsb-release wget software-properties-common gnupg e2fsprogs
# Install fresh clang-18 snapshot, the llvm.sh sets up some repos
curl https://apt.llvm.org/llvm.sh --output /tmp/llvm.sh
bash /tmp/llvm.sh 18
apt install clang-tools-18
ln -s /usr/bin/clang-18 /usr/bin/clang
ln -s /usr/bin/llvm-strip-18 /usr/bin/llvm-strip
# that would be /root inside chroot
cd $HOME
# Get and compile pahole, use instructions from:
# https://git.kernel.org/pub/scm/devel/pahole/pahole.git/about/
git clone https://git.kernel.org/pub/scm/devel/pahole/pahole.git
cd pahole
git submodule update --init --recursive
mkdir build
cd build
cmake -D__LIB=lib ..
make -j
# make it available system-wide
ln -s $(realpath pahole) /usr/local/bin/
cd $HOME
git clone --depth=1 https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git
# Run vmtests, this should download rootfs, build kernel and tests, run test_verifier
# vmtest.sh would ask for root password to mount rootfs image
cd bpf-next/tools/testing/selftests/bpf
./vmtest.sh -- ./test_verifier
# And now run test_progs
./vmtest.sh -- ./test_progs
# One can filter tests too
./vmtest.sh -- ./test_progs -t ringbuf
