PREVAIL

a Polynomial-Runtime EBPF Verifier using an Abstract Interpretation Layer

A new eBPF verifier.

The version discussed in the PLDI paper is available here.

Getting Started

Dependencies from vanilla Ubuntu

sudo apt install build-essential git cmake libboost-dev libyaml-cpp-dev
sudo apt install python3-pip python3-tk
pip3 install matplotlib   # for plotting the graphs

Dependencies from vanilla Windows

• Install git
• Install Visual Studio Build Tools 2019 and choose the "C++ build tools" workload (Visual Studio Build Tools 2019 has support for CMake Version 3.15).
• Install nuget.exe

Installation

Clone:

git clone --recurse-submodules https://github.com/vbpf/ebpf-verifier.git
cd ebpf-verifier

Make on Ubuntu:

cmake -B build -DCMAKE_BUILD_TYPE=Release
cmake --build build

Make on Windows (which uses a multi-configuration generator):

cmake -B build
cmake --build build --config Release

Running with Docker

Build and run:

docker build -t verifier .
docker run -it verifier ebpf-samples/cilium/bpf_lxc.o 2/1 --domain=zoneCrab
1,0.062802,21792
# To run the Linux verifier you'll need a privileged container:
docker run --privileged -it verifier ebpf-samples/linux/cpustat_kern.o --domain=linux

Example:

ebpf-verifier$ ./check ebpf-samples/cilium/bpf_lxc.o 2/1 --domain=zoneCrab
1,0.062802,21792

The output is three comma-separated values:

• 1 or 0, for "pass" and "fail" respectively
• The runtime of the fixpoint algorithm (in seconds)
• The peak memory consumption, in kb, as reflected by the resident-set size (rss)

Usage:

A new eBPF verifier
Usage: ./check [OPTIONS] path [section]

Positionals:
  path FILE REQUIRED          Elf file to analyze
  section SECTION             Section to analyze

Options:
  -h,--help                   Print this help message and exit
  -l                          List sections
  -d,--dom,--domain DOMAIN:{cfg,linux,stats,zoneCrab}
                              Abstract domain
  --termination               Verify termination
  -i                          Print invariants
  -f                          Print verifier's failure logs
  -v                          Print both invariants and failures
  --no-simplify               Do not simplify
  --asm FILE                  Print disassembly to FILE
  --dot FILE                  Export control-flow graph to dot FILE

You can use @headers as the path to instead just show the output field headers.

A standard alternative to the --asm flag is llvm-objdump -S FILE.

The cfg can be viewed using dot and the standard PDF viewer:

./check ebpf-samples/cilium/bpf_lxc.o 2/1 --domain=zoneCrab --dot cfg.dot
dot -Tpdf cfg.dot > cfg.pdf

Step-by-Step Instructions

To get the results for described in Figures 9 and 10, run the following:

scripts/runperf.sh ebpf-samples stats zoneCrab | tee results.csv

The first argument to the script, ebpf-samples, is the root directory in which to search for elf files. You can pass any subdirectory or file, e.g. ebpd-samples/linux.

The rest of the positional arguments are the numerical domains to use.

The output is a large csv file. The first line is a header:

suite,project,file,section,hash,instructions,loads,stores,jumps,joins,zoneCrab?,zoneCrab_sec,zoneCrab_kb
ebpf-samples,cilium,bpf_lxc.o,2/1,69a5e4fc57ca1c94,41,6,10,1,1,1,0.057409,21796

• suite in our case will be "ebpf-samples"
• project is one of the directories in the suite. We currently have bpf_cilium_test, cilium linux, ovs, prototype-kernel and suricata
• file is the elf file containing the programs. The compiled version of a C file
• section is the elf section containing the program checked
• hash is a unique hash of the eBPF code. There are duplicate programs in the benchmark (since we use files from projects "as-is"). To count the real number of programs these duplicates should be removed
• instructions, loads, stores, jumps and joins show the number of these features
• For each domain DOM, there are 3 consecutive columns:
  • "DOM?" is 0 for rejected program, 1 for accepted program
  • "DOM_sec" is the number of seconds that the fixpoint operation took
  • "DOM_kb" is the peak memory resident set size consumed by the analysis, and is an estimate for the amount of additional memory needed by the analysis

Note that in the full benchmark, exactly 2 programs should be rejected by zoneCrab, our domain of choice. Other domain reject different number of programs.

Any subset of the available domains is valid. So in order to compare the two different implementations of the zone domain, one can run

scripts/runperf.sh ebpf-samples/linux stats zoneCrab | tee results.csv
python3 scripts/makeplot.py results.csv stores

The script scripts/makeplot.py takes a csv file in the format described above, and the key to plot against (usually instructions or stores) and plots two graphs: on showing runtime as a function of the number of stores, and the other is the memory consumption as a function of the number of stores.

Caveat

When performed on a VM without sufficient memory, some analyses of some domains are terminated by the OS due to insufficient memory, resulting in "-1" runtime and skewing the graph. To avoid this, the failing cases should be omitted.

Testing the Linux verifier

To run the Linux verifier, you must use sudo:

sudo ./check ebpf-samples/linux/cpustat_kern.o --domain=linux

Counter and Artificial examples

The folder counter/ contains other examples used to demonstrate the usefulness of our tools, compared to the existing verifier. To compile the examples, run

make -C counter
scripts/runperf.sh counter/objects stats zoneCrab

Valid programs that are rejected by a verifier are referred to as false positives. Two examples of real-world false positives are taken from the Linux samples suite. The file xdp_tx_iptunnel_kern.o is valid and passes both the Linux kernel verifier and ours. However, in the original source code there are redundant loads from memory to a variable holding the same value. These were added due to untracked register spilling that led to a false positive. Two fixes are compiled into xdp_tx_iptunnel_1_kern.o and xdp_tx_iptunnel_2_kern.o. Both pass our verifier (without any special effort) but fail the Linux kernel verifier:

$ ./check counter/objects/xdp_tx_iptunnel_2_kern.o
1,0.314213,86740
$ sudo ./check counter/objects/xdp_tx_iptunnel_2_kern.o --domain=linux -v
<... long trace reporting an alleged failure>

Double-strncmp experiment

This experiment quadratic blowup in the Linux verifier, versus linear runtime in our tool. Be sure to run with sudo, since Linux requires special permissions for this.

sudo scripts/experiment.sh | tee blowup.csv
python3 scripts/makeplot.py blowup.csv iterations False

Programs with loops

There are several simple programs with loops in the folder counter/src, called simple_loop_*.c and manual_memset*.c. The Linux verifier rejects them immediately:

$ sudo ./check counter/objects/simple_loop_ptr_backwards.o --domain=linux -v
counter/objects/simple_loop_ptr_backwards.o
	sk_skb/loop-tree_ptr,bpf_load_program(prog_cnt=0) err=22
back-edge from insn 7 to 5

Using our tool, the safety and termination of some loop-based programs can be verified:

$ ./check --termination counter/objects/simple_loop_ptr_backwards.o

(not all the programs in the folder are verified)