BPF Unit and Integration Testing

Our BPF datapath has its own test framework, which allows us to write unit and integration tests that verify that our BPF code works as intended, independently from the other Cilium components. The framework uses the BPF_PROG_RUN feature to run eBPF programs in the kernel without attaching them to actual hooks.

The framework is designed to allow datapath developers to quickly write tests for the code they are working on. The tests themselves are fully written in C to minimize context switching. Tests pass results back to the framework which will outputs the results in Go test output, for optimal integration with CI and other tools.

Running tests

To run the tests in your local environment, execute the following command from the project root:

$ make -C test run_bpf_tests

The output is verbose by default. Verbose mode can be disabled by setting the V option to 0:

$ make -C test run_bpf_tests V=0

Note

Running BPF tests only works on Linux machines and requires admin privileges. The makefile uses sudo implicitly and may prompt you for credentials.

Writing tests

All BPF tests live in the bpf/tests directory. All .c files in this directory are assumed to contain BPF test programs which can be independently compiled, loaded, and executed using BPF_PROG_RUN. All files in this directory are automatically picked up, so all you have to do is create a new .c file and start writing. All other files like .h files are ignored and can be used for sharing code for example.

Each .c file must at least have one CHECK program. The CHECK macro replaces the SEC which is typically used in BPF programs. The CHECK macro takes two arguments, the first being the program type (for example xdp or tc. See the list of recognized types in the Go library), the second being the name of the test which will appear in the output. All macros are defined in bpf/tests/common.h, so all programs should start by including this file: #include "common.h".

Each CHECK program should start with test_init() and end with test_finish(), CHECK programs will return implicitly with the result of the test, a user doesn’t need to add return statements to the code manually. A test will PASS if it reaches test_finish(), unless it is marked as failed(test_fail(), test_fail_now(), test_fatal()) or skipped(test_skip(), test_skip_now()).

The name of the function has no significance for the tests themselves. The function names are still used as indicators in the kernel (at least the first 15 chars), used to populate tail call maps, and should be unique for the purposes of compilation.

#include "common.h"

CHECK("xdp", "nodeport-lb4")
int nodeportLB4(struct __ctx_buff *ctx)
{
        test_init();

    /* ensure preconditions are met */
    /* call the functions you would like to test */
    /* check that everything works as expected */

    test_finish();
}

Sub-tests

Each CHECK program may contain sub-tests, each of which has its own test status. A sub-test is created with the TEST macro like so:

#include "common.h"

#include <bpf/ctx/xdp.h>
#include <lib/jhash.h>
#include "bpf/section.h"

CHECK("xdp", "jhash")
int bpf_test(__maybe_unused struct xdp_md *ctx)
{
    test_init();

    TEST("Non-zero", {
        unsigned int hash = jhash_3words(123, 234, 345, 456);

        if (hash != 2698615579)
            test_fatal("expected '2698615579' got '%lu'", hash);
    });

    TEST("Zero", {
        unsigned int hash = jhash_3words(0, 0, 0, 0);

        if (hash != 459859287)
            test_fatal("expected '459859287' got '%lu'", hash);
    });

    test_finish();
}

Since all sub-tests are part of the same BPF program they are executed consecutively in one BPF_PROG_RUN invocation and can share setup code which can improve run speed and reduce code duplication. The name passed to the TEST macro for each sub-test serves to self-document the steps and makes it easier to spot what part of a test fails.

Integration tests

Writing tests for a single function or small group of functions should be fairly straightforward, only requiring a CHECK program. Testing functionality across tail calls requires an additional step: given that the program does not return to the CHECK function after making a tail call, we can’t check whether it was successful.

The workaround is to use a SETUP program in addition to a CHECK program. A SETUP program will run before the CHECK program with the same name. The BPF context (for example the struct __sk_buff for TC programs), modified by the SETUP program, is then passed to the CHECK program, which can inspect the result. By executing the test setup and executing the tail call in SETUP we can execute complete programs. The return code of the SETUP program is prepended as a u32 to the start of the packet data passed to CHECK, meaning that the CHECK program will find the actual packet data at (void *)data + 4.

This is an abbreviated example showing the key components:

#include "common.h"

#include "bpf/ctx/xdp.h"
#include "bpf_xdp.c"

struct {
    __uint(type, BPF_MAP_TYPE_PROG_ARRAY);
    __uint(key_size, sizeof(__u32));
    __uint(max_entries, 2);
    __array(values, int());
} entry_call_map __section(".maps") = {
    .values = {
        [0] = &cil_xdp_entry,
    },
};

SETUP("xdp", "l2_example")
int test1_setup(struct __ctx_buff *ctx)
{
    /* Create room for our packet to be crafted */
    unsigned int data_len = ctx->data_end - ctx->data;
    int offset = offset = sizeof(struct ethhdr) - data_len;
    bpf_xdp_adjust_tail(ctx, offset);

    void *data = (void *)(long)ctx->data;
    void *data_end = (void *)(long)ctx->data_end;

    if (data + sizeof(struct ethhdr) > data_end)
        return TEST_ERROR;

    /* Writing just the L2 header for brevity */
    struct ethhdr l2 = {
        .h_source = {0xAA, 0xBB, 0xCC, 0xDD, 0xEE, 0xFF},
        .h_dest = {0x12, 0x23, 0x34, 0x45, 0x56, 0x67},
        .h_proto = bpf_htons(ETH_P_IP)
    };
    memcpy(data, &l2, sizeof(struct ethhdr));

    /* OMITTED setting up map state */

    /* Jump into the entrypoint */
    tail_call_static(ctx, &entry_call_map, 0);
    /* Fail if we didn't jump */
    return TEST_ERROR;
}

CHECK("xdp", "l2_example")
int test1_check(__maybe_unused const struct __ctx_buff *ctx)
{
    test_init();

    void *data = (void *)(long)ctx->data;
    void *data_end = (void *)(long)ctx->data_end;

    if (data + sizeof(__u32) > data_end)
        test_fatal("status code out of bounds");

    __u32 *status_code = data;

    if (*status_code != XDP_TX)
        test_fatal("status code != XDP_TX");

    data += sizeof(__u32);

    if (data + sizeof(struct ethhdr) > data_end)
        test_fatal("ctx doesn't fit ethhdr");

    struct ethhdr *l2 = data;

    data += sizeof(struct ethhdr);

    if (memcmp(l2->h_source, fib_smac, sizeof(fib_smac)))
        test_fatal("l2->h_source != fib_smac");

    if (memcmp(l2->h_dest, fib_dmac, sizeof(fib_dmac)))
        test_fatal("l2->h_dest != fib_dmac");

    if (data + sizeof(struct iphdr) > data_end)
        test_fatal("ctx doesn't fit iphdr");

    test_finish();
}

Function reference

  • test_log(fmt, args...) - writes a log message. The conversion specifiers supported by fmt are the same as for bpf_trace_printk(). They are %d, %i, %u, %x, %ld, %li, %lu, %lx, %lld, %lli, %llu, %llx. No modifier (size of field, padding with zeroes, etc.) is available.

  • test_fail() - marks the current test or sub-test as failed but will continue execution.

  • test_fail_now() - marks the current test or sub-test as failed and will stop execution of the test or sub-test (If called in a sub-test, the other sub-tests will still run).

  • test_fatal(fmt, args...) - writes a log and then calls test_fail_now()

  • assert(stmt) - asserts that the statement within is true and call test_fail_now() otherwise. assert will log the file and line number of the assert statement.

  • test_skip() - marks the current test or sub-test as skipped but will continue execution.

  • test_skip_now() - marks the current test or sub-test as skipped and will stop execution of the test or sub-test (If called in a sub-test, the other sub-tests will still run).

  • test_init() - initializes the internal state for the test and must be called before any of the functions above can be called.

  • test_finish() - submits the results and returns from the current function.

Warning

Functions that halt the execution (test_fail_now(), test_fatal(), test_skip_now()) can’t be used within both a sub-test (TEST) and for, while, or switch/case blocks since they use the break keyword to stop a sub-test. These functions can still be used from within for, while and switch/case blocks if no sub-tests are used, because in that case the flow interruption happens via return.

Function mocking

Being able to mock out a function is a great tool to have when creating tests for a number of reasons. You might for example want to test what happens if a specific function returns an error to see if it is handled gracefully. You might want to proxy function calls to record if the function under test actually called specific dependencies. Or you might want to test code that uses helpers which rely on a state we can’t set in BPF, like the routing table.

Mocking is easy with this framework:

  1. Create a function with a unique name and the same signature as the function it is replacing.

  2. Create a macro with the exact same name as the function we want to replace and point it to the function created in step 1. For example #define original_function our_mocked_function`

  3. Include the file which contains the definition we are replacing.

The following example mocks out the fib_lookup helper call and replaces it with our mocked version, since we don’t actually have routes for the IPs we want to test:

#include "common.h"

#include "bpf/ctx/xdp.h"

#define fib_lookup mock_fib_lookup

static const char fib_smac[6] = {0xDE, 0xAD, 0xBE, 0xEF, 0x01, 0x02};
static const char fib_dmac[6] = {0x13, 0x37, 0x13, 0x37, 0x13, 0x37};

long mock_fib_lookup(__maybe_unused void *ctx, struct bpf_fib_lookup *params,
            __maybe_unused int plen, __maybe_unused __u32 flags)
{
    memcpy(params->smac, fib_smac, sizeof(fib_smac));
    memcpy(params->dmac, fib_dmac, sizeof(fib_dmac));
    return 0;
}

#include "bpf_xdp.c"
#include "lib/nodeport.h"

Limitations

For all its benefits there are some limitations to this way of testing:

  • Code must pass the verifier, so our setup and test code has to obey the same rules as other BPF programs. A side effect is that it automatically guarantees that all code that passes will also load. The biggest concern is the complexity limit on older kernels, this can be somewhat mitigated by separating heavy setup work into its own SETUP program and optionally tail calling into the code to be tested, to ensure the testing harness doesn’t push us over the complexity limit.

  • Test functions like test_log(), test_fail(), test_skip() can only be executed within the scope of the main program or a TEST. These functions rely on local variables set by test_init() and will produce errors when used in other functions.

  • Functions that halt the execution (test_fail_now(), test_fatal(), test_skip_now()) can’t be used within both a sub-test (TEST) and for, while, or switch/case blocks since they use the break keyword to stop a sub-test. These functions can still be used from within for, while and switch/case blocks if no sub-tests are used, because in that case the flow interruption happens via return.

  • Sub-test names can’t use more than 127 characters.

  • Log messages can’t use more than 127 characters and have no more than 12 arguments.