| 1 | ============= |
| 2 | BPF Iterators |
| 3 | ============= |
| 4 | |
| 5 | |
| 6 | ---------- |
| 7 | Motivation |
| 8 | ---------- |
| 9 | |
| 10 | There are a few existing ways to dump kernel data into user space. The most |
| 11 | popular one is the ``/proc`` system. For example, ``cat /proc/net/tcp6`` dumps |
| 12 | all tcp6 sockets in the system, and ``cat /proc/net/netlink`` dumps all netlink |
| 13 | sockets in the system. However, their output format tends to be fixed, and if |
| 14 | users want more information about these sockets, they have to patch the kernel, |
| 15 | which often takes time to publish upstream and release. The same is true for popular |
| 16 | tools like `ss <https://man7.org/linux/man-pages/man8/ss.8.html>`_ where any |
| 17 | additional information needs a kernel patch. |
| 18 | |
| 19 | To solve this problem, the `drgn |
| 20 | <https://www.kernel.org/doc/html/latest/bpf/drgn.html>`_ tool is often used to |
| 21 | dig out the kernel data with no kernel change. However, the main drawback for |
| 22 | drgn is performance, as it cannot do pointer tracing inside the kernel. In |
| 23 | addition, drgn cannot validate a pointer value and may read invalid data if the |
| 24 | pointer becomes invalid inside the kernel. |
| 25 | |
| 26 | The BPF iterator solves the above problem by providing flexibility on what data |
| 27 | (e.g., tasks, bpf_maps, etc.) to collect by calling BPF programs for each kernel |
| 28 | data object. |
| 29 | |
| 30 | ---------------------- |
| 31 | How BPF Iterators Work |
| 32 | ---------------------- |
| 33 | |
| 34 | A BPF iterator is a type of BPF program that allows users to iterate over |
| 35 | specific types of kernel objects. Unlike traditional BPF tracing programs that |
| 36 | allow users to define callbacks that are invoked at particular points of |
| 37 | execution in the kernel, BPF iterators allow users to define callbacks that |
| 38 | should be executed for every entry in a variety of kernel data structures. |
| 39 | |
| 40 | For example, users can define a BPF iterator that iterates over every task on |
| 41 | the system and dumps the total amount of CPU runtime currently used by each of |
| 42 | them. Another BPF task iterator may instead dump the cgroup information for each |
| 43 | task. Such flexibility is the core value of BPF iterators. |
| 44 | |
| 45 | A BPF program is always loaded into the kernel at the behest of a user space |
| 46 | process. A user space process loads a BPF program by opening and initializing |
| 47 | the program skeleton as required and then invoking a syscall to have the BPF |
| 48 | program verified and loaded by the kernel. |
| 49 | |
| 50 | In traditional tracing programs, a program is activated by having user space |
| 51 | obtain a ``bpf_link`` to the program with ``bpf_program__attach()``. Once |
| 52 | activated, the program callback will be invoked whenever the tracepoint is |
| 53 | triggered in the main kernel. For BPF iterator programs, a ``bpf_link`` to the |
| 54 | program is obtained using ``bpf_link_create()``, and the program callback is |
| 55 | invoked by issuing system calls from user space. |
| 56 | |
| 57 | Next, let us see how you can use the iterators to iterate on kernel objects and |
| 58 | read data. |
| 59 | |
| 60 | ------------------------ |
| 61 | How to Use BPF iterators |
| 62 | ------------------------ |
| 63 | |
| 64 | BPF selftests are a great resource to illustrate how to use the iterators. In |
| 65 | this section, we’ll walk through a BPF selftest which shows how to load and use |
| 66 | a BPF iterator program. To begin, we’ll look at `bpf_iter.c |
| 67 | <https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/tools/testing/selftests/bpf/prog_tests/bpf_iter.c>`_, |
| 68 | which illustrates how to load and trigger BPF iterators on the user space side. |
| 69 | Later, we’ll look at a BPF program that runs in kernel space. |
| 70 | |
| 71 | Loading a BPF iterator in the kernel from user space typically involves the |
| 72 | following steps: |
| 73 | |
| 74 | * The BPF program is loaded into the kernel through ``libbpf``. Once the kernel |
| 75 | has verified and loaded the program, it returns a file descriptor (fd) to user |
| 76 | space. |
| 77 | * Obtain a ``link_fd`` to the BPF program by calling the ``bpf_link_create()`` |
| 78 | specified with the BPF program file descriptor received from the kernel. |
| 79 | * Next, obtain a BPF iterator file descriptor (``bpf_iter_fd``) by calling the |
| 80 | ``bpf_iter_create()`` specified with the ``bpf_link`` received from Step 2. |
| 81 | * Trigger the iteration by calling ``read(bpf_iter_fd)`` until no data is |
| 82 | available. |
| 83 | * Close the iterator fd using ``close(bpf_iter_fd)``. |
| 84 | * If needed to reread the data, get a new ``bpf_iter_fd`` and do the read again. |
| 85 | |
| 86 | The following are a few examples of selftest BPF iterator programs: |
| 87 | |
| 88 | * `bpf_iter_tcp4.c <https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/tools/testing/selftests/bpf/progs/bpf_iter_tcp4.c>`_ |
| 89 | * `bpf_iter_task_vma.c <https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/tools/testing/selftests/bpf/progs/bpf_iter_task_vma.c>`_ |
| 90 | * `bpf_iter_task_file.c <https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/tools/testing/selftests/bpf/progs/bpf_iter_task_file.c>`_ |
| 91 | |
| 92 | Let us look at ``bpf_iter_task_file.c``, which runs in kernel space: |
| 93 | |
| 94 | Here is the definition of ``bpf_iter__task_file`` in `vmlinux.h |
| 95 | <https://facebookmicrosites.github.io/bpf/blog/2020/02/19/bpf-portability-and-co-re.html#btf>`_. |
| 96 | Any struct name in ``vmlinux.h`` in the format ``bpf_iter__<iter_name>`` |
| 97 | represents a BPF iterator. The suffix ``<iter_name>`` represents the type of |
| 98 | iterator. |
| 99 | |
| 100 | :: |
| 101 | |
| 102 | struct bpf_iter__task_file { |
| 103 | union { |
| 104 | struct bpf_iter_meta *meta; |
| 105 | }; |
| 106 | union { |
| 107 | struct task_struct *task; |
| 108 | }; |
| 109 | u32 fd; |
| 110 | union { |
| 111 | struct file *file; |
| 112 | }; |
| 113 | }; |
| 114 | |
| 115 | In the above code, the field 'meta' contains the metadata, which is the same for |
| 116 | all BPF iterator programs. The rest of the fields are specific to different |
| 117 | iterators. For example, for task_file iterators, the kernel layer provides the |
| 118 | 'task', 'fd' and 'file' field values. The 'task' and 'file' are `reference |
| 119 | counted |
| 120 | <https://facebookmicrosites.github.io/bpf/blog/2018/08/31/object-lifetime.html#file-descriptors-and-reference-counters>`_, |
| 121 | so they won't go away when the BPF program runs. |
| 122 | |
| 123 | Here is a snippet from the ``bpf_iter_task_file.c`` file: |
| 124 | |
| 125 | :: |
| 126 | |
| 127 | SEC("iter/task_file") |
| 128 | int dump_task_file(struct bpf_iter__task_file *ctx) |
| 129 | { |
| 130 | struct seq_file *seq = ctx->meta->seq; |
| 131 | struct task_struct *task = ctx->task; |
| 132 | struct file *file = ctx->file; |
| 133 | __u32 fd = ctx->fd; |
| 134 | |
| 135 | if (task == NULL || file == NULL) |
| 136 | return 0; |
| 137 | |
| 138 | if (ctx->meta->seq_num == 0) { |
| 139 | count = 0; |
| 140 | BPF_SEQ_PRINTF(seq, " tgid gid fd file\n"); |
| 141 | } |
| 142 | |
| 143 | if (tgid == task->tgid && task->tgid != task->pid) |
| 144 | count++; |
| 145 | |
| 146 | if (last_tgid != task->tgid) { |
| 147 | last_tgid = task->tgid; |
| 148 | unique_tgid_count++; |
| 149 | } |
| 150 | |
| 151 | BPF_SEQ_PRINTF(seq, "%8d %8d %8d %lx\n", task->tgid, task->pid, fd, |
| 152 | (long)file->f_op); |
| 153 | return 0; |
| 154 | } |
| 155 | |
| 156 | In the above example, the section name ``SEC(iter/task_file)``, indicates that |
| 157 | the program is a BPF iterator program to iterate all files from all tasks. The |
| 158 | context of the program is ``bpf_iter__task_file`` struct. |
| 159 | |
| 160 | The user space program invokes the BPF iterator program running in the kernel |
| 161 | by issuing a ``read()`` syscall. Once invoked, the BPF |
| 162 | program can export data to user space using a variety of BPF helper functions. |
| 163 | You can use either ``bpf_seq_printf()`` (and BPF_SEQ_PRINTF helper macro) or |
| 164 | ``bpf_seq_write()`` function based on whether you need formatted output or just |
| 165 | binary data, respectively. For binary-encoded data, the user space applications |
| 166 | can process the data from ``bpf_seq_write()`` as needed. For the formatted data, |
| 167 | you can use ``cat <path>`` to print the results similar to ``cat |
| 168 | /proc/net/netlink`` after pinning the BPF iterator to the bpffs mount. Later, |
| 169 | use ``rm -f <path>`` to remove the pinned iterator. |
| 170 | |
| 171 | For example, you can use the following command to create a BPF iterator from the |
| 172 | ``bpf_iter_ipv6_route.o`` object file and pin it to the ``/sys/fs/bpf/my_route`` |
| 173 | path: |
| 174 | |
| 175 | :: |
| 176 | |
| 177 | $ bpftool iter pin ./bpf_iter_ipv6_route.o /sys/fs/bpf/my_route |
| 178 | |
| 179 | And then print out the results using the following command: |
| 180 | |
| 181 | :: |
| 182 | |
| 183 | $ cat /sys/fs/bpf/my_route |
| 184 | |
| 185 | |
| 186 | ------------------------------------------------------- |
| 187 | Implement Kernel Support for BPF Iterator Program Types |
| 188 | ------------------------------------------------------- |
| 189 | |
| 190 | To implement a BPF iterator in the kernel, the developer must make a one-time |
| 191 | change to the following key data structure defined in the `bpf.h |
| 192 | <https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/include/linux/bpf.h>`_ |
| 193 | file. |
| 194 | |
| 195 | :: |
| 196 | |
| 197 | struct bpf_iter_reg { |
| 198 | const char *target; |
| 199 | bpf_iter_attach_target_t attach_target; |
| 200 | bpf_iter_detach_target_t detach_target; |
| 201 | bpf_iter_show_fdinfo_t show_fdinfo; |
| 202 | bpf_iter_fill_link_info_t fill_link_info; |
| 203 | bpf_iter_get_func_proto_t get_func_proto; |
| 204 | u32 ctx_arg_info_size; |
| 205 | u32 feature; |
| 206 | struct bpf_ctx_arg_aux ctx_arg_info[BPF_ITER_CTX_ARG_MAX]; |
| 207 | const struct bpf_iter_seq_info *seq_info; |
| 208 | }; |
| 209 | |
| 210 | After filling the data structure fields, call ``bpf_iter_reg_target()`` to |
| 211 | register the iterator to the main BPF iterator subsystem. |
| 212 | |
| 213 | The following is the breakdown for each field in struct ``bpf_iter_reg``. |
| 214 | |
| 215 | .. list-table:: |
| 216 | :widths: 25 50 |
| 217 | :header-rows: 1 |
| 218 | |
| 219 | * - Fields |
| 220 | - Description |
| 221 | * - target |
| 222 | - Specifies the name of the BPF iterator. For example: ``bpf_map``, |
| 223 | ``bpf_map_elem``. The name should be different from other ``bpf_iter`` target names in the kernel. |
| 224 | * - attach_target and detach_target |
| 225 | - Allows for target specific ``link_create`` action since some targets |
| 226 | may need special processing. Called during the user space link_create stage. |
| 227 | * - show_fdinfo and fill_link_info |
| 228 | - Called to fill target specific information when user tries to get link |
| 229 | info associated with the iterator. |
| 230 | * - get_func_proto |
| 231 | - Permits a BPF iterator to access BPF helpers specific to the iterator. |
| 232 | * - ctx_arg_info_size and ctx_arg_info |
| 233 | - Specifies the verifier states for BPF program arguments associated with |
| 234 | the bpf iterator. |
| 235 | * - feature |
| 236 | - Specifies certain action requests in the kernel BPF iterator |
| 237 | infrastructure. Currently, only BPF_ITER_RESCHED is supported. This means |
| 238 | that the kernel function cond_resched() is called to avoid other kernel |
| 239 | subsystem (e.g., rcu) misbehaving. |
| 240 | * - seq_info |
| 241 | - Specifies the set of seq operations for the BPF iterator and helpers to |
| 242 | initialize/free the private data for the corresponding ``seq_file``. |
| 243 | |
| 244 | `Click here |
| 245 | <https://lore.kernel.org/bpf/20210212183107.50963-2-songliubraving@fb.com/>`_ |
| 246 | to see an implementation of the ``task_vma`` BPF iterator in the kernel. |
| 247 | |
| 248 | --------------------------------- |
| 249 | Parameterizing BPF Task Iterators |
| 250 | --------------------------------- |
| 251 | |
| 252 | By default, BPF iterators walk through all the objects of the specified types |
| 253 | (processes, cgroups, maps, etc.) across the entire system to read relevant |
| 254 | kernel data. But often, there are cases where we only care about a much smaller |
| 255 | subset of iterable kernel objects, such as only iterating tasks within a |
| 256 | specific process. Therefore, BPF iterator programs support filtering out objects |
| 257 | from iteration by allowing user space to configure the iterator program when it |
| 258 | is attached. |
| 259 | |
| 260 | -------------------------- |
| 261 | BPF Task Iterator Program |
| 262 | -------------------------- |
| 263 | |
| 264 | The following code is a BPF iterator program to print files and task information |
| 265 | through the ``seq_file`` of the iterator. It is a standard BPF iterator program |
| 266 | that visits every file of an iterator. We will use this BPF program in our |
| 267 | example later. |
| 268 | |
| 269 | :: |
| 270 | |
| 271 | #include <vmlinux.h> |
| 272 | #include <bpf/bpf_helpers.h> |
| 273 | |
| 274 | char _license[] SEC("license") = "GPL"; |
| 275 | |
| 276 | SEC("iter/task_file") |
| 277 | int dump_task_file(struct bpf_iter__task_file *ctx) |
| 278 | { |
| 279 | struct seq_file *seq = ctx->meta->seq; |
| 280 | struct task_struct *task = ctx->task; |
| 281 | struct file *file = ctx->file; |
| 282 | __u32 fd = ctx->fd; |
| 283 | if (task == NULL || file == NULL) |
| 284 | return 0; |
| 285 | if (ctx->meta->seq_num == 0) { |
| 286 | BPF_SEQ_PRINTF(seq, " tgid pid fd file\n"); |
| 287 | } |
| 288 | BPF_SEQ_PRINTF(seq, "%8d %8d %8d %lx\n", task->tgid, task->pid, fd, |
| 289 | (long)file->f_op); |
| 290 | return 0; |
| 291 | } |
| 292 | |
| 293 | ---------------------------------------- |
| 294 | Creating a File Iterator with Parameters |
| 295 | ---------------------------------------- |
| 296 | |
| 297 | Now, let us look at how to create an iterator that includes only files of a |
| 298 | process. |
| 299 | |
| 300 | First, fill the ``bpf_iter_attach_opts`` struct as shown below: |
| 301 | |
| 302 | :: |
| 303 | |
| 304 | LIBBPF_OPTS(bpf_iter_attach_opts, opts); |
| 305 | union bpf_iter_link_info linfo; |
| 306 | memset(&linfo, 0, sizeof(linfo)); |
| 307 | linfo.task.pid = getpid(); |
| 308 | opts.link_info = &linfo; |
| 309 | opts.link_info_len = sizeof(linfo); |
| 310 | |
| 311 | ``linfo.task.pid``, if it is non-zero, directs the kernel to create an iterator |
| 312 | that only includes opened files for the process with the specified ``pid``. In |
| 313 | this example, we will only be iterating files for our process. If |
| 314 | ``linfo.task.pid`` is zero, the iterator will visit every opened file of every |
| 315 | process. Similarly, ``linfo.task.tid`` directs the kernel to create an iterator |
| 316 | that visits opened files of a specific thread, not a process. In this example, |
| 317 | ``linfo.task.tid`` is different from ``linfo.task.pid`` only if the thread has a |
| 318 | separate file descriptor table. In most circumstances, all process threads share |
| 319 | a single file descriptor table. |
| 320 | |
| 321 | Now, in the userspace program, pass the pointer of struct to the |
| 322 | ``bpf_program__attach_iter()``. |
| 323 | |
| 324 | :: |
| 325 | |
| 326 | link = bpf_program__attach_iter(prog, &opts); iter_fd = |
| 327 | bpf_iter_create(bpf_link__fd(link)); |
| 328 | |
| 329 | If both *tid* and *pid* are zero, an iterator created from this struct |
| 330 | ``bpf_iter_attach_opts`` will include every opened file of every task in the |
| 331 | system (in the namespace, actually.) It is the same as passing a NULL as the |
| 332 | second argument to ``bpf_program__attach_iter()``. |
| 333 | |
| 334 | The whole program looks like the following code: |
| 335 | |
| 336 | :: |
| 337 | |
| 338 | #include <stdio.h> |
| 339 | #include <unistd.h> |
| 340 | #include <bpf/bpf.h> |
| 341 | #include <bpf/libbpf.h> |
| 342 | #include "bpf_iter_task_ex.skel.h" |
| 343 | |
| 344 | static int do_read_opts(struct bpf_program *prog, struct bpf_iter_attach_opts *opts) |
| 345 | { |
| 346 | struct bpf_link *link; |
| 347 | char buf[16] = {}; |
| 348 | int iter_fd = -1, len; |
| 349 | int ret = 0; |
| 350 | |
| 351 | link = bpf_program__attach_iter(prog, opts); |
| 352 | if (!link) { |
| 353 | fprintf(stderr, "bpf_program__attach_iter() fails\n"); |
| 354 | return -1; |
| 355 | } |
| 356 | iter_fd = bpf_iter_create(bpf_link__fd(link)); |
| 357 | if (iter_fd < 0) { |
| 358 | fprintf(stderr, "bpf_iter_create() fails\n"); |
| 359 | ret = -1; |
| 360 | goto free_link; |
| 361 | } |
| 362 | /* not check contents, but ensure read() ends without error */ |
| 363 | while ((len = read(iter_fd, buf, sizeof(buf) - 1)) > 0) { |
| 364 | buf[len] = 0; |
| 365 | printf("%s", buf); |
| 366 | } |
| 367 | printf("\n"); |
| 368 | free_link: |
| 369 | if (iter_fd >= 0) |
| 370 | close(iter_fd); |
| 371 | bpf_link__destroy(link); |
| 372 | return 0; |
| 373 | } |
| 374 | |
| 375 | static void test_task_file(void) |
| 376 | { |
| 377 | LIBBPF_OPTS(bpf_iter_attach_opts, opts); |
| 378 | struct bpf_iter_task_ex *skel; |
| 379 | union bpf_iter_link_info linfo; |
| 380 | skel = bpf_iter_task_ex__open_and_load(); |
| 381 | if (skel == NULL) |
| 382 | return; |
| 383 | memset(&linfo, 0, sizeof(linfo)); |
| 384 | linfo.task.pid = getpid(); |
| 385 | opts.link_info = &linfo; |
| 386 | opts.link_info_len = sizeof(linfo); |
| 387 | printf("PID %d\n", getpid()); |
| 388 | do_read_opts(skel->progs.dump_task_file, &opts); |
| 389 | bpf_iter_task_ex__destroy(skel); |
| 390 | } |
| 391 | |
| 392 | int main(int argc, const char * const * argv) |
| 393 | { |
| 394 | test_task_file(); |
| 395 | return 0; |
| 396 | } |
| 397 | |
| 398 | The following lines are the output of the program. |
| 399 | :: |
| 400 | |
| 401 | PID 1859 |
| 402 | |
| 403 | tgid pid fd file |
| 404 | 1859 1859 0 ffffffff82270aa0 |
| 405 | 1859 1859 1 ffffffff82270aa0 |
| 406 | 1859 1859 2 ffffffff82270aa0 |
| 407 | 1859 1859 3 ffffffff82272980 |
| 408 | 1859 1859 4 ffffffff8225e120 |
| 409 | 1859 1859 5 ffffffff82255120 |
| 410 | 1859 1859 6 ffffffff82254f00 |
| 411 | 1859 1859 7 ffffffff82254d80 |
| 412 | 1859 1859 8 ffffffff8225abe0 |
| 413 | |
| 414 | ------------------ |
| 415 | Without Parameters |
| 416 | ------------------ |
| 417 | |
| 418 | Let us look at how a BPF iterator without parameters skips files of other |
| 419 | processes in the system. In this case, the BPF program has to check the pid or |
| 420 | the tid of tasks, or it will receive every opened file in the system (in the |
| 421 | current *pid* namespace, actually). So, we usually add a global variable in the |
| 422 | BPF program to pass a *pid* to the BPF program. |
| 423 | |
| 424 | The BPF program would look like the following block. |
| 425 | |
| 426 | :: |
| 427 | |
| 428 | ...... |
| 429 | int target_pid = 0; |
| 430 | |
| 431 | SEC("iter/task_file") |
| 432 | int dump_task_file(struct bpf_iter__task_file *ctx) |
| 433 | { |
| 434 | ...... |
| 435 | if (task->tgid != target_pid) /* Check task->pid instead to check thread IDs */ |
| 436 | return 0; |
| 437 | BPF_SEQ_PRINTF(seq, "%8d %8d %8d %lx\n", task->tgid, task->pid, fd, |
| 438 | (long)file->f_op); |
| 439 | return 0; |
| 440 | } |
| 441 | |
| 442 | The user space program would look like the following block: |
| 443 | |
| 444 | :: |
| 445 | |
| 446 | ...... |
| 447 | static void test_task_file(void) |
| 448 | { |
| 449 | ...... |
| 450 | skel = bpf_iter_task_ex__open_and_load(); |
| 451 | if (skel == NULL) |
| 452 | return; |
| 453 | skel->bss->target_pid = getpid(); /* process ID. For thread id, use gettid() */ |
| 454 | memset(&linfo, 0, sizeof(linfo)); |
| 455 | linfo.task.pid = getpid(); |
| 456 | opts.link_info = &linfo; |
| 457 | opts.link_info_len = sizeof(linfo); |
| 458 | ...... |
| 459 | } |
| 460 | |
| 461 | ``target_pid`` is a global variable in the BPF program. The user space program |
| 462 | should initialize the variable with a process ID to skip opened files of other |
| 463 | processes in the BPF program. When you parametrize a BPF iterator, the iterator |
| 464 | calls the BPF program fewer times which can save significant resources. |
| 465 | |
| 466 | --------------------------- |
| 467 | Parametrizing VMA Iterators |
| 468 | --------------------------- |
| 469 | |
| 470 | By default, a BPF VMA iterator includes every VMA in every process. However, |
| 471 | you can still specify a process or a thread to include only its VMAs. Unlike |
| 472 | files, a thread can not have a separate address space (since Linux 2.6.0-test6). |
| 473 | Here, using *tid* makes no difference from using *pid*. |
| 474 | |
| 475 | ---------------------------- |
| 476 | Parametrizing Task Iterators |
| 477 | ---------------------------- |
| 478 | |
| 479 | A BPF task iterator with *pid* includes all tasks (threads) of a process. The |
| 480 | BPF program receives these tasks one after another. You can specify a BPF task |
| 481 | iterator with *tid* parameter to include only the tasks that match the given |
| 482 | *tid*. |