1============= 2BPF Iterators 3============= 4 5 6---------- 7Motivation 8---------- 9 10There are a few existing ways to dump kernel data into user space. The most 11popular one is the ``/proc`` system. For example, ``cat /proc/net/tcp6`` dumps 12all tcp6 sockets in the system, and ``cat /proc/net/netlink`` dumps all netlink 13sockets in the system. However, their output format tends to be fixed, and if 14users want more information about these sockets, they have to patch the kernel, 15which often takes time to publish upstream and release. The same is true for popular 16tools like `ss <https://man7.org/linux/man-pages/man8/ss.8.html>`_ where any 17additional information needs a kernel patch. 18 19To solve this problem, the `drgn 20<https://www.kernel.org/doc/html/latest/bpf/drgn.html>`_ tool is often used to 21dig out the kernel data with no kernel change. However, the main drawback for 22drgn is performance, as it cannot do pointer tracing inside the kernel. In 23addition, drgn cannot validate a pointer value and may read invalid data if the 24pointer becomes invalid inside the kernel. 25 26The 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 28data object. 29 30---------------------- 31How BPF Iterators Work 32---------------------- 33 34A BPF iterator is a type of BPF program that allows users to iterate over 35specific types of kernel objects. Unlike traditional BPF tracing programs that 36allow users to define callbacks that are invoked at particular points of 37execution in the kernel, BPF iterators allow users to define callbacks that 38should be executed for every entry in a variety of kernel data structures. 39 40For example, users can define a BPF iterator that iterates over every task on 41the system and dumps the total amount of CPU runtime currently used by each of 42them. Another BPF task iterator may instead dump the cgroup information for each 43task. Such flexibility is the core value of BPF iterators. 44 45A BPF program is always loaded into the kernel at the behest of a user space 46process. A user space process loads a BPF program by opening and initializing 47the program skeleton as required and then invoking a syscall to have the BPF 48program verified and loaded by the kernel. 49 50In traditional tracing programs, a program is activated by having user space 51obtain a ``bpf_link`` to the program with ``bpf_program__attach()``. Once 52activated, the program callback will be invoked whenever the tracepoint is 53triggered in the main kernel. For BPF iterator programs, a ``bpf_link`` to the 54program is obtained using ``bpf_link_create()``, and the program callback is 55invoked by issuing system calls from user space. 56 57Next, let us see how you can use the iterators to iterate on kernel objects and 58read data. 59 60------------------------ 61How to Use BPF iterators 62------------------------ 63 64BPF selftests are a great resource to illustrate how to use the iterators. In 65this section, we’ll walk through a BPF selftest which shows how to load and use 66a 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>`_, 68which illustrates how to load and trigger BPF iterators on the user space side. 69Later, we’ll look at a BPF program that runs in kernel space. 70 71Loading a BPF iterator in the kernel from user space typically involves the 72following 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 86The 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 92Let us look at ``bpf_iter_task_file.c``, which runs in kernel space: 93 94Here 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>`_. 96Any struct name in ``vmlinux.h`` in the format ``bpf_iter__<iter_name>`` 97represents a BPF iterator. The suffix ``<iter_name>`` represents the type of 98iterator. 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 115In the above code, the field 'meta' contains the metadata, which is the same for 116all BPF iterator programs. The rest of the fields are specific to different 117iterators. For example, for task_file iterators, the kernel layer provides the 118'task', 'fd' and 'file' field values. The 'task' and 'file' are `reference 119counted 120<https://facebookmicrosites.github.io/bpf/blog/2018/08/31/object-lifetime.html#file-descriptors-and-reference-counters>`_, 121so they won't go away when the BPF program runs. 122 123Here 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 156In the above example, the section name ``SEC(iter/task_file)``, indicates that 157the program is a BPF iterator program to iterate all files from all tasks. The 158context of the program is ``bpf_iter__task_file`` struct. 159 160The user space program invokes the BPF iterator program running in the kernel 161by issuing a ``read()`` syscall. Once invoked, the BPF 162program can export data to user space using a variety of BPF helper functions. 163You 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 165binary data, respectively. For binary-encoded data, the user space applications 166can process the data from ``bpf_seq_write()`` as needed. For the formatted data, 167you 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, 169use ``rm -f <path>`` to remove the pinned iterator. 170 171For 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`` 173path: 174 175:: 176 177 $ bpftool iter pin ./bpf_iter_ipv6_route.o /sys/fs/bpf/my_route 178 179And then print out the results using the following command: 180 181:: 182 183 $ cat /sys/fs/bpf/my_route 184 185 186------------------------------------------------------- 187Implement Kernel Support for BPF Iterator Program Types 188------------------------------------------------------- 189 190To implement a BPF iterator in the kernel, the developer must make a one-time 191change 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>`_ 193file. 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 210After filling the data structure fields, call ``bpf_iter_reg_target()`` to 211register the iterator to the main BPF iterator subsystem. 212 213The 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/>`_ 246to see an implementation of the ``task_vma`` BPF iterator in the kernel. 247 248--------------------------------- 249Parameterizing BPF Task Iterators 250--------------------------------- 251 252By default, BPF iterators walk through all the objects of the specified types 253(processes, cgroups, maps, etc.) across the entire system to read relevant 254kernel data. But often, there are cases where we only care about a much smaller 255subset of iterable kernel objects, such as only iterating tasks within a 256specific process. Therefore, BPF iterator programs support filtering out objects 257from iteration by allowing user space to configure the iterator program when it 258is attached. 259 260-------------------------- 261BPF Task Iterator Program 262-------------------------- 263 264The following code is a BPF iterator program to print files and task information 265through the ``seq_file`` of the iterator. It is a standard BPF iterator program 266that visits every file of an iterator. We will use this BPF program in our 267example 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---------------------------------------- 294Creating a File Iterator with Parameters 295---------------------------------------- 296 297Now, let us look at how to create an iterator that includes only files of a 298process. 299 300First, 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 312that only includes opened files for the process with the specified ``pid``. In 313this 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 315process. Similarly, ``linfo.task.tid`` directs the kernel to create an iterator 316that 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 318separate file descriptor table. In most circumstances, all process threads share 319a single file descriptor table. 320 321Now, 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 329If 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 331system (in the namespace, actually.) It is the same as passing a NULL as the 332second argument to ``bpf_program__attach_iter()``. 333 334The 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 398The 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------------------ 415Without Parameters 416------------------ 417 418Let us look at how a BPF iterator without parameters skips files of other 419processes in the system. In this case, the BPF program has to check the pid or 420the tid of tasks, or it will receive every opened file in the system (in the 421current *pid* namespace, actually). So, we usually add a global variable in the 422BPF program to pass a *pid* to the BPF program. 423 424The 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 442The 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 462should initialize the variable with a process ID to skip opened files of other 463processes in the BPF program. When you parametrize a BPF iterator, the iterator 464calls the BPF program fewer times which can save significant resources. 465 466--------------------------- 467Parametrizing VMA Iterators 468--------------------------- 469 470By default, a BPF VMA iterator includes every VMA in every process. However, 471you can still specify a process or a thread to include only its VMAs. Unlike 472files, a thread can not have a separate address space (since Linux 2.6.0-test6). 473Here, using *tid* makes no difference from using *pid*. 474 475---------------------------- 476Parametrizing Task Iterators 477---------------------------- 478 479A BPF task iterator with *pid* includes all tasks (threads) of a process. The 480BPF program receives these tasks one after another. You can specify a BPF task 481iterator with *tid* parameter to include only the tasks that match the given 482*tid*. 483