1.. SPDX-License-Identifier: GPL-2.0 2 3.. _kfuncs-header-label: 4 5============================= 6BPF Kernel Functions (kfuncs) 7============================= 8 91. Introduction 10=============== 11 12BPF Kernel Functions or more commonly known as kfuncs are functions in the Linux 13kernel which are exposed for use by BPF programs. Unlike normal BPF helpers, 14kfuncs do not have a stable interface and can change from one kernel release to 15another. Hence, BPF programs need to be updated in response to changes in the 16kernel. See :ref:`BPF_kfunc_lifecycle_expectations` for more information. 17 182. Defining a kfunc 19=================== 20 21There are two ways to expose a kernel function to BPF programs, either make an 22existing function in the kernel visible, or add a new wrapper for BPF. In both 23cases, care must be taken that BPF program can only call such function in a 24valid context. To enforce this, visibility of a kfunc can be per program type. 25 26If you are not creating a BPF wrapper for existing kernel function, skip ahead 27to :ref:`BPF_kfunc_nodef`. 28 292.1 Creating a wrapper kfunc 30---------------------------- 31 32When defining a wrapper kfunc, the wrapper function should have extern linkage. 33This prevents the compiler from optimizing away dead code, as this wrapper kfunc 34is not invoked anywhere in the kernel itself. It is not necessary to provide a 35prototype in a header for the wrapper kfunc. 36 37An example is given below:: 38 39 /* Disables missing prototype warnings */ 40 __bpf_kfunc_start_defs(); 41 42 __bpf_kfunc struct task_struct *bpf_find_get_task_by_vpid(pid_t nr) 43 { 44 return find_get_task_by_vpid(nr); 45 } 46 47 __bpf_kfunc_end_defs(); 48 49A wrapper kfunc is often needed when we need to annotate parameters of the 50kfunc. Otherwise one may directly make the kfunc visible to the BPF program by 51registering it with the BPF subsystem. See :ref:`BPF_kfunc_nodef`. 52 532.2 kfunc Parameters 54-------------------- 55 56All kfuncs now require trusted arguments by default. This means that all 57pointer arguments must be valid, and all pointers to BTF objects must be 58passed in their unmodified form (at a zero offset, and without having been 59obtained from walking another pointer, with exceptions described below). 60 61There are two types of pointers to kernel objects which are considered "trusted": 62 631. Pointers which are passed as tracepoint or struct_ops callback arguments. 642. Pointers which were returned from a KF_ACQUIRE kfunc. 65 66Pointers to non-BTF objects (e.g. scalar pointers) may also be passed to 67kfuncs, and may have a non-zero offset. 68 69The definition of "valid" pointers is subject to change at any time, and has 70absolutely no ABI stability guarantees. 71 72As mentioned above, a nested pointer obtained from walking a trusted pointer is 73no longer trusted, with one exception. If a struct type has a field that is 74guaranteed to be valid (trusted or rcu, as in KF_RCU description below) as long 75as its parent pointer is valid, the following macros can be used to express 76that to the verifier: 77 78* ``BTF_TYPE_SAFE_TRUSTED`` 79* ``BTF_TYPE_SAFE_RCU`` 80* ``BTF_TYPE_SAFE_RCU_OR_NULL`` 81 82For example, 83 84.. code-block:: c 85 86 BTF_TYPE_SAFE_TRUSTED(struct socket) { 87 struct sock *sk; 88 }; 89 90or 91 92.. code-block:: c 93 94 BTF_TYPE_SAFE_RCU(struct task_struct) { 95 const cpumask_t *cpus_ptr; 96 struct css_set __rcu *cgroups; 97 struct task_struct __rcu *real_parent; 98 struct task_struct *group_leader; 99 }; 100 101In other words, you must: 102 1031. Wrap the valid pointer type in a ``BTF_TYPE_SAFE_*`` macro. 104 1052. Specify the type and name of the valid nested field. This field must match 106 the field in the original type definition exactly. 107 108A new type declared by a ``BTF_TYPE_SAFE_*`` macro also needs to be emitted so 109that it appears in BTF. For example, ``BTF_TYPE_SAFE_TRUSTED(struct socket)`` 110is emitted in the ``type_is_trusted()`` function as follows: 111 112.. code-block:: c 113 114 BTF_TYPE_EMIT(BTF_TYPE_SAFE_TRUSTED(struct socket)); 115 1162.3 Annotating kfunc parameters 117------------------------------- 118 119Similar to BPF helpers, there is sometime need for additional context required 120by the verifier to make the usage of kernel functions safer and more useful. 121Hence, we can annotate a parameter by suffixing the name of the argument of the 122kfunc with a __tag, where tag may be one of the supported annotations. 123 1242.3.1 __sz Annotation 125--------------------- 126 127This annotation is used to indicate a memory and size pair in the argument list. 128An example is given below:: 129 130 __bpf_kfunc void bpf_memzero(void *mem, int mem__sz) 131 { 132 ... 133 } 134 135Here, the verifier will treat first argument as a PTR_TO_MEM, and second 136argument as its size. By default, without __sz annotation, the size of the type 137of the pointer is used. Without __sz annotation, a kfunc cannot accept a void 138pointer. 139 1402.3.2 __k Annotation 141-------------------- 142 143This annotation is only understood for scalar arguments, where it indicates that 144the verifier must check the scalar argument to be a known constant, which does 145not indicate a size parameter, and the value of the constant is relevant to the 146safety of the program. 147 148An example is given below:: 149 150 __bpf_kfunc void *bpf_obj_new(u32 local_type_id__k, ...) 151 { 152 ... 153 } 154 155Here, bpf_obj_new uses local_type_id argument to find out the size of that type 156ID in program's BTF and return a sized pointer to it. Each type ID will have a 157distinct size, hence it is crucial to treat each such call as distinct when 158values don't match during verifier state pruning checks. 159 160Hence, whenever a constant scalar argument is accepted by a kfunc which is not a 161size parameter, and the value of the constant matters for program safety, __k 162suffix should be used. 163 1642.3.3 __uninit Annotation 165------------------------- 166 167This annotation is used to indicate that the argument will be treated as 168uninitialized. 169 170An example is given below:: 171 172 __bpf_kfunc int bpf_dynptr_from_skb(..., struct bpf_dynptr_kern *ptr__uninit) 173 { 174 ... 175 } 176 177Here, the dynptr will be treated as an uninitialized dynptr. Without this 178annotation, the verifier will reject the program if the dynptr passed in is 179not initialized. 180 1812.3.4 __nullable Annotation 182--------------------------- 183 184This annotation is used to indicate that the pointer argument may be NULL. 185The verifier will allow passing NULL for such arguments. 186 187An example is given below:: 188 189 __bpf_kfunc void bpf_task_release(struct task_struct *task__nullable) 190 { 191 ... 192 } 193 194Here, the task pointer may be NULL. The kfunc is responsible for checking if 195the pointer is NULL before dereferencing it. 196 197The __nullable annotation can be combined with other annotations. For example, 198when used with __sz or __szk annotations for memory and size pairs, the 199verifier will skip size validation when a NULL pointer is passed, but will 200still process the size argument to extract constant size information when 201needed:: 202 203 __bpf_kfunc void *bpf_dynptr_slice(..., void *buffer__nullable, 204 u32 buffer__szk) 205 206Here, the buffer may be NULL. If the buffer is not NULL, it must be at least 207buffer__szk bytes in size. The kfunc is responsible for checking if the buffer 208is NULL before using it. 209 2102.3.5 __nonown_allowed Annotation 211--------------------------------- 212 213This annotation is used to indicate that the parameter may be a non-owning reference. 214 215An example is given below:: 216 217 __bpf_kfunc int bpf_list_add(..., struct bpf_list_node 218 *prev__nonown_allowed, ...) 219 { 220 ... 221 } 222 223For the ``prev__nonown_allowed`` parameter (resolved as ``KF_ARG_PTR_TO_LIST_NODE``), 224suffix ``__nonown_allowed`` retains the usual owning-pointer rules and also 225permits a non-owning reference with no ref_obj_id (e.g. the return value of 226bpf_list_front() / bpf_list_back()). 227 2282.3.6 __str Annotation 229---------------------- 230This annotation is used to indicate that the argument is a constant string. 231 232An example is given below:: 233 234 __bpf_kfunc bpf_get_file_xattr(..., const char *name__str, ...) 235 { 236 ... 237 } 238 239In this case, ``bpf_get_file_xattr()`` can be called as:: 240 241 bpf_get_file_xattr(..., "xattr_name", ...); 242 243Or:: 244 245 const char name[] = "xattr_name"; /* This need to be global */ 246 int BPF_PROG(...) 247 { 248 ... 249 bpf_get_file_xattr(..., name, ...); 250 ... 251 } 252 253.. _BPF_kfunc_nodef: 254 2552.4 Using an existing kernel function 256------------------------------------- 257 258When an existing function in the kernel is fit for consumption by BPF programs, 259it can be directly registered with the BPF subsystem. However, care must still 260be taken to review the context in which it will be invoked by the BPF program 261and whether it is safe to do so. 262 2632.5 Annotating kfuncs 264--------------------- 265 266In addition to kfuncs' arguments, verifier may need more information about the 267type of kfunc(s) being registered with the BPF subsystem. To do so, we define 268flags on a set of kfuncs as follows:: 269 270 BTF_KFUNCS_START(bpf_task_set) 271 BTF_ID_FLAGS(func, bpf_get_task_pid, KF_ACQUIRE | KF_RET_NULL) 272 BTF_ID_FLAGS(func, bpf_put_pid, KF_RELEASE) 273 BTF_KFUNCS_END(bpf_task_set) 274 275This set encodes the BTF ID of each kfunc listed above, and encodes the flags 276along with it. Ofcourse, it is also allowed to specify no flags. 277 278kfunc definitions should also always be annotated with the ``__bpf_kfunc`` 279macro. This prevents issues such as the compiler inlining the kfunc if it's a 280static kernel function, or the function being elided in an LTO build as it's 281not used in the rest of the kernel. Developers should not manually add 282annotations to their kfunc to prevent these issues. If an annotation is 283required to prevent such an issue with your kfunc, it is a bug and should be 284added to the definition of the macro so that other kfuncs are similarly 285protected. An example is given below:: 286 287 __bpf_kfunc struct task_struct *bpf_get_task_pid(s32 pid) 288 { 289 ... 290 } 291 2922.5.1 KF_ACQUIRE flag 293--------------------- 294 295The KF_ACQUIRE flag is used to indicate that the kfunc returns a pointer to a 296refcounted object. The verifier will then ensure that the pointer to the object 297is eventually released using a release kfunc, or transferred to a map using a 298referenced kptr (by invoking bpf_kptr_xchg). If not, the verifier fails the 299loading of the BPF program until no lingering references remain in all possible 300explored states of the program. 301 3022.5.2 KF_RET_NULL flag 303---------------------- 304 305The KF_RET_NULL flag is used to indicate that the pointer returned by the kfunc 306may be NULL. Hence, it forces the user to do a NULL check on the pointer 307returned from the kfunc before making use of it (dereferencing or passing to 308another helper). This flag is often used in pairing with KF_ACQUIRE flag, but 309both are orthogonal to each other. 310 3112.5.3 KF_RELEASE flag 312--------------------- 313 314The KF_RELEASE flag is used to indicate that the kfunc releases the pointer 315passed in to it. There can be only one referenced pointer that can be passed 316in. All copies of the pointer being released are invalidated as a result of 317invoking kfunc with this flag. 318 3192.5.4 KF_SLEEPABLE flag 320----------------------- 321 322The KF_SLEEPABLE flag is used for kfuncs that may sleep. Such kfuncs can only 323be called by sleepable BPF programs (BPF_F_SLEEPABLE). 324 3252.5.5 KF_DESTRUCTIVE flag 326-------------------------- 327 328The KF_DESTRUCTIVE flag is used to indicate functions calling which is 329destructive to the system. For example such a call can result in system 330rebooting or panicking. Due to this additional restrictions apply to these 331calls. At the moment they only require CAP_SYS_BOOT capability, but more can be 332added later. 333 3342.5.6 KF_RCU flag 335----------------- 336 337The KF_RCU flag allows kfuncs to opt out of the default trusted args 338requirement and accept RCU pointers with weaker guarantees. The kfuncs marked 339with KF_RCU expect either PTR_TRUSTED or MEM_RCU arguments. The verifier 340guarantees that the objects are valid and there is no use-after-free. The 341pointers are not NULL, but the object's refcount could have reached zero. The 342kfuncs need to consider doing refcnt != 0 check, especially when returning a 343KF_ACQUIRE pointer. Note as well that a KF_ACQUIRE kfunc that is KF_RCU should 344very likely also be KF_RET_NULL. 345 3462.5.7 KF_RCU_PROTECTED flag 347--------------------------- 348 349The KF_RCU_PROTECTED flag is used to indicate that the kfunc must be invoked in 350an RCU critical section. This is assumed by default in non-sleepable programs, 351and must be explicitly ensured by calling ``bpf_rcu_read_lock`` for sleepable 352ones. 353 354If the kfunc returns a pointer value, this flag also enforces that the returned 355pointer is RCU protected, and can only be used while the RCU critical section is 356active. 357 358The flag is distinct from the ``KF_RCU`` flag, which only ensures that its 359arguments are at least RCU protected pointers. This may transitively imply that 360RCU protection is ensured, but it does not work in cases of kfuncs which require 361RCU protection but do not take RCU protected arguments. 362 363.. _KF_deprecated_flag: 364 3652.5.8 KF_DEPRECATED flag 366------------------------ 367 368The KF_DEPRECATED flag is used for kfuncs which are scheduled to be 369changed or removed in a subsequent kernel release. A kfunc that is 370marked with KF_DEPRECATED should also have any relevant information 371captured in its kernel doc. Such information typically includes the 372kfunc's expected remaining lifespan, a recommendation for new 373functionality that can replace it if any is available, and possibly a 374rationale for why it is being removed. 375 376Note that while on some occasions, a KF_DEPRECATED kfunc may continue to be 377supported and have its KF_DEPRECATED flag removed, it is likely to be far more 378difficult to remove a KF_DEPRECATED flag after it's been added than it is to 379prevent it from being added in the first place. As described in 380:ref:`BPF_kfunc_lifecycle_expectations`, users that rely on specific kfuncs are 381encouraged to make their use-cases known as early as possible, and participate 382in upstream discussions regarding whether to keep, change, deprecate, or remove 383those kfuncs if and when such discussions occur. 384 3852.5.9 KF_IMPLICIT_ARGS flag 386------------------------------------ 387 388The KF_IMPLICIT_ARGS flag is used to indicate that the BPF signature 389of the kfunc is different from it's kernel signature, and the values 390for implicit arguments are provided at load time by the verifier. 391 392Only arguments of specific types are implicit. 393Currently only ``struct bpf_prog_aux *`` type is supported. 394 395A kfunc with KF_IMPLICIT_ARGS flag therefore has two types in BTF: one 396function matching the kernel declaration (with _impl suffix in the 397name by convention), and another matching the intended BPF API. 398 399Verifier only allows calls to the non-_impl version of a kfunc, that 400uses a signature without the implicit arguments. 401 402Example declaration: 403 404.. code-block:: c 405 406 __bpf_kfunc int bpf_task_work_schedule_signal(struct task_struct *task, struct bpf_task_work *tw, 407 void *map__map, bpf_task_work_callback_t callback, 408 struct bpf_prog_aux *aux) { ... } 409 410Example usage in BPF program: 411 412.. code-block:: c 413 414 /* note that the last argument is omitted */ 415 bpf_task_work_schedule_signal(task, &work->tw, &arrmap, task_work_callback); 416 4172.6 Registering the kfuncs 418-------------------------- 419 420Once the kfunc is prepared for use, the final step to making it visible is 421registering it with the BPF subsystem. Registration is done per BPF program 422type. An example is shown below:: 423 424 BTF_KFUNCS_START(bpf_task_set) 425 BTF_ID_FLAGS(func, bpf_get_task_pid, KF_ACQUIRE | KF_RET_NULL) 426 BTF_ID_FLAGS(func, bpf_put_pid, KF_RELEASE) 427 BTF_KFUNCS_END(bpf_task_set) 428 429 static const struct btf_kfunc_id_set bpf_task_kfunc_set = { 430 .owner = THIS_MODULE, 431 .set = &bpf_task_set, 432 }; 433 434 static int init_subsystem(void) 435 { 436 return register_btf_kfunc_id_set(BPF_PROG_TYPE_TRACING, &bpf_task_kfunc_set); 437 } 438 late_initcall(init_subsystem); 439 4402.7 Specifying no-cast aliases with ___init 441-------------------------------------------- 442 443The verifier will always enforce that the BTF type of a pointer passed to a 444kfunc by a BPF program, matches the type of pointer specified in the kfunc 445definition. The verifier, does, however, allow types that are equivalent 446according to the C standard to be passed to the same kfunc arg, even if their 447BTF_IDs differ. 448 449For example, for the following type definition: 450 451.. code-block:: c 452 453 struct bpf_cpumask { 454 cpumask_t cpumask; 455 refcount_t usage; 456 }; 457 458The verifier would allow a ``struct bpf_cpumask *`` to be passed to a kfunc 459taking a ``cpumask_t *`` (which is a typedef of ``struct cpumask *``). For 460instance, both ``struct cpumask *`` and ``struct bpf_cpmuask *`` can be passed 461to bpf_cpumask_test_cpu(). 462 463In some cases, this type-aliasing behavior is not desired. ``struct 464nf_conn___init`` is one such example: 465 466.. code-block:: c 467 468 struct nf_conn___init { 469 struct nf_conn ct; 470 }; 471 472The C standard would consider these types to be equivalent, but it would not 473always be safe to pass either type to a trusted kfunc. ``struct 474nf_conn___init`` represents an allocated ``struct nf_conn`` object that has 475*not yet been initialized*, so it would therefore be unsafe to pass a ``struct 476nf_conn___init *`` to a kfunc that's expecting a fully initialized ``struct 477nf_conn *`` (e.g. ``bpf_ct_change_timeout()``). 478 479In order to accommodate such requirements, the verifier will enforce strict 480PTR_TO_BTF_ID type matching if two types have the exact same name, with one 481being suffixed with ``___init``. 482 4832.8 Accessing arena memory through kfunc arguments 484-------------------------------------------------- 485 486A read or write at any address inside an arena does not oops the kernel. 487Unallocated arena pages are lazily backed by a scratch page and the 488access is reported through the program's BPF stream as an error. Only 489the BPF program's correctness is affected; the kernel itself remains 490intact. 491 492The arena is followed by a ``GUARD_SZ / 2`` (32 KiB) guard region that 493is also covered by this recovery. A kfunc handed an arena pointer may 494therefore access up to ``GUARD_SZ / 2`` past it without bounds-checking 495against the arena. Larger accesses must verify the range explicitly. 496 497.. _BPF_kfunc_lifecycle_expectations: 498 4993. kfunc lifecycle expectations 500=============================== 501 502kfuncs provide a kernel <-> kernel API, and thus are not bound by any of the 503strict stability restrictions associated with kernel <-> user UAPIs. This means 504they can be thought of as similar to EXPORT_SYMBOL_GPL, and can therefore be 505modified or removed by a maintainer of the subsystem they're defined in when 506it's deemed necessary. 507 508Like any other change to the kernel, maintainers will not change or remove a 509kfunc without having a reasonable justification. Whether or not they'll choose 510to change a kfunc will ultimately depend on a variety of factors, such as how 511widely used the kfunc is, how long the kfunc has been in the kernel, whether an 512alternative kfunc exists, what the norm is in terms of stability for the 513subsystem in question, and of course what the technical cost is of continuing 514to support the kfunc. 515 516There are several implications of this: 517 518a) kfuncs that are widely used or have been in the kernel for a long time will 519 be more difficult to justify being changed or removed by a maintainer. In 520 other words, kfuncs that are known to have a lot of users and provide 521 significant value provide stronger incentives for maintainers to invest the 522 time and complexity in supporting them. It is therefore important for 523 developers that are using kfuncs in their BPF programs to communicate and 524 explain how and why those kfuncs are being used, and to participate in 525 discussions regarding those kfuncs when they occur upstream. 526 527b) Unlike regular kernel symbols marked with EXPORT_SYMBOL_GPL, BPF programs 528 that call kfuncs are generally not part of the kernel tree. This means that 529 refactoring cannot typically change callers in-place when a kfunc changes, 530 as is done for e.g. an upstreamed driver being updated in place when a 531 kernel symbol is changed. 532 533 Unlike with regular kernel symbols, this is expected behavior for BPF 534 symbols, and out-of-tree BPF programs that use kfuncs should be considered 535 relevant to discussions and decisions around modifying and removing those 536 kfuncs. The BPF community will take an active role in participating in 537 upstream discussions when necessary to ensure that the perspectives of such 538 users are taken into account. 539 540c) A kfunc will never have any hard stability guarantees. BPF APIs cannot and 541 will not ever hard-block a change in the kernel purely for stability 542 reasons. That being said, kfuncs are features that are meant to solve 543 problems and provide value to users. The decision of whether to change or 544 remove a kfunc is a multivariate technical decision that is made on a 545 case-by-case basis, and which is informed by data points such as those 546 mentioned above. It is expected that a kfunc being removed or changed with 547 no warning will not be a common occurrence or take place without sound 548 justification, but it is a possibility that must be accepted if one is to 549 use kfuncs. 550 5513.1 kfunc deprecation 552--------------------- 553 554As described above, while sometimes a maintainer may find that a kfunc must be 555changed or removed immediately to accommodate some changes in their subsystem, 556usually kfuncs will be able to accommodate a longer and more measured 557deprecation process. For example, if a new kfunc comes along which provides 558superior functionality to an existing kfunc, the existing kfunc may be 559deprecated for some period of time to allow users to migrate their BPF programs 560to use the new one. Or, if a kfunc has no known users, a decision may be made 561to remove the kfunc (without providing an alternative API) after some 562deprecation period so as to provide users with a window to notify the kfunc 563maintainer if it turns out that the kfunc is actually being used. 564 565It's expected that the common case will be that kfuncs will go through a 566deprecation period rather than being changed or removed without warning. As 567described in :ref:`KF_deprecated_flag`, the kfunc framework provides the 568KF_DEPRECATED flag to kfunc developers to signal to users that a kfunc has been 569deprecated. Once a kfunc has been marked with KF_DEPRECATED, the following 570procedure is followed for removal: 571 5721. Any relevant information for deprecated kfuncs is documented in the kfunc's 573 kernel docs. This documentation will typically include the kfunc's expected 574 remaining lifespan, a recommendation for new functionality that can replace 575 the usage of the deprecated function (or an explanation as to why no such 576 replacement exists), etc. 577 5782. The deprecated kfunc is kept in the kernel for some period of time after it 579 was first marked as deprecated. This time period will be chosen on a 580 case-by-case basis, and will typically depend on how widespread the use of 581 the kfunc is, how long it has been in the kernel, and how hard it is to move 582 to alternatives. This deprecation time period is "best effort", and as 583 described :ref:`above<BPF_kfunc_lifecycle_expectations>`, circumstances may 584 sometimes dictate that the kfunc be removed before the full intended 585 deprecation period has elapsed. 586 5873. After the deprecation period the kfunc will be removed. At this point, BPF 588 programs calling the kfunc will be rejected by the verifier. 589 5904. Core kfuncs 591============== 592 593The BPF subsystem provides a number of "core" kfuncs that are potentially 594applicable to a wide variety of different possible use cases and programs. 595Those kfuncs are documented here. 596 5974.1 struct task_struct * kfuncs 598------------------------------- 599 600There are a number of kfuncs that allow ``struct task_struct *`` objects to be 601used as kptrs: 602 603.. kernel-doc:: kernel/bpf/helpers.c 604 :identifiers: bpf_task_acquire bpf_task_release 605 606These kfuncs are useful when you want to acquire or release a reference to a 607``struct task_struct *`` that was passed as e.g. a tracepoint arg, or a 608struct_ops callback arg. For example: 609 610.. code-block:: c 611 612 /** 613 * A trivial example tracepoint program that shows how to 614 * acquire and release a struct task_struct * pointer. 615 */ 616 SEC("tp_btf/task_newtask") 617 int BPF_PROG(task_acquire_release_example, struct task_struct *task, u64 clone_flags) 618 { 619 struct task_struct *acquired; 620 621 acquired = bpf_task_acquire(task); 622 if (acquired) 623 /* 624 * In a typical program you'd do something like store 625 * the task in a map, and the map will automatically 626 * release it later. Here, we release it manually. 627 */ 628 bpf_task_release(acquired); 629 return 0; 630 } 631 632 633References acquired on ``struct task_struct *`` objects are RCU protected. 634Therefore, when in an RCU read region, you can obtain a pointer to a task 635embedded in a map value without having to acquire a reference: 636 637.. code-block:: c 638 639 #define private(name) SEC(".data." #name) __hidden __attribute__((aligned(8))) 640 private(TASK) static struct task_struct *global; 641 642 /** 643 * A trivial example showing how to access a task stored 644 * in a map using RCU. 645 */ 646 SEC("tp_btf/task_newtask") 647 int BPF_PROG(task_rcu_read_example, struct task_struct *task, u64 clone_flags) 648 { 649 struct task_struct *local_copy; 650 651 bpf_rcu_read_lock(); 652 local_copy = global; 653 if (local_copy) 654 /* 655 * We could also pass local_copy to kfuncs or helper functions here, 656 * as we're guaranteed that local_copy will be valid until we exit 657 * the RCU read region below. 658 */ 659 bpf_printk("Global task %s is valid", local_copy->comm); 660 else 661 bpf_printk("No global task found"); 662 bpf_rcu_read_unlock(); 663 664 /* At this point we can no longer reference local_copy. */ 665 666 return 0; 667 } 668 669---- 670 671A BPF program can also look up a task from a pid. This can be useful if the 672caller doesn't have a trusted pointer to a ``struct task_struct *`` object that 673it can acquire a reference on with bpf_task_acquire(). 674 675.. kernel-doc:: kernel/bpf/helpers.c 676 :identifiers: bpf_task_from_pid 677 678Here is an example of it being used: 679 680.. code-block:: c 681 682 SEC("tp_btf/task_newtask") 683 int BPF_PROG(task_get_pid_example, struct task_struct *task, u64 clone_flags) 684 { 685 struct task_struct *lookup; 686 687 lookup = bpf_task_from_pid(task->pid); 688 if (!lookup) 689 /* A task should always be found, as %task is a tracepoint arg. */ 690 return -ENOENT; 691 692 if (lookup->pid != task->pid) { 693 /* bpf_task_from_pid() looks up the task via its 694 * globally-unique pid from the init_pid_ns. Thus, 695 * the pid of the lookup task should always be the 696 * same as the input task. 697 */ 698 bpf_task_release(lookup); 699 return -EINVAL; 700 } 701 702 /* bpf_task_from_pid() returns an acquired reference, 703 * so it must be dropped before returning from the 704 * tracepoint handler. 705 */ 706 bpf_task_release(lookup); 707 return 0; 708 } 709 7104.2 struct cgroup * kfuncs 711-------------------------- 712 713``struct cgroup *`` objects also have acquire and release functions: 714 715.. kernel-doc:: kernel/bpf/helpers.c 716 :identifiers: bpf_cgroup_acquire bpf_cgroup_release 717 718These kfuncs are used in exactly the same manner as bpf_task_acquire() and 719bpf_task_release() respectively, so we won't provide examples for them. 720 721---- 722 723Other kfuncs available for interacting with ``struct cgroup *`` objects are 724bpf_cgroup_ancestor() and bpf_cgroup_from_id(), allowing callers to access 725the ancestor of a cgroup and find a cgroup by its ID, respectively. Both 726return a cgroup kptr. 727 728.. kernel-doc:: kernel/bpf/helpers.c 729 :identifiers: bpf_cgroup_ancestor 730 731.. kernel-doc:: kernel/bpf/helpers.c 732 :identifiers: bpf_cgroup_from_id 733 734Eventually, BPF should be updated to allow this to happen with a normal memory 735load in the program itself. This is currently not possible without more work in 736the verifier. bpf_cgroup_ancestor() can be used as follows: 737 738.. code-block:: c 739 740 /** 741 * Simple tracepoint example that illustrates how a cgroup's 742 * ancestor can be accessed using bpf_cgroup_ancestor(). 743 */ 744 SEC("tp_btf/cgroup_mkdir") 745 int BPF_PROG(cgrp_ancestor_example, struct cgroup *cgrp, const char *path) 746 { 747 struct cgroup *parent; 748 749 /* The parent cgroup resides at the level before the current cgroup's level. */ 750 parent = bpf_cgroup_ancestor(cgrp, cgrp->level - 1); 751 if (!parent) 752 return -ENOENT; 753 754 bpf_printk("Parent id is %d", parent->self.id); 755 756 /* Return the parent cgroup that was acquired above. */ 757 bpf_cgroup_release(parent); 758 return 0; 759 } 760 7614.3 struct cpumask * kfuncs 762--------------------------- 763 764BPF provides a set of kfuncs that can be used to query, allocate, mutate, and 765destroy struct cpumask * objects. Please refer to :ref:`cpumasks-header-label` 766for more details. 767