diff options
Diffstat (limited to 'Documentation/bpf')
| -rw-r--r-- | Documentation/bpf/bpf_design_QA.rst | 25 | ||||
| -rw-r--r-- | Documentation/bpf/btf.rst | 49 | ||||
| -rw-r--r-- | Documentation/bpf/index.rst | 1 | ||||
| -rw-r--r-- | Documentation/bpf/instruction-set.rst | 4 | ||||
| -rw-r--r-- | Documentation/bpf/kfuncs.rst | 170 | ||||
| -rw-r--r-- | Documentation/bpf/libbpf/libbpf_naming_convention.rst | 13 | ||||
| -rw-r--r-- | Documentation/bpf/map_hash.rst | 185 |
7 files changed, 427 insertions, 20 deletions
diff --git a/Documentation/bpf/bpf_design_QA.rst b/Documentation/bpf/bpf_design_QA.rst index 437de2a7a5de..a210b8a4df00 100644 --- a/Documentation/bpf/bpf_design_QA.rst +++ b/Documentation/bpf/bpf_design_QA.rst @@ -214,6 +214,12 @@ A: NO. Tracepoints are tied to internal implementation details hence they are subject to change and can break with newer kernels. BPF programs need to change accordingly when this happens. +Q: Are places where kprobes can attach part of the stable ABI? +-------------------------------------------------------------- +A: NO. The places to which kprobes can attach are internal implementation +details, which means that they are subject to change and can break with +newer kernels. BPF programs need to change accordingly when this happens. + Q: How much stack space a BPF program uses? ------------------------------------------- A: Currently all program types are limited to 512 bytes of stack @@ -273,3 +279,22 @@ cc (congestion-control) implementations. If any of these kernel functions has changed, both the in-tree and out-of-tree kernel tcp cc implementations have to be changed. The same goes for the bpf programs and they have to be adjusted accordingly. + +Q: Attaching to arbitrary kernel functions is an ABI? +----------------------------------------------------- +Q: BPF programs can be attached to many kernel functions. Do these +kernel functions become part of the ABI? + +A: NO. + +The kernel function prototypes will change, and BPF programs attaching to +them will need to change. The BPF compile-once-run-everywhere (CO-RE) +should be used in order to make it easier to adapt your BPF programs to +different versions of the kernel. + +Q: Marking a function with BTF_ID makes that function an ABI? +------------------------------------------------------------- +A: NO. + +The BTF_ID macro does not cause a function to become part of the ABI +any more than does the EXPORT_SYMBOL_GPL macro. diff --git a/Documentation/bpf/btf.rst b/Documentation/bpf/btf.rst index 7940da9bc6c1..cf8722f96090 100644 --- a/Documentation/bpf/btf.rst +++ b/Documentation/bpf/btf.rst @@ -74,7 +74,7 @@ sequentially and type id is assigned to each recognized type starting from id #define BTF_KIND_ARRAY 3 /* Array */ #define BTF_KIND_STRUCT 4 /* Struct */ #define BTF_KIND_UNION 5 /* Union */ - #define BTF_KIND_ENUM 6 /* Enumeration */ + #define BTF_KIND_ENUM 6 /* Enumeration up to 32-bit values */ #define BTF_KIND_FWD 7 /* Forward */ #define BTF_KIND_TYPEDEF 8 /* Typedef */ #define BTF_KIND_VOLATILE 9 /* Volatile */ @@ -87,6 +87,7 @@ sequentially and type id is assigned to each recognized type starting from id #define BTF_KIND_FLOAT 16 /* Floating point */ #define BTF_KIND_DECL_TAG 17 /* Decl Tag */ #define BTF_KIND_TYPE_TAG 18 /* Type Tag */ + #define BTF_KIND_ENUM64 19 /* Enumeration up to 64-bit values */ Note that the type section encodes debug info, not just pure types. ``BTF_KIND_FUNC`` is not a type, and it represents a defined subprogram. @@ -101,10 +102,10 @@ Each type contains the following common data:: * bits 24-28: kind (e.g. int, ptr, array...etc) * bits 29-30: unused * bit 31: kind_flag, currently used by - * struct, union and fwd + * struct, union, fwd, enum and enum64. */ __u32 info; - /* "size" is used by INT, ENUM, STRUCT and UNION. + /* "size" is used by INT, ENUM, STRUCT, UNION and ENUM64. * "size" tells the size of the type it is describing. * * "type" is used by PTR, TYPEDEF, VOLATILE, CONST, RESTRICT, @@ -281,10 +282,10 @@ modes exist: ``struct btf_type`` encoding requirement: * ``name_off``: 0 or offset to a valid C identifier - * ``info.kind_flag``: 0 + * ``info.kind_flag``: 0 for unsigned, 1 for signed * ``info.kind``: BTF_KIND_ENUM * ``info.vlen``: number of enum values - * ``size``: 4 + * ``size``: 1/2/4/8 ``btf_type`` is followed by ``info.vlen`` number of ``struct btf_enum``.:: @@ -297,6 +298,10 @@ The ``btf_enum`` encoding: * ``name_off``: offset to a valid C identifier * ``val``: any value +If the original enum value is signed and the size is less than 4, +that value will be sign extended into 4 bytes. If the size is 8, +the value will be truncated into 4 bytes. + 2.2.7 BTF_KIND_FWD ~~~~~~~~~~~~~~~~~~ @@ -364,7 +369,8 @@ No additional type data follow ``btf_type``. * ``name_off``: offset to a valid C identifier * ``info.kind_flag``: 0 * ``info.kind``: BTF_KIND_FUNC - * ``info.vlen``: 0 + * ``info.vlen``: linkage information (BTF_FUNC_STATIC, BTF_FUNC_GLOBAL + or BTF_FUNC_EXTERN) * ``type``: a BTF_KIND_FUNC_PROTO type No additional type data follow ``btf_type``. @@ -375,6 +381,9 @@ type. The BTF_KIND_FUNC may in turn be referenced by a func_info in the :ref:`BTF_Ext_Section` (ELF) or in the arguments to :ref:`BPF_Prog_Load` (ABI). +Currently, only linkage values of BTF_FUNC_STATIC and BTF_FUNC_GLOBAL are +supported in the kernel. + 2.2.13 BTF_KIND_FUNC_PROTO ~~~~~~~~~~~~~~~~~~~~~~~~~~ @@ -493,7 +502,7 @@ the attribute is applied to a ``struct``/``union`` member or a ``func`` argument, and ``btf_decl_tag.component_idx`` should be a valid index (starting from 0) pointing to a member or an argument. -2.2.17 BTF_KIND_TYPE_TAG +2.2.18 BTF_KIND_TYPE_TAG ~~~~~~~~~~~~~~~~~~~~~~~~ ``struct btf_type`` encoding requirement: @@ -516,6 +525,32 @@ type_tag, then zero or more const/volatile/restrict/typedef and finally the base type. The base type is one of int, ptr, array, struct, union, enum, func_proto and float types. +2.2.19 BTF_KIND_ENUM64 +~~~~~~~~~~~~~~~~~~~~~~ + +``struct btf_type`` encoding requirement: + * ``name_off``: 0 or offset to a valid C identifier + * ``info.kind_flag``: 0 for unsigned, 1 for signed + * ``info.kind``: BTF_KIND_ENUM64 + * ``info.vlen``: number of enum values + * ``size``: 1/2/4/8 + +``btf_type`` is followed by ``info.vlen`` number of ``struct btf_enum64``.:: + + struct btf_enum64 { + __u32 name_off; + __u32 val_lo32; + __u32 val_hi32; + }; + +The ``btf_enum64`` encoding: + * ``name_off``: offset to a valid C identifier + * ``val_lo32``: lower 32-bit value for a 64-bit value + * ``val_hi32``: high 32-bit value for a 64-bit value + +If the original enum value is signed and the size is less than 8, +that value will be sign extended into 8 bytes. + 3. BTF Kernel API ================= diff --git a/Documentation/bpf/index.rst b/Documentation/bpf/index.rst index 96056a7447c7..1bc2c5c58bdb 100644 --- a/Documentation/bpf/index.rst +++ b/Documentation/bpf/index.rst @@ -19,6 +19,7 @@ that goes into great technical depth about the BPF Architecture. faq syscall_api helpers + kfuncs programs maps bpf_prog_run diff --git a/Documentation/bpf/instruction-set.rst b/Documentation/bpf/instruction-set.rst index 1de6a57c7e1e..1b0e6711dec9 100644 --- a/Documentation/bpf/instruction-set.rst +++ b/Documentation/bpf/instruction-set.rst @@ -127,7 +127,7 @@ BPF_XOR | BPF_K | BPF_ALU64 means:: Byte swap instructions ---------------------- -The byte swap instructions use an instruction class of ``BFP_ALU`` and a 4-bit +The byte swap instructions use an instruction class of ``BPF_ALU`` and a 4-bit code field of ``BPF_END``. The byte swap instructions operate on the destination register @@ -351,7 +351,7 @@ These instructions have seven implicit operands: * Register R0 is an implicit output which contains the data fetched from the packet. * Registers R1-R5 are scratch registers that are clobbered after a call to - ``BPF_ABS | BPF_LD`` or ``BPF_IND`` | BPF_LD instructions. + ``BPF_ABS | BPF_LD`` or ``BPF_IND | BPF_LD`` instructions. These instructions have an implicit program exit condition as well. When an eBPF program is trying to access the data beyond the packet boundary, the diff --git a/Documentation/bpf/kfuncs.rst b/Documentation/bpf/kfuncs.rst new file mode 100644 index 000000000000..c0b7dae6dbf5 --- /dev/null +++ b/Documentation/bpf/kfuncs.rst @@ -0,0 +1,170 @@ +============================= +BPF Kernel Functions (kfuncs) +============================= + +1. Introduction +=============== + +BPF Kernel Functions or more commonly known as kfuncs are functions in the Linux +kernel which are exposed for use by BPF programs. Unlike normal BPF helpers, +kfuncs do not have a stable interface and can change from one kernel release to +another. Hence, BPF programs need to be updated in response to changes in the +kernel. + +2. Defining a kfunc +=================== + +There are two ways to expose a kernel function to BPF programs, either make an +existing function in the kernel visible, or add a new wrapper for BPF. In both +cases, care must be taken that BPF program can only call such function in a +valid context. To enforce this, visibility of a kfunc can be per program type. + +If you are not creating a BPF wrapper for existing kernel function, skip ahead +to :ref:`BPF_kfunc_nodef`. + +2.1 Creating a wrapper kfunc +---------------------------- + +When defining a wrapper kfunc, the wrapper function should have extern linkage. +This prevents the compiler from optimizing away dead code, as this wrapper kfunc +is not invoked anywhere in the kernel itself. It is not necessary to provide a +prototype in a header for the wrapper kfunc. + +An example is given below:: + + /* Disables missing prototype warnings */ + __diag_push(); + __diag_ignore_all("-Wmissing-prototypes", + "Global kfuncs as their definitions will be in BTF"); + + struct task_struct *bpf_find_get_task_by_vpid(pid_t nr) + { + return find_get_task_by_vpid(nr); + } + + __diag_pop(); + +A wrapper kfunc is often needed when we need to annotate parameters of the +kfunc. Otherwise one may directly make the kfunc visible to the BPF program by +registering it with the BPF subsystem. See :ref:`BPF_kfunc_nodef`. + +2.2 Annotating kfunc parameters +------------------------------- + +Similar to BPF helpers, there is sometime need for additional context required +by the verifier to make the usage of kernel functions safer and more useful. +Hence, we can annotate a parameter by suffixing the name of the argument of the +kfunc with a __tag, where tag may be one of the supported annotations. + +2.2.1 __sz Annotation +--------------------- + +This annotation is used to indicate a memory and size pair in the argument list. +An example is given below:: + + void bpf_memzero(void *mem, int mem__sz) + { + ... + } + +Here, the verifier will treat first argument as a PTR_TO_MEM, and second +argument as its size. By default, without __sz annotation, the size of the type +of the pointer is used. Without __sz annotation, a kfunc cannot accept a void +pointer. + +.. _BPF_kfunc_nodef: + +2.3 Using an existing kernel function +------------------------------------- + +When an existing function in the kernel is fit for consumption by BPF programs, +it can be directly registered with the BPF subsystem. However, care must still +be taken to review the context in which it will be invoked by the BPF program +and whether it is safe to do so. + +2.4 Annotating kfuncs +--------------------- + +In addition to kfuncs' arguments, verifier may need more information about the +type of kfunc(s) being registered with the BPF subsystem. To do so, we define +flags on a set of kfuncs as follows:: + + BTF_SET8_START(bpf_task_set) + BTF_ID_FLAGS(func, bpf_get_task_pid, KF_ACQUIRE | KF_RET_NULL) + BTF_ID_FLAGS(func, bpf_put_pid, KF_RELEASE) + BTF_SET8_END(bpf_task_set) + +This set encodes the BTF ID of each kfunc listed above, and encodes the flags +along with it. Ofcourse, it is also allowed to specify no flags. + +2.4.1 KF_ACQUIRE flag +--------------------- + +The KF_ACQUIRE flag is used to indicate that the kfunc returns a pointer to a +refcounted object. The verifier will then ensure that the pointer to the object +is eventually released using a release kfunc, or transferred to a map using a +referenced kptr (by invoking bpf_kptr_xchg). If not, the verifier fails the +loading of the BPF program until no lingering references remain in all possible +explored states of the program. + +2.4.2 KF_RET_NULL flag +---------------------- + +The KF_RET_NULL flag is used to indicate that the pointer returned by the kfunc +may be NULL. Hence, it forces the user to do a NULL check on the pointer +returned from the kfunc before making use of it (dereferencing or passing to +another helper). This flag is often used in pairing with KF_ACQUIRE flag, but +both are orthogonal to each other. + +2.4.3 KF_RELEASE flag +--------------------- + +The KF_RELEASE flag is used to indicate that the kfunc releases the pointer +passed in to it. There can be only one referenced pointer that can be passed in. +All copies of the pointer being released are invalidated as a result of invoking +kfunc with this flag. + +2.4.4 KF_KPTR_GET flag +---------------------- + +The KF_KPTR_GET flag is used to indicate that the kfunc takes the first argument +as a pointer to kptr, safely increments the refcount of the object it points to, +and returns a reference to the user. The rest of the arguments may be normal +arguments of a kfunc. The KF_KPTR_GET flag should be used in conjunction with +KF_ACQUIRE and KF_RET_NULL flags. + +2.4.5 KF_TRUSTED_ARGS flag +-------------------------- + +The KF_TRUSTED_ARGS flag is used for kfuncs taking pointer arguments. It +indicates that the all pointer arguments will always be refcounted, and have +their offset set to 0. It can be used to enforce that a pointer to a refcounted +object acquired from a kfunc or BPF helper is passed as an argument to this +kfunc without any modifications (e.g. pointer arithmetic) such that it is +trusted and points to the original object. This flag is often used for kfuncs +that operate (change some property, perform some operation) on an object that +was obtained using an acquire kfunc. Such kfuncs need an unchanged pointer to +ensure the integrity of the operation being performed on the expected object. + +2.5 Registering the kfuncs +-------------------------- + +Once the kfunc is prepared for use, the final step to making it visible is +registering it with the BPF subsystem. Registration is done per BPF program +type. An example is shown below:: + + BTF_SET8_START(bpf_task_set) + BTF_ID_FLAGS(func, bpf_get_task_pid, KF_ACQUIRE | KF_RET_NULL) + BTF_ID_FLAGS(func, bpf_put_pid, KF_RELEASE) + BTF_SET8_END(bpf_task_set) + + static const struct btf_kfunc_id_set bpf_task_kfunc_set = { + .owner = THIS_MODULE, + .set = &bpf_task_set, + }; + + static int init_subsystem(void) + { + return register_btf_kfunc_id_set(BPF_PROG_TYPE_TRACING, &bpf_task_kfunc_set); + } + late_initcall(init_subsystem); diff --git a/Documentation/bpf/libbpf/libbpf_naming_convention.rst b/Documentation/bpf/libbpf/libbpf_naming_convention.rst index f86360f734a8..c5ac97f3d4c4 100644 --- a/Documentation/bpf/libbpf/libbpf_naming_convention.rst +++ b/Documentation/bpf/libbpf/libbpf_naming_convention.rst @@ -9,8 +9,8 @@ described here. It's recommended to follow these conventions whenever a new function or type is added to keep libbpf API clean and consistent. All types and functions provided by libbpf API should have one of the -following prefixes: ``bpf_``, ``btf_``, ``libbpf_``, ``xsk_``, -``btf_dump_``, ``ring_buffer_``, ``perf_buffer_``. +following prefixes: ``bpf_``, ``btf_``, ``libbpf_``, ``btf_dump_``, +``ring_buffer_``, ``perf_buffer_``. System call wrappers -------------------- @@ -59,15 +59,6 @@ Auxiliary functions and types that don't fit well in any of categories described above should have ``libbpf_`` prefix, e.g. ``libbpf_get_error`` or ``libbpf_prog_type_by_name``. -AF_XDP functions -------------------- - -AF_XDP functions should have an ``xsk_`` prefix, e.g. -``xsk_umem__get_data`` or ``xsk_umem__create``. The interface consists -of both low-level ring access functions and high-level configuration -functions. These can be mixed and matched. Note that these functions -are not reentrant for performance reasons. - ABI --- diff --git a/Documentation/bpf/map_hash.rst b/Documentation/bpf/map_hash.rst new file mode 100644 index 000000000000..e85120878b27 --- /dev/null +++ b/Documentation/bpf/map_hash.rst @@ -0,0 +1,185 @@ +.. SPDX-License-Identifier: GPL-2.0-only +.. Copyright (C) 2022 Red Hat, Inc. + +=============================================== +BPF_MAP_TYPE_HASH, with PERCPU and LRU Variants +=============================================== + +.. note:: + - ``BPF_MAP_TYPE_HASH`` was introduced in kernel version 3.19 + - ``BPF_MAP_TYPE_PERCPU_HASH`` was introduced in version 4.6 + - Both ``BPF_MAP_TYPE_LRU_HASH`` and ``BPF_MAP_TYPE_LRU_PERCPU_HASH`` + were introduced in version 4.10 + +``BPF_MAP_TYPE_HASH`` and ``BPF_MAP_TYPE_PERCPU_HASH`` provide general +purpose hash map storage. Both the key and the value can be structs, +allowing for composite keys and values. + +The kernel is responsible for allocating and freeing key/value pairs, up +to the max_entries limit that you specify. Hash maps use pre-allocation +of hash table elements by default. The ``BPF_F_NO_PREALLOC`` flag can be +used to disable pre-allocation when it is too memory expensive. + +``BPF_MAP_TYPE_PERCPU_HASH`` provides a separate value slot per +CPU. The per-cpu values are stored internally in an array. + +The ``BPF_MAP_TYPE_LRU_HASH`` and ``BPF_MAP_TYPE_LRU_PERCPU_HASH`` +variants add LRU semantics to their respective hash tables. An LRU hash +will automatically evict the least recently used entries when the hash +table reaches capacity. An LRU hash maintains an internal LRU list that +is used to select elements for eviction. This internal LRU list is +shared across CPUs but it is possible to request a per CPU LRU list with +the ``BPF_F_NO_COMMON_LRU`` flag when calling ``bpf_map_create``. + +Usage +===== + +.. c:function:: + long bpf_map_update_elem(struct bpf_map *map, const void *key, const void *value, u64 flags) + +Hash entries can be added or updated using the ``bpf_map_update_elem()`` +helper. This helper replaces existing elements atomically. The ``flags`` +parameter can be used to control the update behaviour: + +- ``BPF_ANY`` will create a new element or update an existing element +- ``BPF_NOEXIST`` will create a new element only if one did not already + exist +- ``BPF_EXIST`` will update an existing element + +``bpf_map_update_elem()`` returns 0 on success, or negative error in +case of failure. + +.. c:function:: + void *bpf_map_lookup_elem(struct bpf_map *map, const void *key) + +Hash entries can be retrieved using the ``bpf_map_lookup_elem()`` +helper. This helper returns a pointer to the value associated with +``key``, or ``NULL`` if no entry was found. + +.. c:function:: + long bpf_map_delete_elem(struct bpf_map *map, const void *key) + +Hash entries can be deleted using the ``bpf_map_delete_elem()`` +helper. This helper will return 0 on success, or negative error in case +of failure. + +Per CPU Hashes +-------------- + +For ``BPF_MAP_TYPE_PERCPU_HASH`` and ``BPF_MAP_TYPE_LRU_PERCPU_HASH`` +the ``bpf_map_update_elem()`` and ``bpf_map_lookup_elem()`` helpers +automatically access the hash slot for the current CPU. + +.. c:function:: + void *bpf_map_lookup_percpu_elem(struct bpf_map *map, const void *key, u32 cpu) + +The ``bpf_map_lookup_percpu_elem()`` helper can be used to lookup the +value in the hash slot for a specific CPU. Returns value associated with +``key`` on ``cpu`` , or ``NULL`` if no entry was found or ``cpu`` is +invalid. + +Concurrency +----------- + +Values stored in ``BPF_MAP_TYPE_HASH`` can be accessed concurrently by +programs running on different CPUs. Since Kernel version 5.1, the BPF +infrastructure provides ``struct bpf_spin_lock`` to synchronise access. +See ``tools/testing/selftests/bpf/progs/test_spin_lock.c``. + +Userspace +--------- + +.. c:function:: + int bpf_map_get_next_key(int fd, const void *cur_key, void *next_key) + +In userspace, it is possible to iterate through the keys of a hash using +libbpf's ``bpf_map_get_next_key()`` function. The first key can be fetched by +calling ``bpf_map_get_next_key()`` with ``cur_key`` set to +``NULL``. Subsequent calls will fetch the next key that follows the +current key. ``bpf_map_get_next_key()`` returns 0 on success, -ENOENT if +cur_key is the last key in the hash, or negative error in case of +failure. + +Note that if ``cur_key`` gets deleted then ``bpf_map_get_next_key()`` +will instead return the *first* key in the hash table which is +undesirable. It is recommended to use batched lookup if there is going +to be key deletion intermixed with ``bpf_map_get_next_key()``. + +Examples +======== + +Please see the ``tools/testing/selftests/bpf`` directory for functional +examples. The code snippets below demonstrates API usage. + +This example shows how to declare an LRU Hash with a struct key and a +struct value. + +.. code-block:: c + + #include <linux/bpf.h> + #include <bpf/bpf_helpers.h> + + struct key { + __u32 srcip; + }; + + struct value { + __u64 packets; + __u64 bytes; + }; + + struct { + __uint(type, BPF_MAP_TYPE_LRU_HASH); + __uint(max_entries, 32); + __type(key, struct key); + __type(value, struct value); + } packet_stats SEC(".maps"); + +This example shows how to create or update hash values using atomic +instructions: + +.. code-block:: c + + static void update_stats(__u32 srcip, int bytes) + { + struct key key = { + .srcip = srcip, + }; + struct value *value = bpf_map_lookup_elem(&packet_stats, &key); + + if (value) { + __sync_fetch_and_add(&value->packets, 1); + __sync_fetch_and_add(&value->bytes, bytes); + } else { + struct value newval = { 1, bytes }; + + bpf_map_update_elem(&packet_stats, &key, &newval, BPF_NOEXIST); + } + } + +Userspace walking the map elements from the map declared above: + +.. code-block:: c + + #include <bpf/libbpf.h> + #include <bpf/bpf.h> + + static void walk_hash_elements(int map_fd) + { + struct key *cur_key = NULL; + struct key next_key; + struct value value; + int err; + + for (;;) { + err = bpf_map_get_next_key(map_fd, cur_key, &next_key); + if (err) + break; + + bpf_map_lookup_elem(map_fd, &next_key, &value); + + // Use key and value here + + cur_key = &next_key; + } + } |