447 lines
15 KiB
ReStructuredText
447 lines
15 KiB
ReStructuredText
.. SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
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================
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bpftool-gen
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================
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-------------------------------------------------------------------------------
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tool for BPF code-generation
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-------------------------------------------------------------------------------
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:Manual section: 8
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.. include:: substitutions.rst
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SYNOPSIS
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========
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**bpftool** [*OPTIONS*] **gen** *COMMAND*
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*OPTIONS* := { |COMMON_OPTIONS| | { **-L** | **--use-loader** } }
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*COMMAND* := { **object** | **skeleton** | **help** }
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GEN COMMANDS
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=============
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| **bpftool** **gen object** *OUTPUT_FILE* *INPUT_FILE* [*INPUT_FILE*...]
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| **bpftool** **gen skeleton** *FILE* [**name** *OBJECT_NAME*]
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| **bpftool** **gen subskeleton** *FILE* [**name** *OBJECT_NAME*]
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| **bpftool** **gen min_core_btf** *INPUT* *OUTPUT* *OBJECT* [*OBJECT*...]
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| **bpftool** **gen help**
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DESCRIPTION
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===========
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**bpftool gen object** *OUTPUT_FILE* *INPUT_FILE* [*INPUT_FILE*...]
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Statically link (combine) together one or more *INPUT_FILE*'s
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into a single resulting *OUTPUT_FILE*. All the files involved
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are BPF ELF object files.
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The rules of BPF static linking are mostly the same as for
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user-space object files, but in addition to combining data
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and instruction sections, .BTF and .BTF.ext (if present in
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any of the input files) data are combined together. .BTF
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data is deduplicated, so all the common types across
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*INPUT_FILE*'s will only be represented once in the resulting
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BTF information.
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BPF static linking allows to partition BPF source code into
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individually compiled files that are then linked into
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a single resulting BPF object file, which can be used to
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generated BPF skeleton (with **gen skeleton** command) or
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passed directly into **libbpf** (using **bpf_object__open()**
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family of APIs).
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**bpftool gen skeleton** *FILE*
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Generate BPF skeleton C header file for a given *FILE*.
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BPF skeleton is an alternative interface to existing libbpf
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APIs for working with BPF objects. Skeleton code is intended
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to significantly shorten and simplify code to load and work
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with BPF programs from userspace side. Generated code is
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tailored to specific input BPF object *FILE*, reflecting its
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structure by listing out available maps, program, variables,
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etc. Skeleton eliminates the need to lookup mentioned
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components by name. Instead, if skeleton instantiation
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succeeds, they are populated in skeleton structure as valid
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libbpf types (e.g., **struct bpf_map** pointer) and can be
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passed to existing generic libbpf APIs.
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In addition to simple and reliable access to maps and
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programs, skeleton provides a storage for BPF links (**struct
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bpf_link**) for each BPF program within BPF object. When
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requested, supported BPF programs will be automatically
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attached and resulting BPF links stored for further use by
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user in pre-allocated fields in skeleton struct. For BPF
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programs that can't be automatically attached by libbpf,
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user can attach them manually, but store resulting BPF link
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in per-program link field. All such set up links will be
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automatically destroyed on BPF skeleton destruction. This
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eliminates the need for users to manage links manually and
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rely on libbpf support to detach programs and free up
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resources.
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Another facility provided by BPF skeleton is an interface to
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global variables of all supported kinds: mutable, read-only,
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as well as extern ones. This interface allows to pre-setup
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initial values of variables before BPF object is loaded and
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verified by kernel. For non-read-only variables, the same
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interface can be used to fetch values of global variables on
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userspace side, even if they are modified by BPF code.
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During skeleton generation, contents of source BPF object
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*FILE* is embedded within generated code and is thus not
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necessary to keep around. This ensures skeleton and BPF
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object file are matching 1-to-1 and always stay in sync.
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Generated code is dual-licensed under LGPL-2.1 and
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BSD-2-Clause licenses.
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It is a design goal and guarantee that skeleton interfaces
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are interoperable with generic libbpf APIs. User should
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always be able to use skeleton API to create and load BPF
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object, and later use libbpf APIs to keep working with
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specific maps, programs, etc.
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As part of skeleton, few custom functions are generated.
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Each of them is prefixed with object name. Object name can
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either be derived from object file name, i.e., if BPF object
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file name is **example.o**, BPF object name will be
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**example**. Object name can be also specified explicitly
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through **name** *OBJECT_NAME* parameter. The following
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custom functions are provided (assuming **example** as
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the object name):
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- **example__open** and **example__open_opts**.
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These functions are used to instantiate skeleton. It
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corresponds to libbpf's **bpf_object__open**\ () API.
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**_opts** variants accepts extra **bpf_object_open_opts**
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options.
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- **example__load**.
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This function creates maps, loads and verifies BPF
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programs, initializes global data maps. It corresponds to
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libppf's **bpf_object__load**\ () API.
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- **example__open_and_load** combines **example__open** and
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**example__load** invocations in one commonly used
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operation.
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- **example__attach** and **example__detach**
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This pair of functions allow to attach and detach,
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correspondingly, already loaded BPF object. Only BPF
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programs of types supported by libbpf for auto-attachment
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will be auto-attached and their corresponding BPF links
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instantiated. For other BPF programs, user can manually
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create a BPF link and assign it to corresponding fields in
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skeleton struct. **example__detach** will detach both
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links created automatically, as well as those populated by
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user manually.
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- **example__destroy**
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Detach and unload BPF programs, free up all the resources
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used by skeleton and BPF object.
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If BPF object has global variables, corresponding structs
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with memory layout corresponding to global data data section
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layout will be created. Currently supported ones are: *.data*,
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*.bss*, *.rodata*, and *.kconfig* structs/data sections.
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These data sections/structs can be used to set up initial
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values of variables, if set before **example__load**.
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Afterwards, if target kernel supports memory-mapped BPF
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arrays, same structs can be used to fetch and update
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(non-read-only) data from userspace, with same simplicity
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as for BPF side.
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**bpftool gen subskeleton** *FILE*
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Generate BPF subskeleton C header file for a given *FILE*.
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Subskeletons are similar to skeletons, except they do not own
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the corresponding maps, programs, or global variables. They
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require that the object file used to generate them is already
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loaded into a *bpf_object* by some other means.
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This functionality is useful when a library is included into a
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larger BPF program. A subskeleton for the library would have
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access to all objects and globals defined in it, without
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having to know about the larger program.
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Consequently, there are only two functions defined
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for subskeletons:
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- **example__open(bpf_object\*)**
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Instantiates a subskeleton from an already opened (but not
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necessarily loaded) **bpf_object**.
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- **example__destroy()**
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Frees the storage for the subskeleton but *does not* unload
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any BPF programs or maps.
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**bpftool** **gen min_core_btf** *INPUT* *OUTPUT* *OBJECT* [*OBJECT*...]
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Generate a minimum BTF file as *OUTPUT*, derived from a given
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*INPUT* BTF file, containing all needed BTF types so one, or
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more, given eBPF objects CO-RE relocations may be satisfied.
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When kernels aren't compiled with CONFIG_DEBUG_INFO_BTF,
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libbpf, when loading an eBPF object, has to rely on external
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BTF files to be able to calculate CO-RE relocations.
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Usually, an external BTF file is built from existing kernel
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DWARF data using pahole. It contains all the types used by
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its respective kernel image and, because of that, is big.
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The min_core_btf feature builds smaller BTF files, customized
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to one or multiple eBPF objects, so they can be distributed
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together with an eBPF CO-RE based application, turning the
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application portable to different kernel versions.
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Check examples bellow for more information how to use it.
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**bpftool gen help**
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Print short help message.
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OPTIONS
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=======
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.. include:: common_options.rst
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-L, --use-loader
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For skeletons, generate a "light" skeleton (also known as "loader"
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skeleton). A light skeleton contains a loader eBPF program. It does
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not use the majority of the libbpf infrastructure, and does not need
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libelf.
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EXAMPLES
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========
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**$ cat example1.bpf.c**
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::
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#include <stdbool.h>
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#include <linux/ptrace.h>
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#include <linux/bpf.h>
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#include <bpf/bpf_helpers.h>
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const volatile int param1 = 42;
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bool global_flag = true;
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struct { int x; } data = {};
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SEC("raw_tp/sys_enter")
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int handle_sys_enter(struct pt_regs *ctx)
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{
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static long my_static_var;
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if (global_flag)
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my_static_var++;
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else
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data.x += param1;
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return 0;
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}
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**$ cat example2.bpf.c**
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::
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#include <linux/ptrace.h>
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#include <linux/bpf.h>
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#include <bpf/bpf_helpers.h>
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struct {
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__uint(type, BPF_MAP_TYPE_HASH);
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__uint(max_entries, 128);
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__type(key, int);
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__type(value, long);
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} my_map SEC(".maps");
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SEC("raw_tp/sys_exit")
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int handle_sys_exit(struct pt_regs *ctx)
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{
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int zero = 0;
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bpf_map_lookup_elem(&my_map, &zero);
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return 0;
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}
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This is example BPF application with two BPF programs and a mix of BPF maps
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and global variables. Source code is split across two source code files.
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**$ clang -target bpf -g example1.bpf.c -o example1.bpf.o**
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**$ clang -target bpf -g example2.bpf.c -o example2.bpf.o**
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**$ bpftool gen object example.bpf.o example1.bpf.o example2.bpf.o**
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This set of commands compiles *example1.bpf.c* and *example2.bpf.c*
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individually and then statically links respective object files into the final
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BPF ELF object file *example.bpf.o*.
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**$ bpftool gen skeleton example.bpf.o name example | tee example.skel.h**
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::
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/* SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause) */
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/* THIS FILE IS AUTOGENERATED! */
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#ifndef __EXAMPLE_SKEL_H__
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#define __EXAMPLE_SKEL_H__
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#include <stdlib.h>
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#include <bpf/libbpf.h>
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struct example {
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struct bpf_object_skeleton *skeleton;
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struct bpf_object *obj;
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struct {
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struct bpf_map *rodata;
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struct bpf_map *data;
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struct bpf_map *bss;
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struct bpf_map *my_map;
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} maps;
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struct {
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struct bpf_program *handle_sys_enter;
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struct bpf_program *handle_sys_exit;
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} progs;
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struct {
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struct bpf_link *handle_sys_enter;
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struct bpf_link *handle_sys_exit;
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} links;
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struct example__bss {
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struct {
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int x;
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} data;
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} *bss;
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struct example__data {
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_Bool global_flag;
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long int handle_sys_enter_my_static_var;
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} *data;
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struct example__rodata {
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int param1;
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} *rodata;
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};
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static void example__destroy(struct example *obj);
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static inline struct example *example__open_opts(
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const struct bpf_object_open_opts *opts);
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static inline struct example *example__open();
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static inline int example__load(struct example *obj);
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static inline struct example *example__open_and_load();
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static inline int example__attach(struct example *obj);
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static inline void example__detach(struct example *obj);
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#endif /* __EXAMPLE_SKEL_H__ */
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**$ cat example.c**
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::
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#include "example.skel.h"
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int main()
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{
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struct example *skel;
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int err = 0;
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skel = example__open();
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if (!skel)
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goto cleanup;
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skel->rodata->param1 = 128;
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err = example__load(skel);
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if (err)
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goto cleanup;
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err = example__attach(skel);
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if (err)
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goto cleanup;
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/* all libbpf APIs are usable */
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printf("my_map name: %s\n", bpf_map__name(skel->maps.my_map));
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printf("sys_enter prog FD: %d\n",
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bpf_program__fd(skel->progs.handle_sys_enter));
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/* detach and re-attach sys_exit program */
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bpf_link__destroy(skel->links.handle_sys_exit);
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skel->links.handle_sys_exit =
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bpf_program__attach(skel->progs.handle_sys_exit);
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printf("my_static_var: %ld\n",
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skel->bss->handle_sys_enter_my_static_var);
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cleanup:
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example__destroy(skel);
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return err;
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}
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**# ./example**
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::
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my_map name: my_map
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sys_enter prog FD: 8
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my_static_var: 7
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This is a stripped-out version of skeleton generated for above example code.
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min_core_btf
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------------
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**$ bpftool btf dump file 5.4.0-example.btf format raw**
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::
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[1] INT 'long unsigned int' size=8 bits_offset=0 nr_bits=64 encoding=(none)
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[2] CONST '(anon)' type_id=1
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[3] VOLATILE '(anon)' type_id=1
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[4] ARRAY '(anon)' type_id=1 index_type_id=21 nr_elems=2
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[5] PTR '(anon)' type_id=8
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[6] CONST '(anon)' type_id=5
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[7] INT 'char' size=1 bits_offset=0 nr_bits=8 encoding=(none)
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[8] CONST '(anon)' type_id=7
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[9] INT 'unsigned int' size=4 bits_offset=0 nr_bits=32 encoding=(none)
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<long output>
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**$ bpftool btf dump file one.bpf.o format raw**
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::
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[1] PTR '(anon)' type_id=2
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[2] STRUCT 'trace_event_raw_sys_enter' size=64 vlen=4
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'ent' type_id=3 bits_offset=0
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'id' type_id=7 bits_offset=64
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'args' type_id=9 bits_offset=128
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'__data' type_id=12 bits_offset=512
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[3] STRUCT 'trace_entry' size=8 vlen=4
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'type' type_id=4 bits_offset=0
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'flags' type_id=5 bits_offset=16
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'preempt_count' type_id=5 bits_offset=24
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<long output>
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**$ bpftool gen min_core_btf 5.4.0-example.btf 5.4.0-smaller.btf one.bpf.o**
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**$ bpftool btf dump file 5.4.0-smaller.btf format raw**
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::
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[1] TYPEDEF 'pid_t' type_id=6
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[2] STRUCT 'trace_event_raw_sys_enter' size=64 vlen=1
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'args' type_id=4 bits_offset=128
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[3] STRUCT 'task_struct' size=9216 vlen=2
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'pid' type_id=1 bits_offset=17920
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'real_parent' type_id=7 bits_offset=18048
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[4] ARRAY '(anon)' type_id=5 index_type_id=8 nr_elems=6
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[5] INT 'long unsigned int' size=8 bits_offset=0 nr_bits=64 encoding=(none)
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[6] TYPEDEF '__kernel_pid_t' type_id=8
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[7] PTR '(anon)' type_id=3
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[8] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
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<end>
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Now, the "5.4.0-smaller.btf" file may be used by libbpf as an external BTF file
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when loading the "one.bpf.o" object into the "5.4.0-example" kernel. Note that
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the generated BTF file won't allow other eBPF objects to be loaded, just the
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ones given to min_core_btf.
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::
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LIBBPF_OPTS(bpf_object_open_opts, opts, .btf_custom_path = "5.4.0-smaller.btf");
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struct bpf_object *obj;
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obj = bpf_object__open_file("one.bpf.o", &opts);
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...
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