1============================= 2BPF Kernel Functions (kfuncs) 3============================= 4 51. Introduction 6=============== 7 8BPF Kernel Functions or more commonly known as kfuncs are functions in the Linux 9kernel which are exposed for use by BPF programs. Unlike normal BPF helpers, 10kfuncs do not have a stable interface and can change from one kernel release to 11another. Hence, BPF programs need to be updated in response to changes in the 12kernel. 13 142. Defining a kfunc 15=================== 16 17There are two ways to expose a kernel function to BPF programs, either make an 18existing function in the kernel visible, or add a new wrapper for BPF. In both 19cases, care must be taken that BPF program can only call such function in a 20valid context. To enforce this, visibility of a kfunc can be per program type. 21 22If you are not creating a BPF wrapper for existing kernel function, skip ahead 23to :ref:`BPF_kfunc_nodef`. 24 252.1 Creating a wrapper kfunc 26---------------------------- 27 28When defining a wrapper kfunc, the wrapper function should have extern linkage. 29This prevents the compiler from optimizing away dead code, as this wrapper kfunc 30is not invoked anywhere in the kernel itself. It is not necessary to provide a 31prototype in a header for the wrapper kfunc. 32 33An example is given below:: 34 35 /* Disables missing prototype warnings */ 36 __diag_push(); 37 __diag_ignore_all("-Wmissing-prototypes", 38 "Global kfuncs as their definitions will be in BTF"); 39 40 struct task_struct *bpf_find_get_task_by_vpid(pid_t nr) 41 { 42 return find_get_task_by_vpid(nr); 43 } 44 45 __diag_pop(); 46 47A wrapper kfunc is often needed when we need to annotate parameters of the 48kfunc. Otherwise one may directly make the kfunc visible to the BPF program by 49registering it with the BPF subsystem. See :ref:`BPF_kfunc_nodef`. 50 512.2 Annotating kfunc parameters 52------------------------------- 53 54Similar to BPF helpers, there is sometime need for additional context required 55by the verifier to make the usage of kernel functions safer and more useful. 56Hence, we can annotate a parameter by suffixing the name of the argument of the 57kfunc with a __tag, where tag may be one of the supported annotations. 58 592.2.1 __sz Annotation 60--------------------- 61 62This annotation is used to indicate a memory and size pair in the argument list. 63An example is given below:: 64 65 void bpf_memzero(void *mem, int mem__sz) 66 { 67 ... 68 } 69 70Here, the verifier will treat first argument as a PTR_TO_MEM, and second 71argument as its size. By default, without __sz annotation, the size of the type 72of the pointer is used. Without __sz annotation, a kfunc cannot accept a void 73pointer. 74 75.. _BPF_kfunc_nodef: 76 772.3 Using an existing kernel function 78------------------------------------- 79 80When an existing function in the kernel is fit for consumption by BPF programs, 81it can be directly registered with the BPF subsystem. However, care must still 82be taken to review the context in which it will be invoked by the BPF program 83and whether it is safe to do so. 84 852.4 Annotating kfuncs 86--------------------- 87 88In addition to kfuncs' arguments, verifier may need more information about the 89type of kfunc(s) being registered with the BPF subsystem. To do so, we define 90flags on a set of kfuncs as follows:: 91 92 BTF_SET8_START(bpf_task_set) 93 BTF_ID_FLAGS(func, bpf_get_task_pid, KF_ACQUIRE | KF_RET_NULL) 94 BTF_ID_FLAGS(func, bpf_put_pid, KF_RELEASE) 95 BTF_SET8_END(bpf_task_set) 96 97This set encodes the BTF ID of each kfunc listed above, and encodes the flags 98along with it. Ofcourse, it is also allowed to specify no flags. 99 1002.4.1 KF_ACQUIRE flag 101--------------------- 102 103The KF_ACQUIRE flag is used to indicate that the kfunc returns a pointer to a 104refcounted object. The verifier will then ensure that the pointer to the object 105is eventually released using a release kfunc, or transferred to a map using a 106referenced kptr (by invoking bpf_kptr_xchg). If not, the verifier fails the 107loading of the BPF program until no lingering references remain in all possible 108explored states of the program. 109 1102.4.2 KF_RET_NULL flag 111---------------------- 112 113The KF_RET_NULL flag is used to indicate that the pointer returned by the kfunc 114may be NULL. Hence, it forces the user to do a NULL check on the pointer 115returned from the kfunc before making use of it (dereferencing or passing to 116another helper). This flag is often used in pairing with KF_ACQUIRE flag, but 117both are orthogonal to each other. 118 1192.4.3 KF_RELEASE flag 120--------------------- 121 122The KF_RELEASE flag is used to indicate that the kfunc releases the pointer 123passed in to it. There can be only one referenced pointer that can be passed in. 124All copies of the pointer being released are invalidated as a result of invoking 125kfunc with this flag. 126 1272.4.4 KF_KPTR_GET flag 128---------------------- 129 130The KF_KPTR_GET flag is used to indicate that the kfunc takes the first argument 131as a pointer to kptr, safely increments the refcount of the object it points to, 132and returns a reference to the user. The rest of the arguments may be normal 133arguments of a kfunc. The KF_KPTR_GET flag should be used in conjunction with 134KF_ACQUIRE and KF_RET_NULL flags. 135 1362.4.5 KF_TRUSTED_ARGS flag 137-------------------------- 138 139The KF_TRUSTED_ARGS flag is used for kfuncs taking pointer arguments. It 140indicates that the all pointer arguments will always be refcounted, and have 141their offset set to 0. It can be used to enforce that a pointer to a refcounted 142object acquired from a kfunc or BPF helper is passed as an argument to this 143kfunc without any modifications (e.g. pointer arithmetic) such that it is 144trusted and points to the original object. This flag is often used for kfuncs 145that operate (change some property, perform some operation) on an object that 146was obtained using an acquire kfunc. Such kfuncs need an unchanged pointer to 147ensure the integrity of the operation being performed on the expected object. 148 1492.5 Registering the kfuncs 150-------------------------- 151 152Once the kfunc is prepared for use, the final step to making it visible is 153registering it with the BPF subsystem. Registration is done per BPF program 154type. An example is shown below:: 155 156 BTF_SET8_START(bpf_task_set) 157 BTF_ID_FLAGS(func, bpf_get_task_pid, KF_ACQUIRE | KF_RET_NULL) 158 BTF_ID_FLAGS(func, bpf_put_pid, KF_RELEASE) 159 BTF_SET8_END(bpf_task_set) 160 161 static const struct btf_kfunc_id_set bpf_task_kfunc_set = { 162 .owner = THIS_MODULE, 163 .set = &bpf_task_set, 164 }; 165 166 static int init_subsystem(void) 167 { 168 return register_btf_kfunc_id_set(BPF_PROG_TYPE_TRACING, &bpf_task_kfunc_set); 169 } 170 late_initcall(init_subsystem); 171