Interacting With Maps

Interacting with eBPF maps happens through lookup/update/delete primitives.

Userspace

The userspace API map helpers for eBPF are defined in tools/lib/bpf/bpf.h and include the following functions:

/* Userspace helpers */
int bpf_map_lookup_elem(int fd, void *key, void *value);
int bpf_map_update_elem(int fd, void *key, void *value, __u64 flags);
int bpf_map_delete_elem(int fd, void *key);
/* Only userspace: */
int bpf_map_get_next_key(int fd, void *key, void *next_key);

To interact with an eBPF map from userspace, you use the bpf syscall and a file descriptor (fd). The fd serves as the map handle.On success, these functions return zero, while on failure they return -1 and set errno.

• The wrappers for the bpf syscall are implemented in tools/lib/bpf/bpf.c and call functions in kernel/bpf/syscall.c, such as map_lookup_elem.

   

• It’s worth noting that void *key and void *value are passed as void pointers. This is because of the memory separation between kernel and userspace, and it involves making a copy of the value. Kernel primitives like copy_from_user() and copy_to_user() are used for this purpose, as seen in map_lookup_elem, which also allocates and deallocates memory using kmalloc+kfree for a short period.

• From userspace, there is no direct function call to increment or decrement the value in-place. Instead, the bpf_map_update_elem() call will overwrite the existing value with a copy of the value supplied. The overwrite operation depends on the map type and may happen atomically using locking mechanisms specific to the map type.

Kernel-side eBPF program

The eBPF program helpers for kernel-side interaction with maps are defined in the samples/bpf/bpf_helpers.h header file and are implemented in the kernel/bpf/helpers.c file via macros.

/* eBPF program helpers */
void *bpf_map_lookup_elem(void *map, void *key);
int bpf_map_update_elem(void *map, void *key, void *value, unsigned long long flags);
int bpf_map_delete_elem(void *map, void *key);

The bpf_map_lookup_elem() function is a kernel-side helper function that allows eBPF programs to directly access the value stored in a map by providing a pointer to the map and a pointer to the key.

• Unlike the userspace API, which provides a copy of the value, the kernel-side API provides a direct pointer to the memory element inside the kernel where the value is stored.
• This allows eBPF programs to perform atomic operations, such as incrementing or decrementing the value “in-place”, using appropriate compiler primitives like __sync_fetch_and_add(), which are understood by LLVM (Low-Level Virtual Machine) when generating eBPF instructions. -This direct access to the value memory element in the kernel provides more efficient and optimized access to map data structures for eBPF programs running in the kernel. So, the bpf_map_lookup_elem() function in the kernel-side eBPF API enables efficient and direct access to map values from eBPF programs running in the kernel.