libxdp(3)

NAME
libxdp - library for attaching XDP programs and using AF_XDP sockets

SYNOPSIS

This directory contains the files for the libxdp library for attaching XDP programs to network interfaces and using AF_XDP sockets. The library is fairly lightweight and relies on libbpf to do the heavy lifting for processing eBPF object files etc.

Libxdp provides two primary features on top of libbpf. The first is the ability to load multiple XDP programs in sequence on a single network device (which is not natively supported by the kernel). This support relies on the freplace functionality in the kernel, which makes it possible to attach an eBPF program as a replacement for a global function in another (already loaded) eBPF program. The second main feature is helper functions for configuring AF_XDP sockets as well as reading and writing packets from these sockets.

Some of the functionality provided by libxdp depends on particular kernel features; see the "Kernel feature compatibility" section below for details.

  Using libxdp from an application

Basic usage of libxdp from an application is quite straight forward. The following example loads, then unloads, an XDP program from the 'lo' interface:
    #define IFINDEX 1
    struct xdp_program *prog;
    int err;
    prog = xdp_program__open_file("my-program.o", "section_name", NULL);
    err = xdp_program__attach(prog, IFINDEX, XDP_MODE_NATIVE, 0);
    if (!err)
        xdp_program__detach(prog, IFINDEX, XDP_MODE_NATIVE, 0);
    xdp_program__close(prog);

The xdp_program structure is an opaque structure that represents a single XDP program. libxdp contains functions to create such a struct either from a BPF object file on disk, from a libbpf BPF object, or from an identifier of a program that is already loaded into the kernel:
    struct xdp_program *xdp_program__from_bpf_obj(struct bpf_object *obj,
    					      const char *section_name);
    struct xdp_program *xdp_program__find_file(const char *filename,
    					   const char *section_name,
    					   struct bpf_object_open_opts *opts);
    struct xdp_program *xdp_program__open_file(const char *filename,
    					   const char *section_name,
    					   struct bpf_object_open_opts *opts);
    struct xdp_program *xdp_program__from_fd(int fd);
    struct xdp_program *xdp_program__from_id(__u32 prog_id);
    struct xdp_program *xdp_program__from_pin(const char *pin_path);

The functions that open a BPF object or file need the function name of the XDP program as well as the file name or object, since an ELF file can contain multiple XDP programs. The xdp_program__find_file() function takes a filename without a path, and will look for the object in LIBXDP_OBJECT_PATH which defaults to /usr/lib/bpf (or /usr/lib64/bpf on systems using a split library path). This is convenient for applications shipping pre-compiled eBPF object files.

The xdp_program__attach() function will attach the program to an interface, building a dispatcher program to execute it. Multiple programs can be attached at once with xdp_program__attach_multi(); they will be sorted in order of their run priority, and execution from one program to the next will proceed based on the chain call actions defined for each program (see the Program metadata section below). Because the loading process involves modifying the attach type of the program, the attach functions only work with struct xdp_program objects that have not yet been loaded into the kernel.

When using the attach functions to attach to an interface that already has an XDP program loaded, libxdp will attempt to add the program to the list of loaded programs. However, this may fail, either due to missing kernel support, or because the already-attached program was not loaded using a dispatcher compatible with libxdp. If the kernel support for incremental attach (merged in kernel 5.10) is missing, the only way to actually run multiple programs on a single interface is to attach them all at the same time with xdp_program__attach_multi(). If the existing program is not an XDP dispatcher, that program will have to be detached from the interface before libxdp can attach a new one. This can be done by calling xdp_program__detach() with a reference to the loaded program; but note that this will of course break any application relying on that other XDP program to be present.

Program metadata

To support multiple XDP programs on the same interface, libxdp uses two pieces of metadata for each XDP program: Run priority and chain call actions.

  Run priority

This is the priority of the program and is a simple integer used to sort programs when loading multiple programs onto the same interface. Programs that wish to run early (such as a packet filter) should set low values for this, while programs that want to run later (such as a packet forwarder or counter) should set higher values. Note that later programs are only run if the previous programs end with a return code that is part of its chain call actions (see below). If not specified, the default priority value is 50.

  Chain call actions

These are the program return codes that the program indicate for packets that should continue processing. If the program returns one of these actions, later programs in the call chain will be run, whereas if it returns any other action, processing will be interrupted, and the XDP dispatcher will return the verdict immediately. If not set, this defaults to just XDP_PASS, which is likely the value most programs should use.

  Specifying metadata

The metadata outlined above is specified as BTF information embedded in the ELF file containing the XDP program. The xdp_helpers.h file shipped with libxdp contains helper macros to include this information, which can be used as follows:
    #include <bpf/bpf_helpers.h>
    #include <xdp/xdp_helpers.h>
    struct {
    	__uint(priority, 10);
    	__uint(XDP_PASS, 1);
    	__uint(XDP_DROP, 1);
    } XDP_RUN_CONFIG(my_xdp_func);

This example specifies that the XDP program in my_xdp_func should have priority 10 and that its chain call actions are XDP_PASS and XDP_DROP. In a source file with multiple XDP programs in the same file, a definition like the above can be included for each program (main XDP function). Any program that does not specify any config information will use the default values outlined above.

  Inspecting and modifying metadata

libxdp exposes the following functions that an application can use to inspect and modify the metadata on an XDP program. Modification is only possible before a program is attached on an interface. These functions won't modify the BTF information itself, but the new values will be stored as part of the program attachment.
    unsigned int xdp_program__run_prio(const struct xdp_program *xdp_prog);
    int xdp_program__set_run_prio(struct xdp_program *xdp_prog,
    			      unsigned int run_prio);
    bool xdp_program__chain_call_enabled(const struct xdp_program *xdp_prog,
    				     enum xdp_action action);
    int xdp_program__set_chain_call_enabled(struct xdp_program *prog,
    					unsigned int action,
    					bool enabled);
    int xdp_program__print_chain_call_actions(const struct xdp_program *prog,
    					  char *buf,
    					  size_t buf_len);

The dispatcher program

To support multiple non-offloaded programs on the same network interface, libxdp uses a dispatcher program which is a small wrapper program that will call each component program in turn, expect the return code, and then chain call to the next program based on the chain call actions of the previous program (see the Program metadata section above).

While applications using libxdp do not need to know the details of the dispatcher program to just load an XDP program unto an interface, libxdp does expose the dispatcher and its attached component programs, which can be used to list the programs currently attached to an interface.

The structure used for this is struct xdp_multiprog, which can only be constructed from the programs loaded on an interface based on ifindex. The API for getting a multiprog reference and iterating through the attached programs looks like this:
    struct xdp_multiprog *xdp_multiprog__get_from_ifindex(int ifindex);
    struct xdp_program *xdp_multiprog__next_prog(const struct xdp_program *prog,
    					     const struct xdp_multiprog *mp);
    void xdp_multiprog__close(struct xdp_multiprog *mp);
    int xdp_multiprog__detach(struct xdp_multiprog *mp, int ifindex);
    enum xdp_attach_mode xdp_multiprog__attach_mode(const struct xdp_multiprog *mp);
    struct xdp_program *xdp_multiprog__main_prog(const struct xdp_multiprog *mp);
    struct xdp_program *xdp_multiprog__hw_prog(const struct xdp_multiprog *mp);
    bool xdp_multiprog__is_legacy(const struct xdp_multiprog *mp);

If a non-offloaded program is attached to the interface which libxdp doesn't recognise as a dispatcher program, an xdp_multiprog structure will still be returned, and xdp_multiprog__is_legacy() will return true for that program (note that this also holds true if only an offloaded program is loaded). A reference to that (regular) XDP program can be obtained by xdp_multiprog__main_prog(). If the program attached to the interface is a dispatcher program, xdp_multiprog__main_prog() will return a reference to the dispatcher program itself, which is mainly useful for obtaining other data about that program (such as the program ID). A reference to an offloaded program can be acquired using xdp_multiprog_hw_prog(). Function xdp_multiprog__attach_mode() returns the attach mode of the non-offloaded program, whether an offloaded program is attached should be checked through xdp_multiprog_hw_prog().

  Pinning in bpffs

The kernel will automatically detach component programs from the dispatcher once the last reference to them disappears. To prevent this from happening, libxdp will pin the component program references in bpffs before attaching the dispatcher to the network interface. The pathnames generated for pinning is as follows:

—   /sys/fs/bpf/xdp/dispatch-IFINDEX-DID - dispatcher program for IFINDEX with BPF program ID DID

—   /sys/fs/bpf/xdp/dispatch-IFINDEX-DID/prog0-prog - component program 0, program reference

—   /sys/fs/bpf/xdp/dispatch-IFINDEX-DID/prog0-link - component program 0, bpf_link reference

—   /sys/fs/bpf/xdp/dispatch-IFINDEX-DID/prog1-prog - component program 1, program reference

—   /sys/fs/bpf/xdp/dispatch-IFINDEX-DID/prog1-link - component program 1, bpf_link reference

—   etc, up to ten component programs

If set, the LIBXDP_BPFFS environment variable will override the location of bpffs, but the xdp subdirectory is always used.

Using AF_XDP sockets

Libxdp implements helper functions for configuring AF_XDP sockets as well as reading and writing packets from these sockets. AF_XDP sockets can be used to redirect packets to user-space at high rates from an XDP program. Note that this functionality used to reside in libbpf, but has now been moved over to libxdp as it is a better fit for this library. As of the 1.0 release of libbpf, the AF_XDP socket support will be removed and all future development will be performed in libxdp instead.

For an overview of AF_XDP sockets, please refer to this Linux Plumbers paper (http://vger.kernel.org/lpc_net2018_talks/lpc18_pres_af_xdp_perf-v3.pdf) and the documentation in the Linux kernel (Documentation/networking/af_xdp.rst or https://www.kernel.org/doc/html/latest/networking/af_xdp.html).

For an example on how to use the interface, take a look at the sample application in the Linux kernel source tree at samples/bpf/xdpsock_user.c.

  Control path

Libxdp provides helper functions for creating and destroying umems and sockets as shown below. The first thing that a user generally wants to do is to create a umem area. This is the area that will contain all packets received and the ones that are going to be sent. After that, AF_XDP sockets can be created tied to this umem. These can either be sockets that have exclusive ownership of that umem through xsk_socket__create() or shared with other sockets using xsk_socket__create_shared. There is one option called XSK_LIBBPF_FLAGS__INHIBIT_PROG_LOAD that can be set in the libxdp_flags field (also called libbpf_flags for compatibility reasons). This will make libxdp not load any XDP program or set and BPF maps which is a must if users want to add their own XDP program.
    int xsk_umem__create(struct xsk_umem **umem,
    		     void *umem_area, __u64 size,
    		     struct xsk_ring_prod *fill,
    		     struct xsk_ring_cons *comp,
    		     const struct xsk_umem_config *config);
    int xsk_socket__create(struct xsk_socket **xsk,
    		       const char *ifname, __u32 queue_id,
    		       struct xsk_umem *umem,
    		       struct xsk_ring_cons *rx,
    		       struct xsk_ring_prod *tx,
    		       const struct xsk_socket_config *config);
    int xsk_socket__create_shared(struct xsk_socket **xsk_ptr,
    			      const char *ifname,
    			      __u32 queue_id, struct xsk_umem *umem,
    			      struct xsk_ring_cons *rx,
    			      struct xsk_ring_prod *tx,
    			      struct xsk_ring_prod *fill,
    			      struct xsk_ring_cons *comp,
    			      const struct xsk_socket_config *config);
    int xsk_umem__delete(struct xsk_umem *umem);
    void xsk_socket__delete(struct xsk_socket *xsk);

There are also two helper function to get the file descriptor of a umem or a socket. These are needed when using standard Linux syscalls such as poll(), recvmsg(), sendto(), etc.
    int xsk_umem__fd(const struct xsk_umem *umem);
    int xsk_socket__fd(const struct xsk_socket *xsk);

The control path also provides two APIs for setting up AF_XDP sockets when the process that is going to use the AF_XDP socket is non-privileged. These two functions perform the operations that require privileges and can be executed from some form of control process that has the necessary privileges. The xsk_socket__create executed on the non-privileged process will then skip these two steps. For an example on how to use these, please take a look at https://github.com/torvalds/linux/blob/master/samples/bpf/xdpsock_user.c at samples/bpf/xdpsock_user.c and https://github.com/torvalds/linux/blob/master/samples/bpf/xdpsock_ctrl_proc.c at samples/bpf/xdpsock_ctrl_proc.c in the Linux kernel source tree.
    int xsk_setup_xdp_prog(int ifindex, int *xsks_map_fd);
    int xsk_socket__update_xskmap(struct xsk_socket *xsk, int xsks_map_fd);

  Data path

For performance reasons, all the data path functions are static inline functions found in the xsk.h header file so they can be optimized into the target application binary for best possible performance. There are four FIFO rings of two main types: producer rings (fill and Tx) and consumer rings (Rx and completion). The producer rings use xsk_ring_prod functions and consumer rings use xsk_ring_cons functions. For producer rings, you start with reserving one or more slots in a producer ring and then when they have been filled out, you submit them so that the kernel will act on them. For a consumer ring, you peek if there are any new packets in the ring and if so you can read them from the ring. Once you are done reading them, you release them back to the kernel so it can use them for new packets. There is also a cancel operation for consumer rings if the application does not want to consume all packets received with the peek operation.
    __u32 xsk_ring_prod__reserve(struct xsk_ring_prod *prod, __u32 nb, __u32 *idx);
    void xsk_ring_prod__submit(struct xsk_ring_prod *prod, __u32 nb);
    __u32 xsk_ring_cons__peek(struct xsk_ring_cons *cons, __u32 nb, __u32 *idx);
    void xsk_ring_cons__cancel(struct xsk_ring_cons *cons, __u32 nb);
    void xsk_ring_cons__release(struct xsk_ring_cons *cons, __u32 nb);

The functions below are used for reading and writing the descriptors of the rings. xsk_ring_prod__fill_addr() and xsk_ring_prod__tx_desc() writes entries in the fill and Tx rings respectively, while xsk_ring_cons__comp_addr and xsk_ring_cons__rx_desc reads entries from the completion and Rx rings respectively. The idx is the parameter returned in the xsk_ring_prod__reserve or xsk_ring_cons__peek calls. To advance to the next entry, simply do idx++.
    __u64 *xsk_ring_prod__fill_addr(struct xsk_ring_prod *fill, __u32 idx);
    struct xdp_desc *xsk_ring_prod__tx_desc(struct xsk_ring_prod *tx, __u32 idx);
    const __u64 *xsk_ring_cons__comp_addr(const struct xsk_ring_cons *comp, __u32 idx);
    const struct xdp_desc *xsk_ring_cons__rx_desc(const struct xsk_ring_cons *rx, __u32 idx);

The xsk_umem functions are used to get a pointer to the packet data itself, always located inside the umem. In the default aligned mode, you can get the addr variable straight from the Rx descriptor. But in unaligned mode, you need to use the three last function below as the offset used is carried in the upper 16 bits of the addr. Therefore, you cannot use the addr straight from the descriptor in the unaligned case.
    void *xsk_umem__get_data(void *umem_area, __u64 addr);
    __u64 xsk_umem__extract_addr(__u64 addr);
    __u64 xsk_umem__extract_offset(__u64 addr);
    __u64 xsk_umem__add_offset_to_addr(__u64 addr);

There is one more function in the data path and that checks if the need_wakeup flag is set. Use of this flag is highly encouraged and should be enabled by setting XDP_USE_NEED_WAKEUP bit in the xdp_bind_flags field that is provided to the xsk_socket_create_[shared]() calls. If this function returns true, then you need to call recvmsg(), sendto(), or poll() depending on the situation. recvmsg() if you are receiving, or sendto() if you are sending. poll() can be used for both cases and provide the ability to sleep too, as with any other socket. But note that poll is a slower operation than the other two.
    int xsk_ring_prod__needs_wakeup(const struct xsk_ring_prod *r);

For an example on how to use all these APIs, take a look at the sample applications in the Linux kernel source tree at https://github.com/torvalds/linux/blob/master/samples/bpf/xdpsock_user.c at samples/bpf/xdpsock_user.c and https://github.com/torvalds/linux/blob/master/samples/bpf/xsk_fwd.c at samples/bpf/xsk_fwd.c.

Kernel and BPF program feature compatibility

The features exposed by libxdp relies on certain kernel versions and BPF features to work. To get the full benefit of all features, libxdp needs to be used with kernel 5.10 or newer, unless the commits mentioned below have been backported. However, libxdp will probe the kernel and transparently fall back to legacy loading procedures, so it is possible to use the library with older versions, although some features will be unavailable, as detailed below.

The ability to attach multiple BPF programs to a single interface relies on the kernel "BPF program extension" feature which was introduced by commit be8704ff07d2 ("bpf: Introduce dynamic program extensions") in the upstream kernel and first appeared in kernel release 5.6. To incrementally attach multiple programs, a further refinement added by commit 4a1e7c0c63e0 ("bpf: Support attaching freplace programs to multiple attach points") is needed; this first appeared in the upstream kernel version 5.10. The functionality relies on the "BPF trampolines" feature which is unfortunately only available on the x86_64 architecture. In other words, kernels before 5.6 can only attach a single XDP program to each interface, kernels 5.6+ can attach multiple programs if they are all attached at the same time, and kernels 5.10 have full support for XDP multiprog on x86_64. On other architectures, only a single program can be attached to each interface.

To load AF_XDP programs, kernel support for AF_XDP sockets needs to be included and enabled in the kernel build. In addition, when using AF_XDP sockets, an XDP program is also loaded on the interface. The XDP program used for this by libxdp requires the ability to do map lookups into XSK maps, which was introduced with commit fada7fdc83c0 ("bpf: Allow bpf_map_lookup_elem() on an xskmap") in kernel 5.3. This means that the minimum required kernel version for using AF_XDP is kernel 5.3; however, for the AF_XDP XDP program to co-exist with other programs, the same constraints for multiprog applies as outlined above.

Note that some Linux distributions backport features to earlier kernel versions, especially in enterprise kernels; for instance, Red Hat Enterprise Linux kernels include everything needed for libxdp to function since RHEL 8.5.

Finally, XDP programs loaded using the multiprog facility must include type information (using the BPF Type Format, BTF). To get this, compile the programs with a recent version of Clang/LLVM (version 10+), and enable debug information when compiling (using the -g option).

BUGS

Please report any bugs on Github: https://github.com/xdp-project/xdp-tools/issues

AUTHORS

libxdp and this man page were written by Toke Høiland-Jørgensen. AF_XDP support and documentation was contributed by Magnus Karlsson.