[simonmorley]

Closes #591.

A dispatcher component has to be loaded as BPF_PROG_TYPE_EXT, and the kernel only accepts an EXT program that carries BTF describing its own function signature — check_attach_btf_id() compares the replacement's prototype against the target function's via btf_check_func_type_match(). Since a program's BTF is fixed at BPF_PROG_LOAD time, a caller holding an already-loaded BTF-less program cannot add it afterwards, and xdp_multiprog__link_prog() correctly refuses it.

That excludes any project emitting BPF bytecode directly rather than compiling it with clang. Such a program can only take the whole interface, which in turn locks every libxdp user out of that interface.

Every XDP component has the same int (struct xdp_md *) signature, so the BTF is fully determined and libxdp can generate it. This adds insns/insns_cnt to struct xdp_program_opts:

struct bpf_insn insns[] = {
	BPF_MOV64_IMM(BPF_REG_0, XDP_PASS),
	BPF_EXIT_INSN(),
};
DECLARE_LIBXDP_OPTS(xdp_program_opts, opts,
		    .prog_name = "my_program",
		    .insns = insns,
		    .insns_cnt = 2);

prog = xdp_program__create(&opts);

Approach

xdp_elfgen.c synthesises a minimal ELF object — the instructions, a license section, the BTF, and a .BTF.ext func_info record — which is then opened with bpf_object__open_mem(). Wrapping the instructions in an ELF rather than loading them directly means the result is an ordinary struct bpf_object, so the attach, EXT conversion and chain linking paths need no special case at all.

Two details worth flagging for review:

• The struct xdp_md in the generated BTF is deliberately opaque (vlen = 0). btf_check_func_type_match() compares the struct name and never its members, so a component need not agree with the kernel on the layout. That keeps the BTF at ~127 bytes and independent of kernel version.
• libelf references the image rather than copying it, so the buffer is owned by the xdp_program and freed in xdp_program__close(), after bpf_object__close() has stopped reading it.

No new exported symbol, so libxdp.map is unchanged — xdp_program__create() has been exported since LIBXDP_1.3.0 and the opts struct is extended through the existing sz mechanism.

Testing

New test_synth_insns creates a program from a raw instruction stream and checks it attaches as a component of a real dispatcher, rather than falling back to a legacy single-program attach. Full suite on 6.12.107, x86-64:

     [test_link_so]                PASS
     [test_link_a]                 PASS
     [test_old_dispatcher]         PASS
     [test_xdp_devbound]           PASS
     [test_xdp_frags]              PASS
     [test_xsk_prog_refcnt_bpffs]  PASS
     [test_xsk_prog_refcnt_legacy] PASS
     [test_xsk_non_privileged]     SKIPPED
     [test_link_detach]            PASS
     [test_xsk_umem_flags]         PASS
     [test_xsk_map_leak]           PASS
     [test_synth_insns]            PASS

libxdp.3 was updated by hand rather than regenerated, as emacs was not available here; it renders correctly under groff but is worth a check against the org export.

🤖 Generated with Claude Code