bpf, libbpf: Add bpf_tail_call_static helper for bpf programs
Port of tail_call_static() helper function from Cilium's BPF code base [0] to libbpf, so others can easily consume it as well. We've been using this in production code for some time now. The main idea is that we guarantee that the kernel's BPF infrastructure and JIT (here: x86_64) can patch the JITed BPF insns with direct jumps instead of having to fall back to using expensive retpolines. By using inline asm, we guarantee that the compiler won't merge the call from different paths with potentially different content of r2/r3. We're also using Cilium's __throw_build_bug() macro (here as: __bpf_unreachable()) in different places as a neat trick to trigger compilation errors when compiler does not remove code at compilation time. This works for the BPF back end as it does not implement the __builtin_trap(). [0] https://github.com/cilium/cilium/commit/f5537c26020d5297b70936c6b7d03a1e412a1035 Signed-off-by:Daniel Borkmann <daniel@iogearbox.net> Signed-off-by:
Alexei Starovoitov <ast@kernel.org> Acked-by:
Andrii Nakryiko <andriin@fb.com> Link: https://lore.kernel.org/bpf/1656a082e077552eb46642d513b4a6bde9a7dd01.1601477936.git.daniel@iogearbox.net
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