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I put >> this explicit type generation in kernel/bpf/core.c >> to (1) avoid polute net/ipv4/tcp.c and more importantly >> (2) provide a central place to add other types (e.g. in >> bpf/btf uapi header) if they are not referenced in the kernel >> or generated in vmlinux dwarf. >> >> Signed-off-by: Yonghong Song >> --- >>   include/linux/btf.h |  1 + >>   kernel/bpf/core.c   | 19 +++++++++++++++++++ >>   2 files changed, 20 insertions(+) >> >> diff --git a/include/linux/btf.h b/include/linux/btf.h >> index 7fabf1428093..9c1b52738bbe 100644 >> --- a/include/linux/btf.h >> +++ b/include/linux/btf.h >> @@ -9,6 +9,7 @@ >>   #include >>   #define BTF_TYPE_EMIT(type) ((void)(type *)0) >> +#define BTF_TYPE_EMIT_ENUM(enum_val) ((void)enum_val) >>   struct btf; >>   struct btf_member; >> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c >> index 3a283bf97f2f..60551bf68ece 100644 >> --- a/kernel/bpf/core.c >> +++ b/kernel/bpf/core.c >> @@ -2378,3 +2378,22 @@ EXPORT_SYMBOL(bpf_stats_enabled_key); >>   EXPORT_TRACEPOINT_SYMBOL_GPL(xdp_exception); >>   EXPORT_TRACEPOINT_SYMBOL_GPL(xdp_bulk_tx); >> + >> +static int __init bpf_emit_btf_type(void) >> +{ >> +    /* bpf uapi header bpf.h defines an anonymous enum with values >> +     * BPF_TCP_* used by bpf programs. Currently gcc built vmlinux >> +     * is able to emit this enum in dwarf due to the following >> +     * BUILD_BUG_ON test in net/ipv4/tcp.c: >> +     *   BUILD_BUG_ON((int)BPF_TCP_ESTABLISHED != (int)TCP_ESTABLISHED); >> +     * clang built vmlinux does not have this enum in dwarf >> +     * since clang removes the above code before generating >> IR/debuginfo. >> +     * Let us explicitly emit the type debuginfo to ensure the >> +     * above-mentioned anonymous enum in the vmlinux dwarf and hence BTF >> +     * regardless of which compiler is used. >> +     */ >> +    BTF_TYPE_EMIT_ENUM(BPF_TCP_ESTABLISHED); >> + >> +    return 0; >> +} >> +late_initcall(bpf_emit_btf_type); > > Does this have to be late_initcall() given this adds minor init call > overhead, what if this would be exported as symbol for modules instead? If issuing types in module, if I understand correctly, it will not be in main vmlinux btf, so programs will not be able to use unless module is loaded. I would prefer such types always available in vmlinux btf. I am using a separate late_initcall just to cleaner codes. But this BTF_TYPE_EMIT_ENUM can be in any init call. $ grep _initcall *.c btf.c:fs_initcall(btf_module_init); core.c:pure_initcall(bpf_jit_charge_init); cpumap.c:subsys_initcall(cpu_map_init); devmap.c:subsys_initcall(dev_map_init); inode.c:fs_initcall(bpf_init); map_iter.c:late_initcall(bpf_map_iter_init); net_namespace.c:subsys_initcall(netns_bpf_init); prog_iter.c:late_initcall(bpf_prog_iter_init); stackmap.c:subsys_initcall(stack_map_init); sysfs_btf.c:subsys_initcall(btf_vmlinux_init); task_iter.c:late_initcall(task_iter_init); trampoline.c:late_initcall(init_trampolines); $ I think we can use any above in kernel/bpf directory. This way, we will have 0 runtime overhead as the code will be optimized away. Any preference? > > Thanks, > Daniel