warning: "Attempt to use kernel headers from user space about libbpf-bootstrap

[system-thoughts]

This warning can be elimated by add -D__KERNEL__:

# Build BPF code
$(OUTPUT)/%.bpf.o: %.bpf.c $(wildcard %.h) | $(OUTPUT)
    $(call msg,BPF,$@)
    $(Q)$(CLANG) -g -O2 -target bpf \
    -D__KERNEL__ -D__TARGET_ARCH_$(ARCH) \
    $(LINUX_INCLUDE) $(CLANG_BPF_SYS_INCLUDES) -c $(filter %.c,$^) -o $@

However, other undeclared identifier error still exist.

[anakryiko]

The recommendation is to use CO-RE and vmlinux.h. If you can't or don't want to, you'll have to make sure you are including the right headers with the right definitions. bool should come from just <stdbool.h>, there rest would come from either UAPI or kernel-internal headers (and kernel-internal is where you are running into issues, probably).

Check out bootstrap example for vmlinux.h usage. That makes everything much-much simpler (but you'll need kernel BTF, of course). You might want to check https://github.com/aquasecurity/btfhub and follow the BTFGen discussion which is currently happening upstream.

[system-thoughts]

The recommendation is to use CO-RE and vmlinux.h. If you can't or don't want to, you'll have to make sure you are including the right headers with the right definitions. bool should come from just <stdbool.h>, there rest would come from either UAPI or kernel-internal headers (and kernel-internal is where you are running into issues, probably).

Thanks for your reply. Because the BPF program will run Non-BTF OS(CONFIG_DEBUG_INFO_BTF is not set), So I imitated Minimal APP to write this BPF program.
Now, I decided to use vmlinux.h to slove the linux header problem, Howerver, is it possible to run such BPF program on Non-BTF OS?
Even though I delete #include <linux/stddef.h> line, the problem still exist, and the <stdbool.h> you mentioned is from standard libc or kernel internal implementaion?

[anakryiko]

You can safely use vmlinux.h if you generated it from exactly the same vmlinux image that you are going to run on. Or if you are absolutely sure that you are using only types defined in kernel UAPI (which means they won't change their layout). But the latter doesn't seem true in your case, because you've tried to use internal kernel headers.

So, you can use vmlinux.h without relying on CO-RE, you'll just need to make sure that you have a matching vmlinux image. And you'll need to #define BPF_NO_PRESERVE_ACCESS_INDEX before #include "vmlinux.h" to disable CO-RE annotations.

stdbool.h was from standard libc, but you won't need it with vmlinux.h