[PATCH v2 bpf-next 1/4] bpftool: generate NULL definition in vmlinux.h

Given that vmlinux.h is not compatible with headers like stdint.h, NULL poses
an annoying problem: it is defined as #define, so is not captured in BTF, so
is not emitted into vmlinux.h. This leads to users either sticking to explicit
0, or defining their own NULL (as progs/skb_pkt_end.c does).

It's pretty trivial for bpftool to generate NULL definition, though, so let's
just do that. This might cause compilation warning for existing BPF
applications:

  progs/skb_pkt_end.c:7:9: warning: 'NULL' macro redefined [-Wmacro-redefined]
  #define NULL 0
          ^
  /tmp/linux/tools/testing/selftests/bpf/tools/include/vmlinux.h:4:9: note: previous definition is here
  #define NULL ((void *)0)
	  ^

It is trivial to fix, though, so long-term benefits outweight temporary
inconveniences.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
 tools/bpf/bpftool/btf.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/tools/bpf/bpftool/btf.c b/tools/bpf/bpftool/btf.c
index 62953bbf68b4..ff6a76632873 100644
--- a/tools/bpf/bpftool/btf.c
+++ b/tools/bpf/bpftool/btf.c

@@ -405,6 +405,8 @@ static int dump_btf_c(const struct btf *btf,
 	printf("#ifndef __VMLINUX_H__\n");
 	printf("#define __VMLINUX_H__\n");
 	printf("\n");
+	printf("#define NULL ((void *)0)\n");
+	printf("\n");
 	printf("#ifndef BPF_NO_PRESERVE_ACCESS_INDEX\n");
 	printf("#pragma clang attribute push (__attribute__((preserve_access_index)), apply_to = record)\n");
 	printf("#endif\n\n");

-- 
2.30.2