[PATCH v3 bpf-next 1/2] libbpf: provide NULL and KERNEL_VERSION macros in bpf_helpers.h

[Andrii Nakryiko <andrii@kernel.org>]

Given that vmlinux.h is not compatible with headers like stddef.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).

But it's easy for bpf_helpers.h to provide (conditionally) NULL definition.
Similarly, KERNEL_VERSION is another commonly missed macro that came up
multiple times. So this patch adds both of them, along with offsetof(), that
also is typically defined in stddef.h, just like NULL.

This might cause compilation warning for existing BPF applications defining
their own NULL and/or KERNEL_VERSION already:

  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/lib/bpf/bpf_helpers.h | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/tools/lib/bpf/bpf_helpers.h b/tools/lib/bpf/bpf_helpers.h
index 53ff81c49dbd..cc2e51c64a54 100644
--- a/tools/lib/bpf/bpf_helpers.h
+++ b/tools/lib/bpf/bpf_helpers.h
@@ -40,8 +40,22 @@
 #define __weak __attribute__((weak))
 #endif
 
+/* When utilizing vmlinux.h with BPF CO-RE, user BPF programs can't include
+ * any system-level headers (such as stddef.h, linux/version.h, etc), and
+ * commonly-used macros like NULL and KERNEL_VERSION aren't available through
+ * vmlinux.h. This just adds unnecessary hurdles and forces users to re-define
+ * them on their own. So as a convenience, provide such definitions here.
+ */
+#ifndef NULL
+#define NULL ((void *)0)
+#endif
+
+#ifndef KERNEL_VERSION
+#define KERNEL_VERSION(a,b,c) (((a) << 16) + ((b) << 8) + ((c) > 255 ? 255 : (c))
+#endif
+
 /*
- * Helper macro to manipulate data structures
+ * Helper macros to manipulate data structures
  */
 #ifndef offsetof
 #define offsetof(TYPE, MEMBER)	((unsigned long)&((TYPE *)0)->MEMBER)
-- 
2.30.2