Merge branch 'Provide NULL and KERNEL_VERSION macros in bpf_helpers.h'

Andrii Nakryiko says:

====================

Provide NULL and KERNEL_VERSION macros in bpf_helpers.h. Patch #2 removes such
custom NULL definition from one of the selftests.

v2->v3:
  - instead of vmlinux.h, do this in bpf_helpers.h;
  - added KERNEL_VERSION, which comes up periodically as well;
  - I dropped strict compilation patches for now, because we run into new
    warnings (e.g., not checking read() result) in kernel-patches CI, which
    I can't even reproduce locally. Also -Wdiscarded-qualifiers pragma for
    jit_disasm.c is not supported by Clang, it needs to be
    -Wincompatible-pointer-types-discards-qualifiers for Clang; we don't have
    to deal with that in this patch set;
v1->v2:
  - fix few typos and wrong copy/paste;
  - fix #pragma push -> pop.
====================
Signed-off-by: NAlexei Starovoitov <ast@kernel.org>

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)

tools/testing/selftests/bpf/progs/skb_pkt_end.c

@@ -4,7 +4,6 @@
 #include <bpf/bpf_core_read.h>
 #include <bpf/bpf_helpers.h>
 
-#define NULL 0
 #define INLINE __always_inline
 
 #define skb_shorter(skb, len) ((void *)(long)(skb)->data + (len) > (void *)(long)skb->data_end)