[PATCH v3] bpf/scripts: Generate GCC compatible helpers

[James Hilliard <james.hilliard1@gmail.com>]

The current bpf_helper_defs.h helpers are llvm specific and don't work
correctly with gcc.

GCC appears to required kernel helper funcs to have the following
attribute set: __attribute__((kernel_helper(NUM)))

Generate gcc compatible headers based on the format in bpf-helpers.h.

This generates GCC/Clang compatible helpers, for example:
	/* Helper macro for GCC/Clang compatibility */
	#define NOARG
	#if __GNUC__ && !__clang__
	#define BPF_HELPER_DEF(num, ret_star, ret_type, name, ...) \
	ret_type ret_star name(__VA_ARGS__) __attribute__((kernel_helper(num)));
	#else
	#define BPF_HELPER_DEF(num, ret_star, ret_type, name, ...) \
	static ret_type ret_star(*name)(__VA_ARGS__) = (void *) num;
	#endif

	BPF_HELPER_DEF(1, *, void, bpf_map_lookup_elem, void *map, const void *key)

	BPF_HELPER_DEF(2, NOARG, long, bpf_map_update_elem, void *map, const void *key, const void *value, __u64 flags)

See:
https://github.com/gcc-mirror/gcc/blob/releases/gcc-12.1.0/gcc/config/bpf/bpf-helpers.h#L24-L27

This fixes the following build error:
error: indirect call in function, which are not supported by eBPF

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
---
Changes v2 -> v3:
  - use a conditional helper macro
Changes v1 -> v2:
  - more details in commit log
---
 scripts/bpf_doc.py | 21 ++++++++++++++++++---
 1 file changed, 18 insertions(+), 3 deletions(-)

diff --git a/scripts/bpf_doc.py b/scripts/bpf_doc.py
index a0ec321469bd..45f51ff1318c 100755
--- a/scripts/bpf_doc.py
+++ b/scripts/bpf_doc.py
@@ -717,6 +717,16 @@ class PrinterHelpers(Printer):
         header = '''\
 /* This is auto-generated file. See bpf_doc.py for details. */
 
+/* Helper macro for GCC/Clang compatibility */
+#define NOARG
+#if __GNUC__ && !__clang__
+#define BPF_HELPER_DEF(num, ret_star, ret_type, name, ...) \\
+ret_type ret_star name(__VA_ARGS__) __attribute__((kernel_helper(num)));
+#else
+#define BPF_HELPER_DEF(num, ret_star, ret_type, name, ...) \\
+static ret_type ret_star(*name)(__VA_ARGS__) = (void *) num;
+#endif
+
 /* Forward declarations of BPF structs */'''
 
         print(header)
@@ -746,6 +756,11 @@ class PrinterHelpers(Printer):
             return
         self.seen_helpers.add(proto['name'])
 
+        if proto['ret_star']:
+            ret_star = proto['ret_star']
+        else:
+            ret_star = 'NOARG'
+
         print('/*')
         print(" * %s" % proto['name'])
         print(" *")
@@ -762,8 +777,8 @@ class PrinterHelpers(Printer):
                 print(' *{}{}'.format(' \t' if line else '', line))
 
         print(' */')
-        print('static %s %s(*%s)(' % (self.map_type(proto['ret_type']),
-                                      proto['ret_star'], proto['name']), end='')
+        print('BPF_HELPER_DEF(%d, %s, %s, %s, ' % (len(self.seen_helpers),
+            ret_star, self.map_type(proto['ret_type']), proto['name']), end='')
         comma = ''
         for i, a in enumerate(proto['args']):
             t = a['type']
@@ -781,7 +796,7 @@ class PrinterHelpers(Printer):
             comma = ', '
             print(one_arg, end='')
 
-        print(') = (void *) %d;' % len(self.seen_helpers))
+        print(')')
         print('')
 
 ###############################################################################
-- 
2.34.1