[PATCH v2] 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 adds conditional blocks for GCC while leaving clang codepaths
unchanged, for example:
	#if __GNUC__ && !__clang__
	void *bpf_map_lookup_elem(void *map, const void *key) __attribute__((kernel_helper(1)));
	#else
	static void *(*bpf_map_lookup_elem)(void *map, const void *key) = (void *) 1;
	#endif

	#if __GNUC__ && !__clang__
	long bpf_map_update_elem(void *map, const void *key, const void *value, __u64 flags) __attribute__((kernel_helper(2)));
	#else
	static long (*bpf_map_update_elem)(void *map, const void *key, const void *value, __u64 flags) = (void *) 2;
	#endif

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 v1 -> v2:
  - more details in commit log
---
 scripts/bpf_doc.py | 43 ++++++++++++++++++++++++++-----------------
 1 file changed, 26 insertions(+), 17 deletions(-)

diff --git a/scripts/bpf_doc.py b/scripts/bpf_doc.py
index a0ec321469bd..36fb400a5731 100755
--- a/scripts/bpf_doc.py
+++ b/scripts/bpf_doc.py
@@ -739,6 +739,24 @@ class PrinterHelpers(Printer):
 
     seen_helpers = set()
 
+    def print_args(self, proto):
+        comma = ''
+        for i, a in enumerate(proto['args']):
+            t = a['type']
+            n = a['name']
+            if proto['name'] in self.overloaded_helpers and i == 0:
+                    t = 'void'
+                    n = 'ctx'
+            one_arg = '{}{}'.format(comma, self.map_type(t))
+            if n:
+                if a['star']:
+                    one_arg += ' {}'.format(a['star'])
+                else:
+                    one_arg += ' '
+                one_arg += '{}'.format(n)
+            comma = ', '
+            print(one_arg, end='')
+
     def print_one(self, helper):
         proto = helper.proto_break_down()
 
@@ -762,26 +780,17 @@ class PrinterHelpers(Printer):
                 print(' *{}{}'.format(' \t' if line else '', line))
 
         print(' */')
+        print('#if __GNUC__ && !__clang__')
+        print('%s %s%s(' % (self.map_type(proto['ret_type']),
+                                      proto['ret_star'], proto['name']), end='')
+        self.print_args(proto)
+        print(') __attribute__((kernel_helper(%d)));' % len(self.seen_helpers))
+        print('#else')
         print('static %s %s(*%s)(' % (self.map_type(proto['ret_type']),
                                       proto['ret_star'], proto['name']), end='')
-        comma = ''
-        for i, a in enumerate(proto['args']):
-            t = a['type']
-            n = a['name']
-            if proto['name'] in self.overloaded_helpers and i == 0:
-                    t = 'void'
-                    n = 'ctx'
-            one_arg = '{}{}'.format(comma, self.map_type(t))
-            if n:
-                if a['star']:
-                    one_arg += ' {}'.format(a['star'])
-                else:
-                    one_arg += ' '
-                one_arg += '{}'.format(n)
-            comma = ', '
-            print(one_arg, end='')
-
+        self.print_args(proto)
         print(') = (void *) %d;' % len(self.seen_helpers))
+        print('#endif')
         print('')
 
 ###############################################################################
-- 
2.34.1