[PATCH bpf-next v2] bpf: Make bpf_helper_defs.h c++ friendly

[Stanislav Fomichev <sdf@xxxxxxxxxx>]

From: Peng Wei <pengweiprc@xxxxxxxxxx>

Compiling C++ BPF programs with existing bpf_helper_defs.h is not
possible due to stricter C++ type conversions. C++ complains
about (void *) type conversions:

$ clang++ --include linux/types.h ./tools/lib/bpf/bpf_helper_defs.h

bpf_helper_defs.h:57:67: error: invalid conversion from ‘void*’ to ‘void* (*)(void*, const void*)’ [-fpermissive]
   57 | static void *(*bpf_map_lookup_elem)(void *map, const void *key) = (void *) 1;
      |                                                                   ^~~~~~~~~~
      |                                                                   |
      |                                                                   void*

Extend bpf_doc.py to use proper function type instead of void.

Before:
static void *(*bpf_map_lookup_elem)(void *map, const void *key) = (void *) 1;

After:
static void *(*bpf_map_lookup_elem)(void *map, const void *key) = (void *(*)(void *map, const void *key)) 1;

v2:
- add clang++ invocation example (Yonghong)

Cc: Yonghong Song <yhs@xxxxxxxx>
Signed-off-by: Peng Wei <pengweiprc@xxxxxxxxxx>
Signed-off-by: Stanislav Fomichev <sdf@xxxxxxxxxx>
---
 scripts/bpf_doc.py | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/scripts/bpf_doc.py b/scripts/bpf_doc.py
index eaae2ce78381..fa21137a90e7 100755
--- a/scripts/bpf_doc.py
+++ b/scripts/bpf_doc.py
@@ -827,6 +827,9 @@ COMMANDS
                 print(' *{}{}'.format(' \t' if line else '', line))
 
         print(' */')
+        fptr_type = '%s%s(*)(' % (
+            self.map_type(proto['ret_type']),
+            ((' ' + proto['ret_star']) if proto['ret_star'] else ''))
         print('static %s %s(*%s)(' % (self.map_type(proto['ret_type']),
                                       proto['ret_star'], proto['name']), end='')
         comma = ''
@@ -845,8 +848,10 @@ COMMANDS
                 one_arg += '{}'.format(n)
             comma = ', '
             print(one_arg, end='')
+            fptr_type += one_arg
 
-        print(') = (void *) %d;' % helper.enum_val)
+        fptr_type += ')'
+        print(') = (%s) %d;' % (fptr_type, helper.enum_val))
         print('')
 
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-- 
2.40.1.495.gc816e09b53d-goog