[PATCH] bpf: generate const static pointers for kernel helpers

["Jose E. Marchesi" <jose.marchesi@oracle.com>]

The generated bpf_helper_defs.h file currently contains definitions
like this for the kernel helpers, which are static objects:

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

These work well in both clang and GCC because both compilers do
constant propagation with -O1 and higher optimization, resulting in
`call 1' BPF instructions being generated, which are calls to kernel
helpers.

However, there is a discrepancy on how the -Wunused-variable
warning (activated by -Wall) is handled in these compilers:

- clang will not emit -Wunused-variable warnings for static variables
  defined in C header files, be them constant or not constant.

- GCC will not emit -Wunused-variable warnings for _constant_ static
  variables defined in header files, but it will emit warnings for
  non-constant static variables defined in header files.

There is no reason for these bpf_helpers_def.h pointers to not be
declared constant, and it is actually desirable to do so, since their
values are not to be changed.  So this patch modifies bpf_doc.py to
generate prototypes like:

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

This allows GCC to not error while compiling BPF programs with `-Wall
-Werror', while still being able to detect and error on legitimate
unused variables in the program themselves.

This change doesn't impact the desired constant propagation in neither
Clang nor GCC with -O1 and higher.  On the contrary, being declared as
constant may increase the odds they get constant folded when
used/referred to in certain circumstances.

Tested in bpf-next master.
No regressions.

Signed-off-by: Jose E. Marchesi <jose.marchesi@oracle.com>
Cc: alexei.starovoitov@gmail.com
Cc: yonghong.song@linux.dev
Cc: eddyz87@gmail.com
Cc: cupertino.miranda@oracle.com
Cc: david.faust@oracle.com
---
 scripts/bpf_doc.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/scripts/bpf_doc.py b/scripts/bpf_doc.py
index 61b7dddedc46..2b94749c99cc 100755
--- a/scripts/bpf_doc.py
+++ b/scripts/bpf_doc.py
@@ -827,7 +827,7 @@ class PrinterHelpers(Printer):
                 print(' *{}{}'.format(' \t' if line else '', line))
 
         print(' */')
-        print('static %s %s(*%s)(' % (self.map_type(proto['ret_type']),
+        print('static %s %s(* const %s)(' % (self.map_type(proto['ret_type']),
                                       proto['ret_star'], proto['name']), end='')
         comma = ''
         for i, a in enumerate(proto['args']):
-- 
2.30.2