On 1/27/24 10:50 AM, Jose E. Marchesi wrote:
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@xxxxxxxxxx> Cc: alexei.starovoitov@xxxxxxxxx Cc: yonghong.song@xxxxxxxxx Cc: eddyz87@xxxxxxxxx Cc: cupertino.miranda@xxxxxxxxxx Cc: david.faust@xxxxxxxxxx
LGTM. Please add proper tag in the commit subject, e.g., "[PATCH bpf-next]" instead of "[PATCH]", so CI can pick up the patch and do proper test. Acked-by: Yonghong Song <yonghong.song@xxxxxxxxx>
--- 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']):
