Re: [PATCH bpf-next] bpf: Disable -Wmissing-declarations for globally-linked kfuncs

[Daniel Borkmann <daniel@iogearbox.net>]

On 8/17/23 6:01 AM, David Vernet wrote:
> On Wed, Aug 16, 2023 at 08:48:16PM -0700, Alexei Starovoitov wrote:
>> On Wed, Aug 16, 2023 at 8:38 PM Yonghong Song <yonghong.song@linux.dev> wrote:
>>> On 8/16/23 8:06 AM, David Vernet wrote:
>>>> We recently got an lkp warning about missing declarations, as in e.g.
>>>> [0]. This warning is largely redundant with -Wmissing-prototypes, which
>>>> we already disable for kfuncs that have global linkage and are meant to
>>>> be exported in BTF, and called from BPF programs. Let's also disable
>>>> -Wmissing-declarations for kfuncs. For what it's worth, I wasn't able to
>>>> reproduce the warning even on W <= 3, so I can't actually be 100% sure
>>>> this fixes the issue.
>>>>
>>>> [0]: https://lore.kernel.org/all/202308162115.Hn23vv3n-lkp@intel.com/
>>>
>>> Okay, I just got a similar email to [0] which complains
>>>     bpf_obj_new_impl, ..., bpf_cast_to_kern_ctx
>>> missing declarations.
>>>
>>> In the email, the used compiler is
>>> compiler: gcc-7 (Ubuntu 7.5.0-6ubuntu2) 7.5.0
>>>
>>> Unfortunately, I did not have gcc-7 to verify this.
>>> Also, what is the minimum gcc version kernel supports? 5.1?
>>
>> pahole and BTF might be broken in such old GCC too.
>> Maybe we should add:
>> config BPF_SYSCALL
>>          depends on GCC_VERSION >= 90000 || CLANG_VERSION >= 130000
> 
> It seems prudent to formally declare minimum compiler versions. Though
> modern gcc and clang also support -Wmissing-declarations, so maybe we
> should merge this patch regardless? Just unfortunate to have to add even
> more boilerplate just to get the compiler off our backs.

Urgh, to restrict BPF syscall with such `depends on` would be super ugly. Why
can't we just move this boilerplate behind a macro instead of copying this
everywhere? For example the below on top of your patch builds just fine on my
side:

diff --git a/include/linux/btf.h b/include/linux/btf.h
index df64cc642074..6a873a652001 100644
--- a/include/linux/btf.h
+++ b/include/linux/btf.h
@@ -83,6 +83,16 @@
   */
  #define __bpf_kfunc __used noinline

+#define __bpf_kfunc_start	\
+	__diag_push();	\
+	__diag_ignore_all("-Wmissing-prototypes",	\
+			  "Global functions as their definitions will be in vmlinux BTF");	\
+	__diag_ignore_all("-Wmissing-declarations",	\
+			  "Global functions as their definitions will be in vmlinux BTF");
+
+#define __bpf_kfunc_end	\
+	__diag_pop();
+
  /*
   * Return the name of the passed struct, if exists, or halt the build if for
   * example the structure gets renamed. In this way, developers have to revisit
diff --git a/net/core/filter.c b/net/core/filter.c
index c2b32b94c6bd..08dd0dd710dd 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -11724,11 +11724,7 @@ bpf_sk_base_func_proto(enum bpf_func_id func_id)
  	return func;
  }

-__diag_push();
-__diag_ignore_all("-Wmissing-prototypes",
-		  "Global functions as their definitions will be in vmlinux BTF");
-__diag_ignore_all("-Wmissing-declarations",
-		  "Global functions as their definitions will be in vmlinux BTF");
+__bpf_kfunc_start
  __bpf_kfunc int bpf_dynptr_from_skb(struct sk_buff *skb, u64 flags,
  				    struct bpf_dynptr_kern *ptr__uninit)
  {
@@ -11754,7 +11750,7 @@ __bpf_kfunc int bpf_dynptr_from_xdp(struct xdp_buff *xdp, u64 flags,

  	return 0;
  }
-__diag_pop();
+__bpf_kfunc_end

  int bpf_dynptr_from_skb_rdonly(struct sk_buff *skb, u64 flags,
  			       struct bpf_dynptr_kern *ptr__uninit)

Thanks,
Daniel