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

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On 4/25/23 11:35 AM, Stanislav Fomichev wrote:
On Tue, Apr 25, 2023 at 11:29 AM Yonghong Song <yhs@xxxxxxxx> wrote:



On 4/25/23 11:22 AM, Stanislav Fomichev wrote:
On Tue, Apr 25, 2023 at 11:10 AM Yonghong Song <yhs@xxxxxxxx> wrote:



On 4/25/23 10:04 AM, Stanislav Fomichev wrote:
On Mon, Apr 24, 2023 at 6:56 PM Yonghong Song <yhs@xxxxxxxx> wrote:



On 4/24/23 5:01 PM, Stanislav Fomichev wrote:
From: Peng Wei <pengweiprc@xxxxxxxxxx>

Compiling C++ BPF programs with existing bpf_helper_defs.h is not

Just curious, why you want to compile BPF programs with C++?
The patch looks good to me. But it would be great to know
some reasoning since a lot of stuff, e.g., some CORE related
intrinsics, not available for C++.

Can you share more? What's not available? Any pointers to the docs maybe?

Sorry, it is an attribute, instead of instrinsics.

The attribute preserve_access_index/btf_type_tag/btf_decl_tag are all C
only.

Interesting, thanks! I don't think we use btf_type_tag/btf_decl_tag in
the program we want to try c++, but losing preserve_access_index might
be unfortunate :-( But we'll see..
Btw, any reason these are explicitly opted out from c++? Doesn't seem
like there is anything c-specific in them?

Initial use case is C only. If we say to support C++, we will
need to add attribute processing codes in various other places
(member functions, templates, other c++ constructs, etc.)
to convert these attributes to proper debuginfo. There are no use
cases for this, so we didn't do it in the first place.

The c++ we are talking about here is mostly "c with classes +
templates"; no polymorphism / inheritance.

In llvm-project/clang/include/clang/Basic/Attr.td:

def BPFPreserveAccessIndex : InheritableAttr,
                               TargetSpecificAttr<TargetBPF>  {
    let Spellings = [Clang<"preserve_access_index">];
    let Subjects = SubjectList<[Record], ErrorDiag>;
    let Documentation = [BPFPreserveAccessIndexDocs];
    let LangOpts = [COnly];
}

def BTFDeclTag : InheritableAttr {
    let Spellings = [Clang<"btf_decl_tag">];
    let Args = [StringArgument<"BTFDeclTag">];
    let Subjects = SubjectList<[Var, Function, Record, Field, TypedefName],
                               ErrorDiag>;
    let Documentation = [BTFDeclTagDocs];
    let LangOpts = [COnly];
}

def BTFTypeTag : TypeAttr {
    let Spellings = [Clang<"btf_type_tag">];
    let Args = [StringArgument<"BTFTypeTag">];
    let Documentation = [BTFTypeTagDocs];
    let LangOpts = [COnly];
}




People here want to try to use c++ to see if templating helps with v4
vs v6 handling.
We have a bunch of copy-paste around this place and would like to see
whether c++ could make it a bit more readable.

possible due to stricter C++ type conversions. C++ complains
about (void *) type conversions:

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.

Could you specify what exactly the compilation command triggering the
above error?

The following does it for me:
clang++ --include linux/types.h ./tools/lib/bpf/bpf_helper_defs.h

Thanks. It would be good if you add the above compilation command
in the commit message.

Sure, will add.




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;

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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