Linux Virtualization - Sep 2018 - [RFC] UAPI: Check headers by compiling all together as C++

Here's a set of patches that inserts a step into the build process to make
sure that the UAPI headers can all be built together with C++ (if the
compiler being used supports C++).  All but the final patch perform fixups,
including:

 (1) Fix member names that conflict with C++ reserved words by providing
     alternates that can be used anywhere.  An anonymous union is used so
     that that the conflicting name is still available outside of C++.

 (2) Fix the use of flexible arrays in structs that get embedded (which is
     illegal in C++).

 (3) Remove the use of internal kernel structs in UAPI structures.

 (4) Fix symbol collisions.

 (5) Replace usage of u32 and co. with __u32 and co.

 (6) Fix use of sparsely initialised arrays (which g++ doesn't implement).

 (7) Remove some use of PAGE_SIZE since this isn't valid outside of the
     kernel.

And lastly:

 (8) Compile all of the UAPI headers (with a few exceptions) together as
     C++ to catch new errors occurring as part of the regular build
     process.

The patches can also be found here:

	http://git.kernel.org/cgit/linux/kernel/git/dhowells/linux-fs.git/log/?h=uapi-check

Thanks,
David
---
David Howells (11):
      UAPI: drm: Fix use of C++ keywords as structural members
      UAPI: keys: Fix use of C++ keywords as structural members
      UAPI: virtio_net: Fix use of C++ keywords as structural members
      UAPI: bcache: Fix use of embedded flexible array
      UAPI: coda: Don't use internal kernel structs in UAPI
      UAPI: netfilter: Fix symbol collision issues
      UAPI: nilfs2: Fix use of undefined byteswapping functions
      UAPI: sound: Fix use of u32 and co. in UAPI headers
      UAPI: ndctl: Fix g++-unsupported initialisation in headers
      UAPI: ndctl: Remove use of PAGE_SIZE
      UAPI: Check headers build for C++


 Makefile                                          |    1 
 include/linux/ndctl.h                             |   22 ++++
 include/uapi/drm/i810_drm.h                       |    7 +
 include/uapi/drm/msm_drm.h                        |    7 +
 include/uapi/linux/bcache.h                       |    2 
 include/uapi/linux/coda_psdev.h                   |    4 +
 include/uapi/linux/keyctl.h                       |    7 +
 include/uapi/linux/ndctl.h                        |   20 ++-
 include/uapi/linux/netfilter/nfnetlink_cthelper.h |    2 
 include/uapi/linux/netfilter_ipv4/ipt_ECN.h       |    9 --
 include/uapi/linux/nilfs2_ondisk.h                |   21 ++--
 include/uapi/linux/virtio_net.h                   |    7 +
 include/uapi/sound/skl-tplg-interface.h           |  106 +++++++++---------
 scripts/headers-c++.sh                            |  124 +++++++++++++++++++++
 14 files changed, 255 insertions(+), 84 deletions(-)
 create mode 100644 include/linux/ndctl.h
 create mode 100755 scripts/headers-c++.sh

The virtio_net_ctrl_hdr struct uses a C++ keyword as structural members.  Fix
this by inserting an anonymous union that provides an alternative name and
then hide the reserved name in C++.

Signed-off-by: David Howells <dhowells at redhat.com>
cc: "Michael S. Tsirkin" <mst at redhat.com>
cc: Jason Wang <jasowang at redhat.com>
cc: virtualization at lists.linux-foundation.org
---

 include/uapi/linux/virtio_net.h |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/virtio_net.h b/include/uapi/linux/virtio_net.h
index a3715a3224c1..967142bc0e05 100644
--- a/include/uapi/linux/virtio_net.h
+++ b/include/uapi/linux/virtio_net.h
@@ -150,7 +150,12 @@ struct virtio_net_hdr_mrg_rxbuf {
  * command goes in between.
  */
 struct virtio_net_ctrl_hdr {
-	__u8 class;
+	union {
+#ifndef __cplusplus
+		__u8 class;
+#endif
+		__u8 _class;
+	};
 	__u8 cmd;
 } __attribute__((packed));

On Wed, Sep 05, 2018 at 04:54:55PM +0100, David Howells
wrote:
> The virtio_net_ctrl_hdr struct uses a C++ keyword as structural members. 
Fix
> this by inserting an anonymous union that provides an alternative name and
> then hide the reserved name in C++.
> 
> Signed-off-by: David Howells <dhowells at redhat.com>
> cc: "Michael S. Tsirkin" <mst at redhat.com>
> cc: Jason Wang <jasowang at redhat.com>
> cc: virtualization at lists.linux-foundation.org
> ---
> 
>  include/uapi/linux/virtio_net.h |    7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/include/uapi/linux/virtio_net.h
b/include/uapi/linux/virtio_net.h
> index a3715a3224c1..967142bc0e05 100644
> --- a/include/uapi/linux/virtio_net.h
> +++ b/include/uapi/linux/virtio_net.h
> @@ -150,7 +150,12 @@ struct virtio_net_hdr_mrg_rxbuf {
>   * command goes in between.
>   */
>  struct virtio_net_ctrl_hdr {
> -	__u8 class;
> +	union {
> +#ifndef __cplusplus
> +		__u8 class;
> +#endif
> +		__u8 _class;
> +	};
Ugh, ick, no!

Come on now, either put the whole C namespace stuff around the file, or
don't care about this at all.  Doing this whack-a-mole style is a mess.

"class" is a fine variable name for C code, there's no reason this
has
to change here at all.

greg k-h

On Wed, Sep 05, 2018 at 04:54:27PM +0100, David Howells
wrote:
> 
> Here's a set of patches that inserts a step into the build process to
make
> sure that the UAPI headers can all be built together with C++ (if the
> compiler being used supports C++).  All but the final patch perform fixups,
> including:
Wait, why do we care?  What has recently changed to start to directly
import kernel uapi files into C++ code?

And if userspace wants to do this, can't they do the C namespace trick
themselves when they do the import?  That must be how they are doing it
today, right?

thanks,

greg k-h

Greg KH <gregkh at linuxfoundation.org> wrote:
> Come on now, either put the whole C namespace stuff around the file,
You mean wrap it with 'extern "C" { ... }'?  That doesn't
fix it.  That only
affects the symbols generated by the compiler.
> "class" is a fine variable name for C code, there's no reason
this has
> to change here at all.
I'm trying to prevent future accidents like the one in linux/keyctl.h.  The
easiest way to do this[**] is to pass the entire set of UAPI headers[*]
through the compiler together.

Besides I still have my dark plan to C++-ise the kernel[***] :-D

David

[*] with some obvious exceptions

[**] and it catches other errors too

[***] https://lkml.org/lkml/2018/4/1/116

On Wed, Sep 05, 2018 at 04:54:55PM +0100, David Howells
wrote:
> The virtio_net_ctrl_hdr struct uses a C++ keyword as structural members. 
Fix
> this by inserting an anonymous union that provides an alternative name and
> then hide the reserved name in C++.
> 
> Signed-off-by: David Howells <dhowells at redhat.com>
> cc: "Michael S. Tsirkin" <mst at redhat.com>
> cc: Jason Wang <jasowang at redhat.com>
> cc: virtualization at lists.linux-foundation.org
> ---
> 
>  include/uapi/linux/virtio_net.h |    7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/include/uapi/linux/virtio_net.h
b/include/uapi/linux/virtio_net.h
> index a3715a3224c1..967142bc0e05 100644
> --- a/include/uapi/linux/virtio_net.h
> +++ b/include/uapi/linux/virtio_net.h
> @@ -150,7 +150,12 @@ struct virtio_net_hdr_mrg_rxbuf {
>   * command goes in between.
>   */
>  struct virtio_net_ctrl_hdr {
> -	__u8 class;
> +	union {
> +#ifndef __cplusplus
> +		__u8 class;
> +#endif
> +		__u8 _class;
> +	};
>  	__u8 cmd;
>  } __attribute__((packed));

As long as you do not intend to use any classes, how about
simply adding

-Dclass=_class

to your command line?

Seems to work fine with gcc 8.1.1 on Fedora.

-- 
MST

On Wed, Sep 05, 2018 at 07:33:38PM +0200, Yann Droneaud
wrote:
> Hi,
> 
> Le mercredi 05 septembre 2018 ? 18:55 +0200, Greg KH a ?crit :
> > On Wed, Sep 05, 2018 at 04:54:27PM +0100, David Howells wrote:
> > > 
> > > Here's a set of patches that inserts a step into the build
process to make
> > > sure that the UAPI headers can all be built together with C++ (if
the
> > > compiler being used supports C++).  All but the final patch
perform fixups,
> > > including:
> > 
> > Wait, why do we care?  What has recently changed to start to directly
> > import kernel uapi files into C++ code?
> > 
> > And if userspace wants to do this, can't they do the C namespace
trick
> > themselves when they do the import?  That must be how they are doing
it
> > today, right?
> > 
> 
> They can't.
> 
> 
> Adding extern "C" { } doesn't magically make
"class" a non keyword.
> Even if it was the case, writing C++ code using whatever->class would
> probably broke because class is a keyword in C++.
I think it's a bug in the language TBH.
> -- 
> Yann Droneaud
> OPTEYA
>

Greg KH <greg at kroah.com> wrote:
> > Here's a set of patches that inserts a step into the build process
to make
> > sure that the UAPI headers can all be built together with C++ (if the
> > compiler being used supports C++).  All but the final patch perform
fixups,
> > including:
> 
> Wait, why do we care?  What has recently changed to start to directly
> import kernel uapi files into C++ code?
There's at least one outstanding bug due to a C++ identifier in the kernel
UAPI headers.

Are you saying you explicitly don't want people to be able to use the kernel
UAPI headers in C++?
> And if userspace wants to do this, can't they do the C namespace trick
> themselves when they do the import?  That must be how they are doing it
> today, right?
No, because there's no such trick (except with the preprocessor).

David

On Wednesday 2018-09-05 18:55, Greg KH wrote:
>On Wed, Sep 05, 2018 at 04:54:27PM +0100, David Howells wrote:
>> 
>> Here's a set of patches that inserts a step into the build process
to make
>> sure that the UAPI headers can all be built together with C++ (if the
>> compiler being used supports C++).  All but the final patch perform
fixups,
>> including:
>
>Wait, why do we care?  What has recently changed to start to directly
>import kernel uapi files into C++ code?
With C++11, C++ has become a much nicer language to use (for userspace, anyway).
>And if userspace wants to do this, can't they do the C namespace trick
>themselves when they do the import?
The only trick is to use an extra C source file and extensively wrap things.

Michael S. Tsirkin <mst at redhat.com> wrote:
> As long as you do not intend to use any classes, how about
> simply adding
> 
> -Dclass=_class
> 
> to your command line?
That kind of misses the point;-).  It's not reasonable to expect all
userspace
C++ users to do this.

David