[RFC] UAPI: Check headers by compiling all together as C++

[RFC] UAPI: Check headers by compiling all together as C++

[David Howells]

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

[PATCH 04/11] UAPI: bcache: Fix use of embedded flexible array

[David Howells]

The bkey struct defined by bcache is embedded in the jset struct.  However,
this is illegal in C++ as there's a "flexible array" at the end of the
struct.  Change this to be a 0-length struct instead.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Coly Li <colyli@suse.de>
cc: Kent Overstreet <kent.overstreet@gmail.com>
cc: linux-bcache@vger.kernel.org
---

 include/uapi/linux/bcache.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/uapi/linux/bcache.h b/include/uapi/linux/bcache.h
index 5d4f58e059fd..11863e903bff 100644
--- a/include/uapi/linux/bcache.h
+++ b/include/uapi/linux/bcache.h
@@ -23,7 +23,7 @@ static inline void SET_##name(type *k, __u64 v)			\
 struct bkey {
 	__u64	high;
 	__u64	low;
-	__u64	ptr[];
+	__u64	ptr[0];
 };
 
 #define KEY_FIELD(name, field, offset, size)				\

Re: [PATCH 04/11] UAPI: bcache: Fix use of embedded flexible array

[Jan Engelhardt]

On Wed, 05 Sep 2018 16:55:03 +0100, David Howells wrote:
>
>The bkey struct defined by bcache is embedded in the jset struct. However,
>this is illegal in C++ as there's a "flexible array" at the end of the struct.
>Change this to be a 0-length struct instead.
>
>-	__u64	ptr[];
>+	__u64	ptr[0];

As per the C++ standard, it is _also_ illegal to declare an array of size zero.

"""it [the array size expression] shall be a converted constant expression of
type std::size_t and its value shall be greater than zero."""
—http://eel.is/c++draft/dcl.array

That makes both "__u64 ptr[]" and "__u64 ptr[0]" *implementation-specific
extensions*.


3rd party tooling (concerns both C and C++):

Coverity Scan (IIRC) treats "__u64 ptr[0]" as an array of "definitely-zero"
size. Writing to any element will outright flag an out-of-bounds violation.
That is sensible, since only "ptr[]" was standardized.


Conclusion:

So please, do never use __u64 ptr[0].

Re: [PATCH 04/11] UAPI: bcache: Fix use of embedded flexible array

[David Howells]

Jan Engelhardt <jengelh@inai.de> wrote:

> """it [the array size expression] shall be a converted constant expression of
> type std::size_t and its value shall be greater than zero."""
> —http://eel.is/c++draft/dcl.array

Interesting.  You're not actually quoting the full sentence:

	If the constant-expression is present, it shall be a converted
	constant expression of type std​::​size_­t and its value shall be
	greater than zero.

This suggests that:

	__u64 ptr[]

is actually valid since:

	D1 [ constant-expressionopt ] attribute-specifier-seqopt

suggests that the part between the brackets is optional.

David

Re: [PATCH 04/11] UAPI: bcache: Fix use of embedded flexible array

[Jan Engelhardt]

On Tuesday 2018-10-09 17:41, David Howells wrote:

>Jan Engelhardt <jengelh@inai.de> wrote:
>
>> """it [the array size expression] shall be a converted constant expression of
>> type std::size_t and its value shall be greater than zero."""
>> —http://eel.is/c++draft/dcl.array
>
>Interesting.  You're not actually quoting the full sentence:
>
>	If the constant-expression is present, it shall be a converted
>	constant expression of type std​::​size_­t and its value shall be
>	greater than zero.
>
>This suggests that:
>
>	__u64 ptr[]
>
>is actually valid

I think that kind of validity only goes for this kind of standalone 
decl:

	extern int myints[];

but not for []-inside-struct.

Re: [RFC] UAPI: Check headers by compiling all together as C++

[Greg KH]

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

Re: [RFC] UAPI: Check headers by compiling all together as C++

[Yann Droneaud]

Hi,

Le mercredi 05 septembre 2018 à 18:55 +0200, Greg KH a écrit :
> 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++.

-- 
Yann Droneaud
OPTEYA


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Re: [RFC] UAPI: Check headers by compiling all together as C++

[Michael S. Tsirkin]

On Wed, Sep 05, 2018 at 07:33:38PM +0200, Yann Droneaud wrote:
> Hi,
> 
> Le mercredi 05 septembre 2018 à 18:55 +0200, Greg KH a écrit :
> > 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
> 
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Re: [RFC] UAPI: Check headers by compiling all together as C++

[Yann Droneaud]

Le mercredi 05 septembre 2018 à 19:33 +0200, Yann Droneaud a écrit :
> Le mercredi 05 septembre 2018 à 18:55 +0200, Greg KH a écrit :
> > 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++.
> 

For the record, libX11 has to handle the kink pf issue with C++
keyword:


https://gitlab.freedesktop.org/xorg/lib/libx11/blob/733f64bfeb311c1d040b2f751bfdef9c9d0f89ef/include/X11/Xlib.h#L227

typedef struct {
	XExtData *ext_data;	/* hook for extension to hang data */
	VisualID visualid;	/* visual id of this visual */
#if defined(__cplusplus) || defined(c_plusplus)
	int c_class;		/* C++ class of screen (monochrome, etc.) */
#else
	int class;		/* class of screen (monochrome, etc.) */
#endif
	unsigned long red_mask, green_mask, blue_mask;	/* mask values */
	int bits_per_rgb;	/* log base 2 of distinct color values */
	int map_entries;	/* color map entries */
} Visual;


Regards.

-- 
Yann Droneaud
OPTEYA


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Re: [RFC] UAPI: Check headers by compiling all together as C++

[Jan Engelhardt]

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.