Re: [PATCH libnetfilter_queue v2] src: examples: Use libnetfilter_queue cached linux headers throughout

[Duncan Roe <duncan_roe@optusnet.com.au>]

On Wed, Jul 07, 2021 at 12:52:31AM +0200, Pablo Neira Ayuso wrote:
> On Tue, Jul 06, 2021 at 03:36:48PM +1000, Duncan Roe wrote:
> > A user will typically copy nf-queue.c, make changes and compile: picking up
> > /usr/include/linux/nfnetlink_queue.h rather than
> > /usr/include/libnetfilter_queue/linux_nfnetlink_queue.h as is recommended.
> >
> > libnetfilter_queue.h already includes linux_nfnetlink_queue.h so we only need
> > to delete the errant line.
> >
> > (Running `make nf-queue` from within libnetfilter_queue/examples will get
> > the private cached version of nfnetlink_queue.h which is not distributed).
> >
> > Signed-off-by: Duncan Roe <duncan_roe@optusnet.com.au>
> > ---
> > v2: Don't insert a new #include
> >     Doesn't clash with other nearby patch
> >  examples/nf-queue.c | 1 -
> >  1 file changed, 1 deletion(-)
> >
> > diff --git a/examples/nf-queue.c b/examples/nf-queue.c
> > index 3da2c24..e4b33b5 100644
> > --- a/examples/nf-queue.c
> > +++ b/examples/nf-queue.c
> > @@ -11,7 +11,6 @@
> >  #include <linux/netfilter/nfnetlink.h>
> >
> >  #include <linux/types.h>
> > -#include <linux/netfilter/nfnetlink_queue.h>
>
> I remember now what the intention was.
>
> This include is fine as is: new applications should cache a copy of
> nfnetlink_queue.h in their own tree, that's the recommended way to go.
> This is the approach that we follow in other existing userspace
> netfilter codebase (ie. the userspace program caches the kernel UAPI
> header in the tree). The linux_nfnetlink_queue.h header is a legacy
> file only for backward compatibility, it should not be used for new
> software. This is not documented, the use of this include in
> examples/nf-queue.c was intentional.
>
> This approach also allows to fall back to the UAPI kernel headers that
> are installed in your system.
>
> Thanks.

Thanks for explaining. But I do see a glitch: if you put
> #include <libnetfilter_queue/libnetfilter_queue.h>
before
> #include <linux/netfilter/nfnetlink_queue.h>
then you will get libnetfilter_queue/linux_nfnetlink_queue.h because
libnetfilter_queue.h includes it.

IMHO it's highly undesirable for the order of #include stmts to make a
difference and we should do something about it.

If you like, I can submit a patch to remove the linux_nfnetlink_queue.h include
from libnetfilter_queue.h and add nfnetlink_queue.h to any sources which then
fail to compile.

Should I?

Cheers ... Duncan.