[iproute PATCH RFC] libnetlink: introduce DECLARE_NLREQ

[iproute PATCH RFC] libnetlink: introduce DECLARE_NLREQ

[Phil Sutter]

This macro aims to simplify most netlink users' pattern to prepare a
request, which is to create an unnamed struct and initialize it:

| struct {
|	struct nlmsghdr n;
|	struct whatever foo;
|	char buf[arbitrary number];
| } req;
|
| memset(&req, 0, sizeof(req));
| req.n.nlmsg_len = NLMSG_LENGTH(sizeof(struct whatever));
| req.n.nlmsg_flags = NLM_F_REQUEST;

Having this patch applied, the above can be replaced by a static
initializer like so:

| DECLARE_NLREQ(req, n, struct whatever foo, arbitrary number);

There is an added benefit, as well: Due to explicit alignment, the
requested tailroom is really as big as requested no matter what size
struct whatever really is.

Signed-off-by: Phil Sutter <phil@nwl.cc>
---
This patch is RFC because I want to wait for peer review and upstream
acceptance before sending in the big refactoring patch itself.
---
 include/libnetlink.h | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/include/libnetlink.h b/include/libnetlink.h
index 2280c39c670a4..7e12a7b12b55d 100644
--- a/include/libnetlink.h
+++ b/include/libnetlink.h
@@ -196,5 +196,16 @@ int rtnl_from_file(FILE *, rtnl_listen_filter_t handler,
  * messages from dump file */
 #define NLMSG_TSTAMP	15
 
+/* declare a netlink request with initial payload */
+#define DECLARE_NLREQ(name, hdrname, payload, tailroom)                       \
+	struct {                                                              \
+		struct nlmsghdr hdrname;                                      \
+		payload;                                                      \
+		char __b[tailroom] __attribute__((aligned(NLMSG_ALIGNTO)));   \
+	} name = { .hdrname = {                                               \
+		.nlmsg_len = (unsigned long)&name.__b - (unsigned long)&name, \
+		.nlmsg_flags = NLM_F_REQUEST,                                 \
+	}}
+
 #endif /* __LIBNETLINK_H__ */
 
-- 
2.5.0

Re: [iproute PATCH RFC] libnetlink: introduce DECLARE_NLREQ

[Hannes Frederic Sowa]

Phil Sutter <phil@nwl.cc> writes:

> This macro aims to simplify most netlink users' pattern to prepare a
> request, which is to create an unnamed struct and initialize it:
>
> | struct {
> |	struct nlmsghdr n;
> |	struct whatever foo;
> |	char buf[arbitrary number];
> | } req;
> |
> | memset(&req, 0, sizeof(req));
> | req.n.nlmsg_len = NLMSG_LENGTH(sizeof(struct whatever));
> | req.n.nlmsg_flags = NLM_F_REQUEST;
>
> Having this patch applied, the above can be replaced by a static
> initializer like so:
>
> | DECLARE_NLREQ(req, n, struct whatever foo, arbitrary number);
>
> There is an added benefit, as well: Due to explicit alignment, the
> requested tailroom is really as big as requested no matter what size
> struct whatever really is.
>
> Signed-off-by: Phil Sutter <phil@nwl.cc>
> ---
> This patch is RFC because I want to wait for peer review and upstream
> acceptance before sending in the big refactoring patch itself.
> ---
>  include/libnetlink.h | 11 +++++++++++
>  1 file changed, 11 insertions(+)
>
> diff --git a/include/libnetlink.h b/include/libnetlink.h
> index 2280c39c670a4..7e12a7b12b55d 100644
> --- a/include/libnetlink.h
> +++ b/include/libnetlink.h
> @@ -196,5 +196,16 @@ int rtnl_from_file(FILE *, rtnl_listen_filter_t handler,
>   * messages from dump file */
>  #define NLMSG_TSTAMP	15
>  
> +/* declare a netlink request with initial payload */
> +#define DECLARE_NLREQ(name, hdrname, payload, tailroom)                       \
> +	struct {                                                              \
> +		struct nlmsghdr hdrname;                                      \
> +		payload;                                                      \
> +		char __b[tailroom] __attribute__((aligned(NLMSG_ALIGNTO)));   \
> +	} name = { .hdrname = {                                               \
> +		.nlmsg_len = (unsigned long)&name.__b - (unsigned long)&name, \

offsetof(typeof(name), __b) ?

Re: [iproute PATCH RFC] libnetlink: introduce DECLARE_NLREQ

[Phil Sutter]

On Thu, Nov 26, 2015 at 02:56:30PM +0100, Hannes Frederic Sowa wrote:
> > +#define DECLARE_NLREQ(name, hdrname, payload, tailroom)                       \
> > +	struct {                                                              \
> > +		struct nlmsghdr hdrname;                                      \
> > +		payload;                                                      \
> > +		char __b[tailroom] __attribute__((aligned(NLMSG_ALIGNTO)));   \
> > +	} name = { .hdrname = {                                               \
> > +		.nlmsg_len = (unsigned long)&name.__b - (unsigned long)&name, \
> 
> offsetof(typeof(name), __b) ?

Ah, thanks! I already considered offsetof(), but the fact it needs a
type name and the declared struct is anonymous appeared unsolvable to
me. Good to know typeof() can be used this way!

Thanks, Phil

Re: [iproute PATCH RFC] libnetlink: introduce DECLARE_NLREQ

[Stephen Hemminger]

On Thu, 26 Nov 2015 14:26:05 +0100
Phil Sutter <phil@nwl.cc> wrote:

> This macro aims to simplify most netlink users' pattern to prepare a
> request, which is to create an unnamed struct and initialize it:
> 
> | struct {
> |	struct nlmsghdr n;
> |	struct whatever foo;
> |	char buf[arbitrary number];
> | } req;
> |
> | memset(&req, 0, sizeof(req));
> | req.n.nlmsg_len = NLMSG_LENGTH(sizeof(struct whatever));
> | req.n.nlmsg_flags = NLM_F_REQUEST;
> 
> Having this patch applied, the above can be replaced by a static
> initializer like so:
> 
> | DECLARE_NLREQ(req, n, struct whatever foo, arbitrary number);
> 
> There is an added benefit, as well: Due to explicit alignment, the
> requested tailroom is really as big as requested no matter what size
> struct whatever really is.
> 
> Signed-off-by: Phil Sutter <phil@nwl.cc>
> ---
> This patch is RFC because I want to wait for peer review and upstream
> acceptance before sending in the big refactoring patch itself.
> ---
>  include/libnetlink.h | 11 +++++++++++
>  1 file changed, 11 insertions(+)

I am not a fan of complex macros. But netlink seems to get lots of them.
You need to add more parens round arguments (like name).

Really longterm would rather iproute2 switched to a cleaner library like libmnl

Re: [iproute PATCH RFC] libnetlink: introduce DECLARE_NLREQ

[Phil Sutter]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

On Sun, Nov 29, 2015 at 12:07:52PM -0800, Stephen Hemminger wrote:
> On Thu, 26 Nov 2015 14:26:05 +0100
> Phil Sutter <phil@nwl.cc> wrote:
> 
> > This macro aims to simplify most netlink users' pattern to prepare a
> > request, which is to create an unnamed struct and initialize it:
> > 
> > | struct {
> > |	struct nlmsghdr n;
> > |	struct whatever foo;
> > |	char buf[arbitrary number];
> > | } req;
> > |
> > | memset(&req, 0, sizeof(req));
> > | req.n.nlmsg_len = NLMSG_LENGTH(sizeof(struct whatever));
> > | req.n.nlmsg_flags = NLM_F_REQUEST;
> > 
> > Having this patch applied, the above can be replaced by a static
> > initializer like so:
> > 
> > | DECLARE_NLREQ(req, n, struct whatever foo, arbitrary number);
> > 
> > There is an added benefit, as well: Due to explicit alignment, the
> > requested tailroom is really as big as requested no matter what size
> > struct whatever really is.
> > 
> > Signed-off-by: Phil Sutter <phil@nwl.cc>
> > ---
> > This patch is RFC because I want to wait for peer review and upstream
> > acceptance before sending in the big refactoring patch itself.
> > ---
> >  include/libnetlink.h | 11 +++++++++++
> >  1 file changed, 11 insertions(+)
> 
> I am not a fan of complex macros. But netlink seems to get lots of them.
> You need to add more parens round arguments (like name).
> 
> Really longterm would rather iproute2 switched to a cleaner library like libmnl

Ah, indeed! Someone else suggested that once, as well. I'll have a look
into that before continuing this path.

Thanks, Phil
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Re: [iproute PATCH RFC] libnetlink: introduce DECLARE_NLREQ

[Phil Sutter]

On Sun, Nov 29, 2015 at 12:07:52PM -0800, Stephen Hemminger wrote:
> On Thu, 26 Nov 2015 14:26:05 +0100
> Phil Sutter <phil@nwl.cc> wrote:
> 
> > This macro aims to simplify most netlink users' pattern to prepare a
> > request, which is to create an unnamed struct and initialize it:
> > 
> > | struct {
> > |	struct nlmsghdr n;
> > |	struct whatever foo;
> > |	char buf[arbitrary number];
> > | } req;
> > |
> > | memset(&req, 0, sizeof(req));
> > | req.n.nlmsg_len = NLMSG_LENGTH(sizeof(struct whatever));
> > | req.n.nlmsg_flags = NLM_F_REQUEST;
> > 
> > Having this patch applied, the above can be replaced by a static
> > initializer like so:
> > 
> > | DECLARE_NLREQ(req, n, struct whatever foo, arbitrary number);
> > 
> > There is an added benefit, as well: Due to explicit alignment, the
> > requested tailroom is really as big as requested no matter what size
> > struct whatever really is.
> > 
> > Signed-off-by: Phil Sutter <phil@nwl.cc>
> > ---
> > This patch is RFC because I want to wait for peer review and upstream
> > acceptance before sending in the big refactoring patch itself.
> > ---
> >  include/libnetlink.h | 11 +++++++++++
> >  1 file changed, 11 insertions(+)
> 
> I am not a fan of complex macros. But netlink seems to get lots of them.
> You need to add more parens round arguments (like name).
> 
> Really longterm would rather iproute2 switched to a cleaner library like libmnl

libmnl looks nice and simple (unlike libnl I was initially looking at by
accident). Now how to pull this off:

I don't think mandatorily depending on libmnl will be acceptable, do
you? So I can imagine two ways to do this:

A) Have a libmnl version of lib/libnetlink.c which is used instead of
   the old one if libmnl is present.

B) Pull a copy of libmnl into iproute2 sources so it's always available
   (as fallback) and make it replace lib/libnetlink.c. This sounds worse
   than it is, using git-subtree allows to do this without imposing user
   knowledge about it (like git-submodule does).

What do you think?

Cheers, Phil

Re: [iproute PATCH RFC] libnetlink: introduce DECLARE_NLREQ

[Stephen Hemminger]

On Mon, 30 Nov 2015 16:47:25 +0100
Phil Sutter <phil@nwl.cc> wrote:

> libmnl looks nice and simple (unlike libnl I was initially looking at by
> accident). Now how to pull this off:
> 
> I don't think mandatorily depending on libmnl will be acceptable, do
> you? So I can imagine two ways to do this:

Having libmnl be mandatory is fine, but please put in net-next.
Every distro has libmnl and as long as it is documented not a big deal.

> A) Have a libmnl version of lib/libnetlink.c which is used instead of
>    the old one if libmnl is present.
> 
> B) Pull a copy of libmnl into iproute2 sources so it's always available
>    (as fallback) and make it replace lib/libnetlink.c. This sounds worse
>    than it is, using git-subtree allows to do this without imposing user
>    knowledge about it (like git-submodule does).

Just incrementally change code to use libmnl instead of libnetlink.
Start with simple stuff.