| Message ID | 1346338894-12600-1-git-send-email-ljaenicke@innominate.com |
|---|---|
| State | Accepted, archived |
| Delegated to: | stephen hemminger |
| Headers | show |
> + /* attribute has to be NLMSG aligned */ > + struct rtattr ext_req __attribute__((aligned(NLMSG_ALIGNTO))); Would it be better to apply the attribute to the definition of 'struct rtattr' itself ? David -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Thu, Aug 30, 2012 at 04:51:49PM +0100, David Laight wrote: > > + /* attribute has to be NLMSG aligned */ > > + struct rtattr ext_req > __attribute__((aligned(NLMSG_ALIGNTO))); > > Would it be better to apply the attribute to the definition > of 'struct rtattr' itself ? 'struct rtattr' is defined in a kernel header and I am not really sure what kind of side effects it would have. The problem I intend to solve with my page is rather caused by the use case: The request is sent as a single structure, the processing is however performed step by step: first the nlmsg is parsed, then the pointer is moved on to the parsing of the content. This pointer is hoever not oriented at the structure element but by using size information which is then adjust by NLMSG_ALIGN() etc. Consequently one should actually use the same method in sending (generate the header first, then addattr_() which enforces protocol conform alignment). This is the way all other instances in iproute2 generate their messages btw. I hence consider my proposed patch to be the least intrusive solution. Best regards, Lutz
diff --git a/lib/libnetlink.c b/lib/libnetlink.c index 8e8c8b9..09b4277 100644 --- a/lib/libnetlink.c +++ b/lib/libnetlink.c @@ -94,8 +94,8 @@ int rtnl_wilddump_request(struct rtnl_handle *rth, int family, int type) struct { struct nlmsghdr nlh; struct rtgenmsg g; - __u16 align_rta; /* attribute has to be 32bit aligned */ - struct rtattr ext_req; + /* attribute has to be NLMSG aligned */ + struct rtattr ext_req __attribute__ ((aligned(NLMSG_ALIGNTO))); __u32 ext_filter_mask; } req;
Platforms have different alignment requirements which need to be fulfilled by the compiler. If the structure elements are already 4 byte (NLMGS_ALIGNTO) aligned by the compiler adding an explicit padding element (align_rta) is not allowed. Use __attribute__ ((aligned (NLMSG_ALIGNTO))) in order to achieve the required alignment. Experienced on ARM (xscale) with symptom netlink: 12 bytes leftover after parsing attributes Tested on: ARM (32bit Big Endian) PowerPC (32bit Big Endian) x86_64 (64bit Little Endian) Each with different aligment requirments. Signed-off-by: Lutz Jaenicke <ljaenicke@innominate.com> --- lib/libnetlink.c | 4 ++-- 1 files changed, 2 insertions(+), 2 deletions(-)