| Message ID | e9409957290af5249750afa0f10de3a6@linux.vnet.ibm.com |
|---|---|
| State | Not Applicable, archived |
| Delegated to: | David Miller |
| Headers | show |
> Lets change the example so others don't propagate the problem further. > > Signed-off-by David Wilder <dwilder@us.ibm.com> > > --- man7/netlink.7.orig 2016-11-14 13:30:36.522101156 -0800 > +++ man7/netlink.7 2016-11-14 13:30:51.002086354 -0800 > @@ -511,7 +511,7 @@ > .in +4n > .nf > int len; > -char buf[4096]; > +char buf[8192]; Since there doesn't seem to be a define one could use in the user space linux/netlink.h (?), but there are comments in the example code in the manpage, how about also including a brief comment to the effect that using 8192 bytes will avoid message truncation problems on platforms with a large PAGE_SIZE? /* avoid msg truncation on > 4096 byte PAGE_SIZE platforms */ or something like that. rick jones
On 11/14/2016 11:20 PM, dwilder wrote: > The example code in netlink(7) (for reading netlink message) suggests > using > a 4k read buffer with recvmsg. This can cause truncated messages on > systems > using a page size is >4096. Please see: > linux/include/linux/netlink.h (in the kernel source) > > <snip> > /* > * skb should fit one page. This choice is good for headerless > malloc. > * But we should limit to 8K so that userspace does not have to > * use enormous buffer sizes on recvmsg() calls just to avoid > * MSG_TRUNC when PAGE_SIZE is very large. > */ > #if PAGE_SIZE < 8192UL > #define NLMSG_GOODSIZE SKB_WITH_OVERHEAD(PAGE_SIZE) > #else > #define NLMSG_GOODSIZE SKB_WITH_OVERHEAD(8192UL) > #endif > > #define NLMSG_DEFAULT_SIZE (NLMSG_GOODSIZE - NLMSG_HDRLEN) > <snip> > > I was troubleshooting some up-stream code on a ppc64le system > (page:size of 64k) This code had duplicated the example from netlink(7) > and > was using a 4k buffer. On x86-64 with a 4k page size this is not a > problem, > however on the 64k page system some messages were truncated. Using an > 8k buffer > as implied in netlink.h prevents problems with any page size. > > Lets change the example so others don't propagate the problem further. > > Signed-off-by David Wilder <dwilder@us.ibm.com> Thanks, David. Patch applied. Cheers, Michael
On 11/14/2016 11:36 PM, Rick Jones wrote: >> Lets change the example so others don't propagate the problem further. >> >> Signed-off-by David Wilder <dwilder@us.ibm.com> >> >> --- man7/netlink.7.orig 2016-11-14 13:30:36.522101156 -0800 >> +++ man7/netlink.7 2016-11-14 13:30:51.002086354 -0800 >> @@ -511,7 +511,7 @@ >> .in +4n >> .nf >> int len; >> -char buf[4096]; >> +char buf[8192]; > > Since there doesn't seem to be a define one could use in the user space > linux/netlink.h (?), but there are comments in the example code in the > manpage, how about also including a brief comment to the effect that > using 8192 bytes will avoid message truncation problems on platforms > with a large PAGE_SIZE? > > /* avoid msg truncation on > 4096 byte PAGE_SIZE platforms */ > > or something like that. Thanks for the suggestion, Rick. Done! Cheers, Michael
--- man7/netlink.7.orig 2016-11-14 13:30:36.522101156 -0800 +++ man7/netlink.7 2016-11-14 13:30:51.002086354 -0800 @@ -511,7 +511,7 @@ .in +4n .nf int len; -char buf[4096]; +char buf[8192]; struct iovec iov = { buf, sizeof(buf) }; struct sockaddr_nl sa; struct msghdr msg;