Re: recvmsg/sendmsg broken on mips64

[Rich Felker <dalias@libc.org>]

On Thu, Apr 21, 2016 at 11:36:37AM -0400, Rich Felker wrote:
> > >I've managed to track down the cause of the breakage. Somehow your
> > >iproute2 has been miscompiled. What I did was add debug logic to
> > >libc.so to print the contents of the msghdr struct passed in before
> > >fixups, after fixups, and after the syscall. The output I got was:
> > >
> > >msghdr: 0xffffd58e08 12 0xffffd58df8 1 0 0 0 0 0
> > >msghdr: 0xffffd58e08 12 0xffffd58df8 0 0 0 0 0 0
> > >msghdr: 0xffffd58e08 12 0xffffd58df8 0 0 0 0 0 32
> > >
> > >The fields (including __pad1 and __pad2) are printed in order. So as
> > >you can see, ip passed in a structure with a 1 in __pad1 and a 0 in
> > >msg_iovlen. The source (libnetlink.c) stores 1 to msg_iovlen, so my
> > >guess is that somehow it ended up getting the wrong-endian version of
> > >the structure definition. You could confirm this by adding #error to
> > >the little-endian case in arch/mips64/bits/socket.h and recompiling. I
> > >suspect it's going to take some additional work to track down the
> > >cause, which is likely specific to something in your toolchain (it
> > >didn't happen for me when I built my own iproute2).
> > i tried that already before i contacted you. the #error case never
> > raises within the little endian case
> 
> Was that when compiling musl or iproute2? The problem is in how
> iproute2 was built; your libc.so seems fine.
> 
> > so your guess doesnt match reality. (i even tried it again right
> > now. all is fine. it only uses the big endian case)
> 
> If it's not the endian tests, I don't know what else would have caused
> this. I'll get a disassembly dump of the function to show you. Is
> there any way I can reproduce your exact toolchain to see if I can get
> the same miscompilation to happen?

OK, I finally found the source you're building from and tracked down
the problem, which is simply that you have a buggy, 10-year-outdated
version of iproute2's libnetlink.c. The relevant code is here:

https://github.com/mirror/dd-wrt/blob/25e48ec1931daf4ef98a91ada9623638d128f34d/src/router/iproute2/lib/libnetlink.c#L156

Rather than using designated initializers as the current code does:

http://git.kernel.org/cgit/linux/kernel/git/shemminger/iproute2.git/tree/lib/libnetlink.c?id=4bf138d6d2747b198fc0a78f5fe4e1c9287e9e90#n220

it's simply assuming an order for the members of struct msghdr. There
are several ways you could fix this:

1. Update to a modern version of iproute2. This would probably fix a
   lot of other bugs too.

2. Copy the designated-initializers approach from the modern code into
   your version.

3. Just use a zero-initializer for the structure and then assign
   values to individual members by name with ordinary assignments.

Let me know if you need any more info.

Rich