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Date: Wed, 20 Apr 2016 21:37:16 -0400
From: Rich Felker <>
Subject: Re: recvmsg/sendmsg broken on mips64

On Sun, Apr 10, 2016 at 10:35:22PM -0400, Rich Felker wrote:
> On Mon, Apr 11, 2016 at 12:33:07AM +0200, Sebastian Gottschall wrote:
> > Am 11.04.2016 um 00:29 schrieb Rich Felker:
> > >On Mon, Apr 11, 2016 at 12:24:49AM +0200, Sebastian Gottschall wrote:
> > >>>I think what nsz was asking for, and what I'd like to see, is a way to
> > >>>reproduce the bug. I'm going to try building iproute2 for mips64 and
> > >>>running it on a prebuilt kernel from Aboriginal Linux under
> > >>>qemu-system-mips64, but I don't know what specific commands are needed
> > >>>to hit the affected code path.
> > >>any command since all is netlink based
> > >>ip add add  dev eth0
> > >>
> > >>yo will see that nothing will happen. ip will just return a error
> > >>message (i wrote this message already in the first entry on this
> > >>mailinglist)
> > >>"EOF on netlink" is the error which is shown
> > >OK, I'll try this.
> > >
> > >>>>its all resulting in the same failing recvmsg / sendmsg call.. so
> > >>>>yes libnetlink.c does not work with musl on mips64 (it does work on
> > >>>>x64 and everything else, just not mips64) unless the hack i offered
> > >>>>was applied which again fixed all.
> > >>>>before you ask again for a problem description, just read again. it
> > >>>>wont change the description if you ask again and just makes people
> > >>>>tired on this list.
> > >>>Both versions of the struct (musl's and your modified one that matches
> > >>>the kernel) have the exact same layout, but due to having a member
> > >>>with 64-bit type, yours has 8-byte alignment and musl's only has
> > >>>4-byte alignment. This means, at least:
> > >>>
> > >>>1. When musl's sendmsg.c makes its copy to zero out the padding, the
> > >>>    copy may not be correctly aligned for 64-bit writes, and the kernel
> > >>>    faults or manually produces an error for this case, causing the
> > >>>    whole operation to fail. However, I don't see where iproute2 is
> > >>>    actually passing control messages to sendmsg, so while this is a
> > >>>    problem, I don't think it's the cause. Maybe I'm missing the
> > >>>    affected call point; this is why I'd like steps to reproduce the
> > >>>    issue so I can see it.
> > >>>
> > >>>2. iproute2's libnetlink.c's rtnl_listen function does not properly
> > >>>    declare its cmsgbuf with the alignment of cmsghdr; it has type
> > >>>    char[] so the compiler is free not to align it at all. This is
> > >>>    presumably a bug in iproute2, but I can't find any good
> > >>>    documentation (in the standards or Linux-specific) for how you're
> > >>>    supposed to allocate this space, so maybe the kernel is able to
> > >>>    handle aligning the buffer itself. I don't see any way the
> > >>>    alignment of musl's cmsghdr type affects recvmsg though.
> > >>>
> > >>>Maybe there are other effects I'm missing? I'll follow up again once I
> > >>>get a test build/run of iproute2 and let you know whether I can see
> > >>>the problem.
> > >>okay. if you need a remote access to a octeon system using musl (my
> > >>fixed variant), just tell me.
> > >That would be really helpful. Something's wrong with the userspace for
> > >the Aboriginal mips64 binaries (SIGBUS in init) and debugging that
> > >would be a big distraction.
> > >
> > >BTW do you have gdb and strace available?
> > not on the system itself. i'm not sure if strace works on mips64.
> > never tried it.
> > but you're free to copy any binary to the /tmp dir. it has 2 gb ram.
> > so enough space for static binaries if you want to play with.
> > i will send you the ssh data in a private email
> I haven't been able to reproduce the error on your system. I've tried
> building my own static-linked version of the "ip" utility with a
> mips64-linux-musl softfloat compiler, and uploading my and
> using it to run both your version of ip and a dynamic-linked one I
> just built. They all work fine for adding/removing a address
> to the "lo" interface.
> Next I'm going to try to get a minimal testcase that tries to
> intentionally misalign the control message buffers. I suspect I'm just
> "getting lucky" and my buffer happens to be aligned the way the kernel
> wants by chance.

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 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).

In case you or anyone else would like to use the struct dumping in
testing, or just understand precisely what it's printing, I'm
attaching the patch I used.


View attachment "recvmsg-dumper.diff" of type "text/plain" (847 bytes)

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