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Date: Sun, 10 Apr 2016 18:18:13 -0400
From: Rich Felker <>
Subject: Re: recvmsg/sendmsg broken on mips64

On Fri, Apr 08, 2016 at 01:33:51AM +0200, Sebastian Gottschall wrote:
> Am 07.04.2016 um 20:46 schrieb Szabolcs Nagy:
> >* Sebastian Gottschall <> [2016-04-07 13:42:17 +0200]:
> >>>ok so the failure is in sendmsg and in the msg_control copy.
> >>>
> >>>does the call fail with ENOMEM (because >1024 bytes of ancillary data)?
> >>>that would be easy to fix..
> >>>
> >>>(libc has to make a copy, the struct is const and might be in
> >>>readonly memory. a detailed bug report of the failure would
> >>>be more useful than speculations about broken compilers..
> >>>e.g. strace log with and without the msg_control copying.)
> >>how to make a more detailed report than just that all netlink operations in
> >>iproute2 fail. so the whole ip command doesnt work.
> >there are only two places where msg->msg_control
> >is used in iproute2: bpf_scm.h and libnetlink.c,
> >they both use a fixed char[1024] buffer, which
> >should work with musl.
> >
> >one thing i noticed is that iproute2 fails to
> >take cmsghdr alignment requirements into account,
> >so it only works by accident.
> >
> >i think the musl struct has different alignment
> >(4 byte instead of 8 byte) which may cause problems
> >because the copy uses the musl alignment, i'm
> >not sure if this can cause what you observed.
> >
> >so we still don't know what your problem was
> >and what fails exactly.

> easy again. iproute2 doesnt work on mips64 targets since recvmsg /
> sendmsg call does fail, caused by a wrong structure alignment, bad
> structure at all or bad musl code at all.

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.

> 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.


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