Re: recvmsg/sendmsg broken on mips64

[Rich Felker <dalias@libc.org>]

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 192.168.1.1/24  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 libc.so 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 127.0.0.2 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.

Rich