Re: recvmsg/sendmsg broken on mips64

[Sebastian Gottschall <s.gottschall@dd-wrt.com>]

Am 11.04.2016 um 00:18 schrieb Rich Felker:
> 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 <s.gottschall@dd-wrt.com> [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.
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

>
>> 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.
>
> Rich
>