Date: Sun, 10 Apr 2016 22:35:22 -0400 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: recvmsg/sendmsg broken on mips64 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
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