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Message-ID: <278cc21f-683e-89c0-75f6-e57d9c3bd82c@dd-wrt.com> Date: Mon, 11 Apr 2016 08:35:00 +0200 From: Sebastian Gottschall <s.gottschall@...wrt.com> To: musl@...ts.openwall.com Subject: Re: recvmsg/sendmsg broken on mips64 Am 11.04.2016 um 04:35 schrieb Rich Felker: > 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. i can install a broken musl libc again if that helps. (its plain openwrt toolchain result) > > 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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