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Message-ID: <a7ccbb4a-4b2b-1c4a-6a05-ac3f8f155101@dd-wrt.com> Date: Mon, 11 Apr 2016 00:24:49 +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 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@...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 >
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