Date: Thu, 21 Apr 2016 21:30:37 +0200 From: Sebastian Gottschall <s.gottschall@...wrt.com> To: musl@...ts.openwall.com Subject: Re: recvmsg/sendmsg broken on mips64 Am 21.04.2016 um 19:16 schrieb Rich Felker: > On Thu, Apr 21, 2016 at 11:36:37AM -0400, Rich Felker wrote: >>>> I've managed to track down the cause of the breakage. Somehow your >>>> iproute2 has been miscompiled. What I did was add debug logic to >>>> libc.so to print the contents of the msghdr struct passed in before >>>> fixups, after fixups, and after the syscall. The output I got was: >>>> >>>> msghdr: 0xffffd58e08 12 0xffffd58df8 1 0 0 0 0 0 >>>> msghdr: 0xffffd58e08 12 0xffffd58df8 0 0 0 0 0 0 >>>> msghdr: 0xffffd58e08 12 0xffffd58df8 0 0 0 0 0 32 >>>> >>>> The fields (including __pad1 and __pad2) are printed in order. So as >>>> you can see, ip passed in a structure with a 1 in __pad1 and a 0 in >>>> msg_iovlen. The source (libnetlink.c) stores 1 to msg_iovlen, so my >>>> guess is that somehow it ended up getting the wrong-endian version of >>>> the structure definition. You could confirm this by adding #error to >>>> the little-endian case in arch/mips64/bits/socket.h and recompiling. I >>>> suspect it's going to take some additional work to track down the >>>> cause, which is likely specific to something in your toolchain (it >>>> didn't happen for me when I built my own iproute2). >>> i tried that already before i contacted you. the #error case never >>> raises within the little endian case >> Was that when compiling musl or iproute2? The problem is in how >> iproute2 was built; your libc.so seems fine. >> >>> so your guess doesnt match reality. (i even tried it again right >>> now. all is fine. it only uses the big endian case) >> If it's not the endian tests, I don't know what else would have caused >> this. I'll get a disassembly dump of the function to show you. Is >> there any way I can reproduce your exact toolchain to see if I can get >> the same miscompilation to happen? > OK, I finally found the source you're building from and tracked down > the problem, which is simply that you have a buggy, 10-year-outdated > version of iproute2's libnetlink.c. The relevant code is here: > > https://github.com/mirror/dd-wrt/blob/25e48ec1931daf4ef98a91ada9623638d128f34d/src/router/iproute2/lib/libnetlink.c#L156 > > Rather than using designated initializers as the current code does: > > http://git.kernel.org/cgit/linux/kernel/git/shemminger/iproute2.git/tree/lib/libnetlink.c?id=4bf138d6d2747b198fc0a78f5fe4e1c9287e9e90#n220 > > it's simply assuming an order for the members of struct msghdr. There > are several ways you could fix this: > > 1. Update to a modern version of iproute2. This would probably fix a > lot of other bugs too. > > 2. Copy the designated-initializers approach from the modern code into > your version. > > 3. Just use a zero-initializer for the structure and then assign > values to individual members by name with ordinary assignments. okay. will try > > Let me know if you need any more info. > > Rich >
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