On Sat, Jan 09, 2021 at 05:11:18PM +0000, Alexander Lobakin wrote: > Machine: MIPS32 R2 Big Endian (interAptiv (multi)) > > While testing MIPS with LLVM, I found a weird and very rare bug with > MIPS relocs that LLVM emits into kernel modules. It happens on both > 11.0.0 and latest git snapshot and applies, as I can see, only to > references to static symbols. > > When the kernel loads the module, it allocates a space for every > section and then manually apply the relocations relative to the > new address. > > Let's say we have a function phy_probe() in drivers/net/phy/libphy.ko. > It's static and referenced only in phy_register_driver(), where it's > used to fill callback pointer in a structure. > > The real function address after module loading is 0xc06c1444, that > is observed in its ELF st_value field. > There are two relocs related to this usage in phy_register_driver(): > > R_MIPS_HI16 refers to 0x3c010000 > R_MIPS_LO16 refers to 0x24339444 > > The address of .text is 0xc06b8000. So the destination is calculated > as follows: > > 0x00000000 from hi16; > 0xffff9444 from lo16 (sign extend as it's always treated as signed); > 0xc06b8000 from base. > > = 0xc06b1444. The value is lower than the real phy_probe() address > (0xc06c1444) by 0x10000 and is lower than the base address of > module's .text, so it's 100% incorrect. > > This results in: > > [ 2.204022] CPU 3 Unable to handle kernel paging request at virtual > address c06b1444, epc == c06b1444, ra == 803f1090 > > The correct instructions should be: > > R_MIPS_HI16 0x3c010001 > R_MIPS_LO16 0x24339444 > > so there'll be 0x00010000 from hi16. > > I tried to catch those bugs in arch/mips/kernel/module.c (by checking > if the destination is lower than the base address, which should never > happen), and seems like I have only 3 such places in libphy.ko (and > one in nf_tables.ko). > I don't think it should be handled somehow in mentioned source code > as it would look rather ugly and may break kernels build with GNU > stack, which seems to not produce such bad codes. > > If I should report this to any other resources, please let me know. > I chose clang-built-linux and LKML as it may not happen with userland > (didn't tried to catch). > > Thanks, > Al > Hi Alexander, Doubling back around to this as I was browsing through the LLVM 12.0.1 blockers on LLVM's bug tracker and I noticed a commit that could resolve this? It refers to the same relocations that you reference here. https://bugs.llvm.org/show_bug.cgi?id=49821 http://github.com/llvm/llvm-project/commit/7e83a7f1fdfcc2edde61f0a535f9d7a56f531db9 I think that Debian's apt.llvm.org repository should have a build available with that commit in it. Otherwise, building it from source is not too complicated with my script: https://github.com/ClangBuiltLinux/tc-build $ ./build-llvm.py --build-stage1-only --install-stage1-only --projects "clang;lld" --targets "Mips;X86" would get you a working toolchain relatively quickly. Cheers, Nathan
