From: Nathan Chancellor <nathan@xxxxxxxxxx> Date: Fri, 7 May 2021 00:47:14 -0700 > 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. > > > > =3D 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 =3D=3D c06b1444, ra =3D=3D 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, Hi! > 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=3D49821 > > http://github.com/llvm/llvm-project/commit/7e83a7f1fdfcc2edde61f0a535f9d7a= > 56f531db9 This really seems very related to the bug I encountered. Currently I don't have a MIPS setup to try since I've moved to another country, but I should "deploy" it again soon. So I'll definitely take a look a bit later, thanks for pointing on this commit! > 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 "cl= > ang;lld" --targets "Mips;X86" > > would get you a working toolchain relatively quickly. I could just build llvm-git from Arch Linux User Repository :) I did that last time when was checking if the latest snapshot also suffers from the bug, and I think it didn't take much time to build. > Cheers, > Nathan Thanks, Al
