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    SubjectRe: [BUG mips llvm] MIPS: malformed R_MIPS_{HI16,LO16} with LLVM
    From: Nathan Chancellor <nathan@kernel.org>
    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

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