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SubjectRe: [BUG mips llvm] MIPS: malformed R_MIPS_{HI16,LO16} with LLVM
Hi Alexander,
I'm genuinely trying to reproduce/understand this report, questions below:

On Sat, Jan 9, 2021 at 11:15 AM Alexander Lobakin <alobakin@pm.me> wrote:
>
> From: Nick Desaulniers <ndesaulniers@google.com>
> Date: Sat, 9 Jan 2021 09:50:44 -0800
>
> > On Sat, Jan 9, 2021 at 9:11 AM Alexander Lobakin <alobakin@pm.me> 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

Sorry, how are these calculated? (Explicit shell commands invoked
would be appreciated)

I'm doing:
$ ARCH=mips CROSS_COMPILE=mips-linux-gnu- make CC=clang -j71 32r2_defconfig
$ ARCH=mips CROSS_COMPILE=mips-linux-gnu- make CC=clang -j71 modules
$ llvm-nm --format=sysv drivers/net/phy/phy_device.o | grep phy_probe
$ llvm-objdump -Dr --disassemble-symbols=phy_driver_register
drivers/net/phy/phy_device.o
$ llvm-readelf -r drivers/net/phy/phy_device.o | grep -e R_MIPS_HI16
-e R_MIPS_LO16

for some of the commands trying to verify.

> >>
> >> 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.

The disassembly for me produces:
399c: 3c 03 00 00 lui $3, 0 <phy_device_free>
0000399c: R_MIPS_HI16 .text
...
39a8: 24 63 3a 5c addiu $3, $3, 14940 <phy_probe>
000039a8: R_MIPS_LO16 .text

I'm not really sure how to manually resolve the relocations; Fangrui
do you have any tips? (I'm coincidentally reading through Linkers &
Loaders currently, but only just started chpt. 4).

> >>
> >> 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 for the report. Sounds like we may indeed be producing an
> > incorrect relocation. This is only seen for big endian triples?
>
> Unfortunately I don't have a LE board to play with, so can confirm
> only Big Endian.
>
> (BTW, if someone can say if it's possible for MIPS (and how if it is)
> to launch a LE kernel from BE-booted preloader and U-Boot, that would
> be super cool)
>
> > Getting a way for us to deterministically reproduce would be a good
> > first step. Which config or configs beyond defconfig, and which
> > relocations specifically are you observing this with?
>
> I use `make 32r2_defconfig` which combines several configs from
> arch/mips/configs:
> - generic_defconfig;
> - generic/32r2.config;
> - generic/eb.config.
>
> Aside from that, I enable a bunch of my WIP drivers and the
> Netfilter. On my setup, this bug is always present in libphy.ko,
> so CONFIG_PHYLIB=m (with all deps) should be enough.
>
> The three failed relocs belongs to this part of code: [0]
>
> llvm-readelf on them:
>
> Relocation section '.rel.text' at offset 0xbf60 contains 2281 entries:¬
> [...]
> 00005740 00029305 R_MIPS_HI16 00000000 .text
> 00005744 00029306 R_MIPS_LO16 00000000 .text
> 00005720 00029305 R_MIPS_HI16 00000000 .text
> 00005748 00029306 R_MIPS_LO16 00000000 .text
> 0000573c 00029305 R_MIPS_HI16 00000000 .text
> 0000574c 00029306 R_MIPS_LO16 00000000 .text
>
> The first pair is the one from my first mail:
> 0x3c010000 <-- should be 0x3c010001 to work properly
> 0x24339444
>
> I'm planning to hunt for more now, will let you know.
>
> [0] https://elixir.bootlin.com/linux/v5.11-rc2/source/drivers/net/phy/phy_device.c#L2989
>
> > Thanks,
> > ~Nick Desaulniers
>
> Thanks,
> Al
>


--
Thanks,
~Nick Desaulniers

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