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SubjectRe: [BUG mips llvm] MIPS: malformed R_MIPS_{HI16,LO16} with LLVM
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
>>
>> 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 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

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