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SubjectRe: [BUG mips llvm] MIPS: malformed R_MIPS_{HI16,LO16} with LLVM
From: Nick Desaulniers <ndesaulniers@google.com>
Date: Mon, 11 Jan 2021 12:03:29 -0800

> Hi Alexander,
> I'm genuinely trying to reproduce/understand this report, questions below:

Hi Nick!

> 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)

Just look at the disassembly below and you'll see the similar
instructions in your module.

> 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

So, in your case the values of the instructions that relocs refer are:

0x3c030000 R_MIPS_HI16
0x24633a5c R_MIPS_LO16

Mine were:

0x3c010000
0x24339444

Your second one doesn't have bit 15 set, so I think this pair won't
break the code.
Try to hunt for R_MIPS_LO16 that have this bit set, i.e. they have
'8', '9', 'a', 'b', 'c', 'd' or 'e' as their [15:12].

> 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

Thanks,
Al

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