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SubjectRe: [BUG mips llvm] MIPS: malformed R_MIPS_{HI16,LO16} with LLVM
From: Alexander Lobakin <alobakin@pm.me>
Date: Sat, 09 Jan 2021 19:15:31 +0000

> 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 =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 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.

Unfortunately, R_MIPS_32 also suffers from that. And unlikely
R_MIPS_{HI,LO}16, they can't be handled runtime as the values
are pure random.
I expanded arch/mips/kernel/module.c a bit, so it tries to find
the actual symbol in .symtab after each applied relocation and
print the detailed info. Here's an example from nf_defrag_ipv6
loading:

[ 429.789793] nf_defrag_ipv6: final section addresses:
[ 429.795409] 0xc07214fc __ksymtab_gpl
[ 429.799574] 0xc0720000 .text
[ 429.802902] 0xc07216b0 .data
[ 429.806249] 0xc0721790 .bss
[ 429.809474] 0xc0721508 __ksymtab_strings
[ 429.813977] 0xc0728000 .init.text
[ 429.817781] 0xc07214c0 .exit.text
[ 429.821606] 0xc0721520 .rodata
[ 429.825120] 0xc0721578 .rodata.str1.1
[ 429.829322] 0xc0721638 .note.Linux
[ 429.833226] 0xc0721800 .gnu.linkonce.this_module
[ 429.838503] 0xc0721650 .MIPS.abiflags
[ 429.842702] 0xc0721668 .reginfo
[ 429.846326] 0xc0721680 .note.gnu.build-id
[ 429.851129] nf_defrag_ipv6: R_MIPS_32 [0x00000008]: 0xc07216b0 -> 0xc07216b8 is broken
[ 429.860017] nf_defrag_ipv6: R_MIPS_32 [0x00000008]: 0xc07216b0 -> 0xc07216b8 is broken
[ 429.868875] nf_defrag_ipv6: R_MIPS_32 [0x00000138]: 0xc0720000 -> 0xc0720138 is defrag6_net_exit
[ 429.878706] nf_defrag_ipv6: R_MIPS_32 [0x000012c8]: 0xc0720000 -> 0xc07212c8 is nf_ct_net_init
[ 429.888335] nf_defrag_ipv6: R_MIPS_32 [0x0000142c]: 0xc0720000 -> 0xc072142c is nf_ct_net_pre_exit
[ 429.898367] nf_defrag_ipv6: R_MIPS_32 [0x00001440]: 0xc0720000 -> 0xc0721440 is nf_ct_net_exit
[ 429.907994] nf_defrag_ipv6: R_MIPS_32 [0x00000057]: 0xc0721578 -> 0xc07215cf is broken
[ 429.916872] nf_defrag_ipv6: R_MIPS_32 [0x00000000]: 0x80f297f0 -> 0x80f297f0 is proc_dointvec_jiffies
[ 429.927177] nf_defrag_ipv6: R_MIPS_32 [0x00000039]: 0xc0721578 -> 0xc07215b1 is broken
[ 429.936044] nf_defrag_ipv6: R_MIPS_32 [0x00000000]: 0x80f29374 -> 0x80f29374 is proc_doulongvec_minmax
[ 429.946453] nf_defrag_ipv6: R_MIPS_32 [0x00000072]: 0xc0721578 -> 0xc07215ea is broken
[ 429.955320] nf_defrag_ipv6: R_MIPS_32 [0x00000000]: 0x80f29374 -> 0x80f29374 is proc_doulongvec_minmax
[ 429.965737] nf_defrag_ipv6: R_MIPS_32 [0x000000a4]: 0xc0720000 -> 0xc07200a4 is ipv6_defrag
[ 429.975094] nf_defrag_ipv6: R_MIPS_32 [0x000000a4]: 0xc0720000 -> 0xc07200a4 is ipv6_defrag
[ 429.984431] nf_defrag_ipv6: R_MIPS_32 [0x0000106c]: 0xc0720000 -> 0xc072106c is ip6frag_key_hashfn
[ 429.994470] nf_defrag_ipv6: R_MIPS_32 [0x00001090]: 0xc0720000 -> 0xc0721090 is ip6frag_obj_hashfn
[ 430.004486] nf_defrag_ipv6: R_MIPS_32 [0x000010b8]: 0xc0720000 -> 0xc07210b8 is ip6frag_obj_cmpfn
[ 430.014425] nf_defrag_ipv6: R_MIPS_32 [0x00000000]: 0xc0720000 -> 0xc0720000 is nf_defrag_ipv6_enable
[ 430.024742] nf_defrag_ipv6: R_MIPS_32 [0x00000001]: 0xc0721508 -> 0xc0721509 is __kstrtab_nf_defrag_ipv6_enable
[ 430.036074] nf_defrag_ipv6: R_MIPS_32 [0x00000000]: 0xc0721508 -> 0xc0721508 is __kstrtabns_nf_defrag_ipv6_enable
[ 430.047561] nf_defrag_ipv6: R_MIPS_32 [0x00000000]: 0xc0728000 -> 0xc0728000 is init_module
[ 430.056930] nf_defrag_ipv6: R_MIPS_32 [0x00000000]: 0xc07214c0 -> 0xc07214c0 is cleanup_module

At least five symbols are broken and lead to nowhere: two from .data
and three from .rodata. Values in square braces are initial references
that can be observed via `nm -n` -- and for broken ones they really
don't correspond to any symbols, mismatching the neighbours' addresses
by 0x40-0x50.

So for now seems like it's really an LLVM problem and there can't be
any simple workaround for it in the kernel.

> [0] https://elixir.bootlin.com/linux/v5.11-rc2/source/drivers/net/phy/phy_device.c#L2989
>
>> Thanks,
>> ~Nick Desaulniers
>
> Thanks,
> Al

Al

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