Date:   Wed, 14 Mar 2018 20:20:47 +0800
From:   Zong Li <zongbox@...il.com>
To:     Shea Levy <shea@...alevy.com>
Cc:     Palmer Dabbelt <palmer@...ive.com>, Zong Li <zong@...estech.com>,
        albert@...ive.com, linux-riscv@...ts.infradead.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        greentime@...estech.com
Subject: Re: [PATCH 00/11] RISC-V: Resolve the issue of loadable module on 64-bit

2018-03-14 19:56 GMT+08:00 Shea Levy <shea@...alevy.com>:
> Zong Li <zongbox@...il.com> writes:
>
>> 2018-03-14 11:07 GMT+08:00 Palmer Dabbelt <palmer@...ive.com>:
>>> On Tue, 13 Mar 2018 18:34:19 PDT (-0700), zongbox@...il.com wrote:
>>>>
>>>> 2018-03-14 5:30 GMT+08:00 Shea Levy <shea@...alevy.com>:
>>>>>
>>>>> Hi Palmer,
>>>>>
>>>>> Palmer Dabbelt <palmer@...ive.com> writes:
>>>>>
>>>>>> On Tue, 13 Mar 2018 01:35:05 PDT (-0700), zong@...estech.com wrote:
>>>>>>>
>>>>>>> These patches resolve the some issues of loadable module.
>>>>>>>   - symbol out of ranges
>>>>>>>   - unknown relocation types
>>>>>>>
>>>>>>> The reference of external variable and function symbols
>>>>>>> cannot exceed 32-bit offset ranges in kernel module.
>>>>>>> The module only can work on the 32-bit OS or the 64-bit
>>>>>>> OS with sv32 virtual addressing.
>>>>>>>
>>>>>>> These patches will generate the .got, .got.plt and
>>>>>>> .plt sections during loading module, let it can refer
>>>>>>> to the symbol which locate more than 32-bit offset.
>>>>>>> These sections depend on the relocation types:
>>>>>>>  - R_RISCV_GOT_HI20
>>>>>>>  - R_RISCV_CALL_PLT
>>>>>>>
>>>>>>> These patches also support more relocation types
>>>>>>>  - R_RISCV_CALL
>>>>>>>  - R_RISCV_HI20
>>>>>>>  - R_RISCV_LO12_I
>>>>>>>  - R_RISCV_LO12_S
>>>>>>>  - R_RISCV_RVC_BRANCH
>>>>>>>  - R_RISCV_RVC_JUMP
>>>>>>>  - R_RISCV_ALIGN
>>>>>>>  - R_RISCV_ADD32
>>>>>>>  - R_RISCV_SUB32
>>>>>>>
>>>>>>> Zong Li (11):
>>>>>>>   RISC-V: Add sections of PLT and GOT for kernel module
>>>>>>>   RISC-V: Add section of GOT.PLT for kernel module
>>>>>>>   RISC-V: Support GOT_HI20/CALL_PLT relocation type in kernel module
>>>>>>>   RISC-V: Support CALL relocation type in kernel module
>>>>>>>   RISC-V: Support HI20/LO12_I/LO12_S relocation type in kernel module
>>>>>>>   RISC-V: Support RVC_BRANCH/JUMP relocation type in kernel modulewq
>>>>>>>   RISC-V: Support ALIGN relocation type in kernel module
>>>>>>>   RISC-V: Support ADD32 relocation type in kernel module
>>>>>>>   RISC-V: Support SUB32 relocation type in kernel module
>>>>>>>   RISC-V: Enable module support in defconfig
>>>>>>>   RISC-V: Add definition of relocation types
>>>>>>>
>>>>>>>  arch/riscv/Kconfig                  |   5 ++
>>>>>>>  arch/riscv/Makefile                 |   3 +
>>>>>>>  arch/riscv/configs/defconfig        |   2 +
>>>>>>>  arch/riscv/include/asm/module.h     | 112 +++++++++++++++++++++++
>>>>>>>  arch/riscv/include/uapi/asm/elf.h   |  24 +++++
>>>>>>>  arch/riscv/kernel/Makefile          |   1 +
>>>>>>>  arch/riscv/kernel/module-sections.c | 156
>>>>>>> ++++++++++++++++++++++++++++++++
>>>>>>>  arch/riscv/kernel/module.c          | 175
>>>>>>> ++++++++++++++++++++++++++++++++++--
>>>>>>>  arch/riscv/kernel/module.lds        |   8 ++
>>>>>>>  9 files changed, 480 insertions(+), 6 deletions(-)
>>>>>>>  create mode 100644 arch/riscv/include/asm/module.h
>>>>>>>  create mode 100644 arch/riscv/kernel/module-sections.c
>>>>>>>  create mode 100644 arch/riscv/kernel/module.lds
>>>>>>
>>>>>>
>>>>>> This is the second set of patches that turn on modules, and it has the
>>>>>> same
>>>>>> R_RISCV_ALIGN problem as the other one
>>>>>>
>>>>>>
>>>>>> http://lists.infradead.org/pipermail/linux-riscv/2018-February/000081.html
>>>>>>
>>>>>> It looks like this one uses shared libraries for modules instead of
>>>>>> static
>>>>>> objects.  I think using shared objects is the right thing to do, as
>>>>>> it'll allow
>>>>>> us to place modules anywhere in the address space by having multiple
>>>>>> GOTs and
>>>>>> PLTs.
>>>>>
>>>>>
>>>>> Can you expand on this? It was my understanding that outside of the
>>>>> context of multiple address spaces sharing code the GOT and PLT were
>>>>> simply unnecessary overhead, what benefit would they bring here?
>>>>>
>>>>>> That's kind of complicated, though, so we can start with something
>>>>>> simpler like this.
>>>>
>>>>
>>>> Hi,
>>>>
>>>> The kernel module is a object file, it is not be linked by linker, the
>>>> GOT and PLT
>>>> sections will not be generated through -fPIC option, but it will
>>>> generate the relative
>>>> relocation type. As Palmer mention before, If we have GOT and PLT
>>>> sections,
>>>> we can put the module anywhere, even we support the KASLR in the kernel.
>>>
>>>
>>> Sorry, I guess I meant PIC objects not shared objects (I keep forgetting
>>> about
>>> PIE).  We'll probably eventually add large code model targets, but they
>>> might
>>> end up just being functionally equilivant to PIE with multi-GOT and
>>> multi-PLT
>>> so it might not matter.
>>>
>>> Either way, this is the sanest way to do it for now.
>>
>> Actually, I try to use the large code model and without PIC before.
>> (The compiler with mcmodel=large obtain from my colleague development)
>> On this compiler version, the `-mcmodel=large` uses the constant pool
>> mechanism to
>> puts the addresses of data symbols at the function tail. It can resolve
>> the reference about out of range of data symbol, but this code generation not
>> apply to function call. For the compiler code generation and no linker to do
>> relax reason, kernel module still needs the PLT section to jump to far target.
>> On the other hand, the ARM64 mailing list has the patches to remove
>> the large code model for cache performance.
>>
>> https://marc.info/?l=linux-arm-kernel&m=151860828416766
>>
>> so maybe we can use the `medany + fPIC` for now.
>>
>>>> For the ALIGN problem, the kernel module loader is difficult to remove
>>>> or migrate
>>>> the module's code like relax doing, so the remnant nop instructions harm
>>>> the
>>>> performance,  I agree the point that adding the mno-relax option and
>>>> checking
>>>> the alignment in ALIGN type in module loader.
>>>
>>>
>>> Sounds good.  I just merged the mno-relax stuff, it'll show up when I get
>>> around to generating a 7.3.0 backport branch.  For now I think you should
>>> just
>>> fail on R_RISCV_ALIGN and attempt to pass -mno-relax to the compiler (via
>>> something like "$(call cc-option,-mno-relax)", like we do for
>>> "-mstrict-align").  I don't think it's worth handling R_RISCV_ALIGN in the
>>> kernel, as that's essentially the same as full relaxation support.
>>
>> OK. I will send the v2 patches with the modification as you mention about
>> R_RISCV_ALIGN type?
>
> Just to avoid work duplication, do you think you'll get to this before
> this weekend? I was planning on bringing my patches up-to-date then, but
> since we're going the PIC route I can base my no-relax/ALIGN support on
> top of your series instead.
>

Hi Shea,

I'm not sure whether there are any problems before finishing discussion.
If only the alignment problem, I think that it is quick modification.

Thanks a lot.

>>
>>> If I understand your code correctly, you're currently generating one GOT and
>>> one PLT per loaded module.  If that's the case, then this is correct, it's
>>> just
>>> possible to save some memory by merging these tables.  It's probably not
>>> worth
>>> the complexity for kernel modules for a while.
>>
>> Yes, there are one GOT and one PLT per loaded module.
>> In the [PATCH 02/11], I generate the third section .got.plt for saving
>> more memory of
>> each PLT entry.
>>
>> Thanks a lot.

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