[PATCH 0/2] riscv: support ELF format binaries in nommu mode

[Damien Le Moal]

On 3/1/23 09:24, Greg Ungerer wrote:
> Hi Damien,
> 
> On 1/3/23 08:49, Damien Le Moal wrote:
>> On 3/1/23 06:52, Palmer Dabbelt wrote:
>>> On Tue, 28 Feb 2023 05:51:24 PST (-0800), gerg at kernel.org wrote:
>>>> The following changes add the ability to run ELF format binaries when
>>>> running RISC-V in nommu mode. That support is actually part of the
>>>> ELF-FDPIC loader, so these changes are all about making that work on
>>>> RISC-V.
>>>>
>>>> The first issue to deal with is making the ELF-FDPIC loader capable of
>>>> handling 64-bit ELF files. As coded right now it only supports 32-bit
>>>> ELF files.
>>>>
>>>> Secondly some changes are required to enable and compile the ELF-FDPIC
>>>> loader on RISC-V and to pass the ELF-FDPIC mapping addresses through to
>>>> user space when execing the new program.
>>>>
>>>> These changes have not been used to run actual ELF-FDPIC binaries.
>>>> It is used to load and run normal ELF - compiled -pie format. Though the
>>>> underlying changes are expected to work with full ELF-FDPIC binaries if
>>>> or when that is supported on RISC-V in gcc.
>>>>
>>>> To avoid needing changes to the C-library (tested with uClibc-ng
>>>> currently) there is a simple runtime dynamic loader (interpreter)
>>>> available to do the final relocations, https://github.com/gregungerer/uldso.
>>>> The nice thing about doing it this way is that the same program
>>>> binary can also be loaded with the usual ELF loader in MMU linux.
>>>>
>>>> The motivation here is to provide an easy to use alternative to the
>>>> flat format binaries normally used for RISC-V nommu based systems.
>>>>
>>>> Signed-off-by: Greg Ungerer <gerg at kernel.org>
>>>> ---
>>>>
>>>>   arch/riscv/include/asm/elf.h         |   11 +++++++++-
>>>>   arch/riscv/include/asm/mmu.h         |    4 +++
>>>>   arch/riscv/include/uapi/asm/ptrace.h |    5 ++++
>>>>   fs/Kconfig.binfmt                    |    2 -
>>>>   fs/binfmt_elf_fdpic.c                |   38 +++++++++++++++++------------------
>>>>   include/linux/elf-fdpic.h            |   14 +++++++++++-
>>>>   include/uapi/linux/elf-fdpic.h       |   15 +++++++++++++
>>>>   7 files changed, 67 insertions(+), 22 deletions(-)
>>>
>>> Adding Damien, as IIRC he's had some hacked up userspace bits for the
>>> K210.  I'm yet to get anything running, but it'd be great if we get this
>>> to a point where I can actually boot test this on QEMU (I'm just doing
>>> builds now).
>>
>> +Niklas
>>
>> Niklas, didn't you add a nommu qemu build for buildroot ? Can't find the config
>> though...
>>
>>>
>>> Given that it's the second week of the merge window and this is a bunch
>>> of new uABI it seems best to hold off until the next cycle.  I poked
>>> around and don't see anything wrong, but I'll try and take a more
>>> detailed look after the merge window.
>>
>> Does any riscv compiler support nommu fdpic now ? When doing the work on the
>> k210, there was no support at all, hence the statically linked user binaries
>> used (no shared text for libraries).
> 
> I was wondering if anyone was working on it. Couldn't find anything.

Maciej (added to this thread) has done some work on gcc, but nothing that is
upstream yet.

> The way I use this support doesn't improve the statically linked situation.
> The -pie ELF files are still statically linked. But it does avoid the
> use of flat format binaries. That makes the toolchain generation a little
> simpler.

Right. I was still thinking in terms of flat bin. The keyword here is "elf"...
Need more coffe :)

We could try this using either qemu or the k210 with buildroot, with the
addition of your uldso loader. Will try to put that in the pipeline (pretty full
as usual, so may take a while).

> 
> Regards
> Greg
> 

-- 
Damien Le Moal
Western Digital Research