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

[Damien Le Moal]

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).


-- 
Damien Le Moal
Western Digital Research