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

[Greg Ungerer]

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@xxxxxxxxxx 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@xxxxxxxxxx>
> > > ---
> > >
> > >   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.

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.

Regards
Greg