Re: [PATCH v2] sh: select BINFMT_FLAT_NO_DATA_START_OFFSET for noMMU

[Waldemar Brodkorb <wbx@openadk.org>]

Hi Adrian,
John Paul Adrian Glaubitz wrote,

> Hi Waldemar,
> 
> On Wed, 2026-10-21 at 02:02 +0200, Waldemar Brodkorb wrote:
> > binfmt_flat offsets the in-memory start of the data segment by
> > MAX_SHARED_LIBS words and then aligns it to FLAT_DATA_ALIGN, so FLAT_FLAG_RAM
> > binaries end up with a gap between text and data (commit 2217b9826246
> > ("binfmt_flat: revert "binfmt_flat: don't offset the data start"")).
> > 
> > SH position-independent code finds the GOT with a PC-relative mova of
> > _GLOBAL_OFFSET_TABLE_, which assumes data follows text directly, as elf2flt
> > lays out the image. With the gap, the GOT register points below the real GOT,
> > so every GOT load reads the neighbouring slot and gets a wrong pointer. On a
> > J2 (sh2eb, noMMU) this made userland crash or hang right after start-up.
> > In my case the data was moved up by 0x20 bytes.
> > 
> > Commit 04d82a6d0881 ("binfmt_flat: allow not offsetting data start") added
> > CONFIG_BINFMT_FLAT_NO_DATA_START_OFFSET for architectures that cannot
> > tolerate such a gap; currently only riscv noMMU selects it. Select it on SH
> > as well when building without an MMU.
> > 
> > Tested on a J2 board (Numato Mimas V2, sh2eb, noMMU) with Linux 7.3 and
> > FLAT binaries built with elf2flt: with this change the data segment starts
> > exactly at the end of text and init runs.
> > 
> > Without the patch bootup fails with:
> > [   12.080472] Fixing up unaligned userspace access in "init" pid=1 pc=0x(ptrval) ins=0x6583
> > [   12.123609] Fixing up unaligned userspace access in "init" pid=1 pc=0x(ptrval) ins=0x6583
> > [   12.175153] Kernel panic - not syncing: Attempted to kill init! exitcode=0x00007f00
> > 
> > Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
> > Assisted-by: LLM
> > Fixes: 2217b9826246 ("binfmt_flat: revert "binfmt_flat: don't offset the data start"")
> > 
> > ---
> > v1->v2:
> >   - add fixes tag (Geert)
> >   - tested with 7.3 + Florian's uaccess patch (Geert)
> >   - added failing bootup messages to commit message
> > ---
> >  arch/sh/Kconfig | 1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/arch/sh/Kconfig b/arch/sh/Kconfig
> > index d60f1d5a94c0..b501153551c0 100644
> > --- a/arch/sh/Kconfig
> > +++ b/arch/sh/Kconfig
> > @@ -15,6 +15,7 @@ config SUPERH
> >  	select ARCH_MIGHT_HAVE_PC_PARPORT
> >  	select ARCH_WANT_IPC_PARSE_VERSION
> >  	select ARCH_NEED_CMPXCHG_1_EMU
> > +	select BINFMT_FLAT_NO_DATA_START_OFFSET if !MMU
> >  	select CPU_NO_EFFICIENT_FFS
> >  	select DMA_DECLARE_COHERENT
> >  	select GENERIC_ATOMIC64
> 
> I have not tested this yet but I will do so later this week. Personally, I
> test my kernel images on J2 with ToyBox which builds a minimal rootfs with
> busybox.
> 
> I am not sure if the binaries are flat binaries in this case, do you know?

They are not. Toybox uses musl C library, which is FDPIC.
I use OpenADK (https://openadk.org) to build a J2 image with
uClibc-ng and FLAT binaries. You could either try OpenADK or just
believe me :).
For OpenADK you need to choose experimental support and uClibc-ng
git version, as there was another bug in uClibc-ng to let it work.

best regards
 Waldemar