[PATCH v2] sh: select BINFMT_FLAT_NO_DATA_START_OFFSET for noMMU

[Waldemar Brodkorb <wbx@openadk.org>]

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
-- 
2.47.3