[PATCH] ARM: vmlinux.lds: assert that ROM and RAM don't overlap when XIP_KERNEL=y

On Monday 01 February 2016 18:02:34 Ard Biesheuvel wrote:

When building an XIP kernel, the linker produces two disjoint VMA regions,
where the first is mapped onto ROM and the second onto RAM. For this reason,
the linker output pointer '.' is updated halfway through the linker script,
and set to a value that corresponds with the start of the RAM region.

However, in some cases, the ROM region exceeds the expected size, and the
assignment of the output pointer results in a decrement rather than an
increment, causing the virtual addresses of the .data region to clash with
the .text region. Such a kernel cannot boot normally, but it also confuses
the hell out of kallsyms, since .data symbols may appear inside the
[_stext, _etext] or [_sinittext, _einittext] intervals in the first pass,
but not in the second (or vice versa), resulting in inconsistent kallsyms
data.

So let's make sure that the output pointer only advances, and never jumps
back into the ROM region.

Cc: arnd at arndb.de
Signed-off-by: Ard Biesheuvel <redacted>

Acked-by: Arnd Bergmann <arnd@arndb.de>

I've tested this in multiple configurations, and it reliably prints
a useful error message in broken configurations, while it has no
effect on working configurations. This is more helpful than the
mysterious

   Inconsistent kallsyms data
   Try make KALLSYMS_EXTRA_PASS=1 as a workaround

message that we get without the patch.

	Arnd