[Bug target/81863] [7 regression] -mword-relocations is unreliable

[ard.biesheuvel at linaro dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81863

--- Comment #13 from ard.biesheuvel at linaro dot org ---
The kernel does not currently use -mword-relocations. We are looking into it as
an alternative to -fpic when building the kernel image as a PIE executable so
we can self-relocate at boot time. This allows for randomization of the load
offset (KASLR) which is an often requested security feature.

The problem with -fpic (and even -fpie) is that the compiler infers from its
presence that you are building a shared library which requires preemptible
external symbols and should not contain any text relocations. None of these
concerns apply to the kernel, and the current workaround is to -include a .h
file that sets the visibility 'hidden' pragma, which is  a bit tedious. Since
we are only interested in absolute references that are runtime relocatable,
-mword-relocations seems like the right choice, but it is broken currently on
v7 (due to this bug)

Ideally, we'd have some logic that takes -ffreestanding into account when
deciding how to deal with -fpie/-fpic

Also, when generating position independent code for v7 or later, we should
probably attempt to use place relative movw/movt pairs rather than relative
literals to emit PC relative references.