Since commit 35fa91ee ("ARM: kernel: merge core and init PLTs"),
the ARM module PLT code allocates all PLT entries in a single core
section, since the overhead of having a separate init PLT section is
not justified by the small number of PLT entries usually required for
init code.

However, the core and init module regions are allocated independently,
and there is a corner case where the core region may be allocated from
the VMALLOC region if the dedicated module region is exhausted, but the
init region, being much smaller, can still be allocated from the module
region. This puts the PLT entries out of reach of the relocated branch
instructions, defeating the whole purpose of PLTs.

So split the core and init PLT regions, and name the latter ".init.plt"
so it gets allocated along with (and sufficiently close to) the .init
sections that it serves. Also, given that init PLT entries may need to
be emitted for branches that target the core module, modify the logic
that disregards defined symbols to only disregard symbols that are
defined in the same section.

Fixes: 35fa91ee ("ARM: kernel: merge core and init PLTs")
Cc: <stable@vger.kernel.org> # v4.9+
Reported-by: Angus Clark <angus@angusclark.org>
Tested-by: Angus Clark <angus@angusclark.org>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>