arm64: add support for module PLTs

This adds support for emitting PLTs at module load time for relative
branches that are out of range. This is a prerequisite for KASLR, which
may place the kernel and the modules anywhere in the vmalloc area,
making it more likely that branch target offsets exceed the maximum
range of +/- 128 MB.

In this version, I removed the distinction between relocations against
.init executable sections and ordinary executable sections. The reason
is that it is hardly worth the trouble, given that .init.text usually
does not contain that many far branches, and this version now only
reserves PLT entry space for jump and call relocations against undefined
symbols (since symbols defined in the same module can be assumed to be
within +/- 128 MB)

For example, the mac80211.ko module (which is fairly sizable at ~400 KB)
built with -mcmodel=large gives the following relocation counts:

                    relocs    branches   unique     !local
  .text              3925       3347       518        219
  .init.text           11          8         7          1
  .exit.text            4          4         4          1
  .text.unlikely       81         67        36         17

('unique' means branches to unique type/symbol/addend combos, of which
!local is the subset referring to undefined symbols)

IOW, we are only emitting a single PLT entry for the .init sections, and
we are better off just adding it to the core PLT section instead.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index cfa1cc90ebf4..c85c29d660bd 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -395,6 +395,7 @@ config ARM64_ERRATUM_843419
         bool "Cortex-A53: 843419: A load or store might access an incorrect address"
         depends on MODULES
         default y
+        select ARM64_MODULE_CMODEL_LARGE
         help
           This option builds kernel modules using the large memory model in
           order to avoid the use of the ADRP instruction, which can cause
@@ -778,6 +779,14 @@ config ARM64_UAO
           regular load/store instructions if the cpu does not implement the
           feature.
 
+config ARM64_MODULE_CMODEL_LARGE
+        bool
+
+config ARM64_MODULE_PLTS
+        bool
+        select ARM64_MODULE_CMODEL_LARGE
+        select HAVE_MOD_ARCH_SPECIFIC
+
 endmenu
 
 menu "Boot options"