[PATCH v6 0/2] ARM: Implement MODULE_PLT support in FTRACE

From: Alexander X Sverdlin
Date: Mon Feb 17 2020 - 09:11:29 EST

From: Alexander Sverdlin <alexander.sverdlin@xxxxxxxxx>

FTRACE's function tracer currently doesn't always work on ARM with
MODULE_PLT option enabled. If the module is loaded too far, FTRACE's
code modifier cannot cope with introduced veneers and turns the
function tracer off globally.

ARM64 already has a solution for the problem, refer to the following

arm64: ftrace: emit ftrace-mod.o contents through code
arm64: module-plts: factor out PLT generation code for ftrace
arm64: ftrace: fix !CONFIG_ARM64_MODULE_PLTS kernels
arm64: ftrace: fix building without CONFIG_MODULES
arm64: ftrace: add support for far branches to dynamic ftrace
arm64: ftrace: don't validate branch via PLT in ftrace_make_nop()

But the presented ARM variant has just a half of the footprint in terms of
the changed LoCs. It also retains the code validation-before-modification
instead of switching it off.

* rebased
* BUILD_BUG_ON() ensures fixed_plts[] always fits one PLT block
* use "for" loop instead of "while"
* scripts/recordmcount is filtering reloc types
* Fixed build without CONFIG_FUNCTION_TRACER
* Reorganized pre-allocated PLTs handling in get_module_plt(),
now compiler eliminates the whole FTRACE-related handling code
if ARRAY_SIZE(fixed_plts) == 0
* Only extend struct dyn_arch_ftrace when ARM_MODULE_PLTS is enabled
* As suggested by Steven Rostedt, refrain from tree-wide API modification,
save module pointer in struct dyn_arch_ftrace instead (PowerPC way)

Alexander Sverdlin (2):
ARM: PLT: Move struct plt_entries definition to header
ARM: ftrace: Add MODULE_PLTS support

arch/arm/include/asm/ftrace.h | 3 +++
arch/arm/include/asm/module.h | 10 +++++++++
arch/arm/kernel/ftrace.c | 46 ++++++++++++++++++++++++++++++++++++++--
arch/arm/kernel/module-plts.c | 49 +++++++++++++++++++++++++++++++++----------
4 files changed, 95 insertions(+), 13 deletions(-)