On 3/30/21 4:40 AM, Alexander A Sverdlin wrote:

From: Alexander Sverdlin <redacted>

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
patches:

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.

Changelog:
v8:
* Add warn suppress parameter to arm_gen_branch_link()
v7:
* rebased
v6:
* rebased
v5:
* BUILD_BUG_ON() ensures fixed_plts[] always fits one PLT block
* use "for" loop instead of "while"
* scripts/recordmcount is filtering reloc types
v4:
* 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
    v3:
    * Only extend struct dyn_arch_ftrace when ARM_MODULE_PLTS is enabled
    v2:
    * As suggested by Steven Rostedt, refrain from tree-wide API modification,
      save module pointer in struct dyn_arch_ftrace instead (PowerPC way)

FWIW, ftracetest did not pick up new failures (nor were new tests fixed)
with this patch series.
-- 
Florian

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