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arm64: ftrace: add support for far branches to dynamic ftrace

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Currently, dynamic ftrace support in the arm64 kernel assumes that all
core kernel code is within range of ordinary branch instructions that
occur in module code, which is usually the case, but is no longer
guaranteed now that we have support for module PLTs and address space
randomization.

Since on arm64, all patching of branch instructions involves function
calls to the same entry point [ftrace_caller()], we can emit the modules
with a trampoline that has unlimited range, and patch both the trampoline
itself and the branch instruction to redirect the call via the trampoline.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
[will: minor clarification to smp_wmb() comment]
Signed-off-by: Will Deacon <will.deacon@arm.com>
This commit is contained in:
Ard Biesheuvel
2017-06-06 17:00:22 +00:00
committed by Will Deacon
parent f8af0b364e
commit e71a4e1beb
7 changed files with 84 additions and 2 deletions
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18
arch/arm64/kernel/ftrace-mod.S Normal file
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/*
* Copyright (C) 2017 Linaro Ltd <ard.biesheuvel@linaro.org>
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License version 2 as
* published by the Free Software Foundation.
*/
#include <linux/linkage.h>
#include <asm/assembler.h>
.section ".text.ftrace_trampoline", "ax"
.align 3
0: .quad 0
__ftrace_trampoline:
ldr x16, 0b
br x16
ENDPROC(__ftrace_trampoline)