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arm64: ftrace: don't validate branch via PLT in ftrace_make_nop()

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When turning branch instructions into NOPs, we attempt to validate the
action by comparing the old value at the call site with the opcode of
a direct relative branch instruction pointing at the old target.

However, these call sites are statically initialized to call _mcount(),
and may be redirected via a PLT entry if the module is loaded far away
from the kernel text, leading to false negatives and spurious errors.

So skip the validation if CONFIG_ARM64_MODULE_PLTS is configured.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
This commit is contained in:
Ard Biesheuvel 2017-06-06 17:00:21 +00:00 committed by Will Deacon
parent dbbb08f500
commit f8af0b364e
1 changed files with 43 additions and 3 deletions
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46
arch/arm64/kernel/ftrace.c
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@ -84,12 +84,52 @@ int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec,
unsigned long addr)
{
unsigned long pc = rec->ip;
u32 old, new;
long offset = (long)pc - (long)addr;
bool validate = true;
u32 old = 0, new;
if (IS_ENABLED(CONFIG_ARM64_MODULE_PLTS) &&
(offset < -SZ_128M || offset >= SZ_128M)) {
u32 replaced;
/*
* 'mod' is only set at module load time, but if we end up
* dealing with an out-of-range condition, we can assume it
* is due to a module being loaded far away from the kernel.
*/
if (!mod) {
preempt_disable();
mod = __module_text_address(pc);
preempt_enable();
if (WARN_ON(!mod))
return -EINVAL;
}
/*
* The instruction we are about to patch may be a branch and
* link instruction that was redirected via a PLT entry. In
* this case, the normal validation will fail, but we can at
* least check that we are dealing with a branch and link
* instruction that points into the right module.
*/
if (aarch64_insn_read((void *)pc, &replaced))
return -EFAULT;
if (!aarch64_insn_is_bl(replaced) ||
!within_module(pc + aarch64_get_branch_offset(replaced),
mod))
return -EINVAL;
validate = false;
} else {
old = aarch64_insn_gen_branch_imm(pc, addr,
AARCH64_INSN_BRANCH_LINK);
}
old = aarch64_insn_gen_branch_imm(pc, addr, AARCH64_INSN_BRANCH_LINK);
new = aarch64_insn_gen_nop();
return ftrace_modify_code(pc, old, new, true);
return ftrace_modify_code(pc, old, new, validate);
}
void arch_ftrace_update_code(int command)