arm64: ftrace: don't validate branch via PLT in ftrace_make_nop()
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>
diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c
index 40ad08a..4cb5763 100644
--- a/arch/arm64/kernel/ftrace.c
+++ b/arch/arm64/kernel/ftrace.c
@@ -84,12 +84,52 @@
unsigned long addr)
{
unsigned long pc = rec->ip;
- u32 old, new;
+ long offset = (long)pc - (long)addr;
+ bool validate = true;
+ u32 old = 0, new;
- old = aarch64_insn_gen_branch_imm(pc, addr, AARCH64_INSN_BRANCH_LINK);
+ 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);
+ }
+
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)