[PATCH 1/2] arm64: ftrace: don't validate branch via PLT in ftrace_make_nop()

[Ard Biesheuvel]

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 in this case if CONFIG_ARM64_MODULE_PLTS is
configured.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
---
 arch/arm64/kernel/ftrace.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c
index 40ad08ac569a..a8db6857cad6 100644
--- a/arch/arm64/kernel/ftrace.c
+++ b/arch/arm64/kernel/ftrace.c
@@ -84,12 +84,15 @@ int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec,
 		    unsigned long addr)
 {
 	unsigned long pc = rec->ip;
-	u32 old, new;
+	u32 old = 0, new;
 
-	old = aarch64_insn_gen_branch_imm(pc, addr, AARCH64_INSN_BRANCH_LINK);
+	if (!IS_ENABLED(CONFIG_ARM64_MODULE_PLTS))
+		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,
+				  !IS_ENABLED(CONFIG_ARM64_MODULE_PLTS));
 }
 
 void arch_ftrace_update_code(int command)
-- 
2.9.3