[PATCH] arm64: ftrace: fix !CONFIG_ARM64_MODULE_PLTS kernels

[Mark Rutland]

When a kernel is built without CONFIG_ARM64_MODULE_PLTS, we don't
generate the expected branch instruction in ftrace_make_nop(). This
means we pass zero (rather than a valid branch) to ftrace_modify_code()
as the expected instruction to validate. This causes us to return
-EINVAL to the core ftrace code for a valid case, resulting in a splat
at boot time.

This was an unintended effect of commit:

  687644209a6e9557 ("arm64: ftrace: fix building without CONFIG_MODULES")

... which incorrectly moved the generation of the branch instruction
into the ifdef for CONFIG_ARM64_MODULE_PLTS.

This patch fixes the issue by moving the ifdef inside of the relevant
if-else case, and always checking that the branch is in range,
regardless of CONFIG_ARM64_MODULE_PLTS. This ensures that we generate
the expected branch instruction, and also improves our sanity checks.

For consistency, both ftrace_make_nop() and ftrace_make_call() are
updated with this pattern.

Fixes: 687644209a6e9557 ("arm64: ftrace: fix building without CONFIG_MODULES")
Signed-off-by: Mark Rutland <mark.rutland at arm.com>
Reported-by: Marc Zyngier <marc.zyngier at arm.com>
Cc: Ard Biesheuvel <ard.biesheuvel at linaro.org>
Cc: Arnd Bergmann <arnd at arndb.de>
Cc: Catalin Marinas <catalin.marinas at arm.com>
Cc: Will Deacon <will.deacon at arm.com>
---
 arch/arm64/kernel/ftrace.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

Marc spotted this breakage atop of the arm64 for-next/core branch when ftrace
was enabled.

I've given this fix a go with all combinations of MODULES and RANDOMIZE_BASE,
with the ftrace boot time self test, and everything seems happy in all
combinations.

Mark.

diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c
index 401aa27..945f506 100644
--- a/arch/arm64/kernel/ftrace.c
+++ b/arch/arm64/kernel/ftrace.c
@@ -73,10 +73,10 @@ int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
 	unsigned long pc = rec->ip;
 	u32 old, new;
 
-#ifdef CONFIG_ARM64_MODULE_PLTS
 	long offset = (long)pc - (long)addr;
 
 	if (offset < -SZ_128M || offset >= SZ_128M) {
+#ifdef CONFIG_ARM64_MODULE_PLTS
 		unsigned long *trampoline;
 		struct module *mod;
 
@@ -121,8 +121,10 @@ int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
 			smp_wmb();
 		}
 		addr = (unsigned long)&trampoline[1];
-	}
+#else /* CONFIG_ARM64_MODULE_PLTS */
+		return -EINVAL;
 #endif /* CONFIG_ARM64_MODULE_PLTS */
+	}
 
 	old = aarch64_insn_gen_nop();
 	new = aarch64_insn_gen_branch_imm(pc, addr, AARCH64_INSN_BRANCH_LINK);
@@ -140,10 +142,10 @@ int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec,
 	bool validate = true;
 	u32 old = 0, new;
 
-#ifdef CONFIG_ARM64_MODULE_PLTS
 	long offset = (long)pc - (long)addr;
 
 	if (offset < -SZ_128M || offset >= SZ_128M) {
+#ifdef CONFIG_ARM64_MODULE_PLTS
 		u32 replaced;
 
 		/*
@@ -176,11 +178,13 @@ int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec,
 			return -EINVAL;
 
 		validate = false;
+#else /* CONFIG_ARM64_MODULE_PLTS */
+		return -EINVAL;
+#endif /* CONFIG_ARM64_MODULE_PLTS */
 	} else {
 		old = aarch64_insn_gen_branch_imm(pc, addr,
 						  AARCH64_INSN_BRANCH_LINK);
 	}
-#endif /* CONFIG_ARM64_MODULE_PLTS */
 
 	new = aarch64_insn_gen_nop();
 
-- 
1.9.1