arm64/ftrace: fix inadvertent BUG() in trampoline check

The ftrace trampoline code (which deals with modules loaded out of
BL range of the core kernel) uses plt_entries_equal() to check whether
the per-module trampoline equals a zero buffer, to decide whether the
trampoline has already been initialized.

This triggers a BUG() in the opcode manipulation code, since we end
up checking the ADRP offset of a 0x0 opcode, which is not an ADRP
instruction.

So instead, add a helper to check whether a PLT is initialized, and
call that from the frace code.

Cc: <stable@vger.kernel.org> # v5.0
Fixes: bdb85cd1 ("arm64/module: switch to ADRP/ADD sequences for PLT entries")
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/include/asm/module.h

@@ -73,4 +73,9 @@ static inline bool is_forbidden_offset_for_adrp(void *place)
 struct plt_entry get_plt_entry(u64 dst, void *pc);
 bool plt_entries_equal(const struct plt_entry *a, const struct plt_entry *b);
 
+static inline bool plt_entry_is_initialized(const struct plt_entry *e)
+{
+	return e->adrp || e->add || e->br;
+}
+
 #endif /* __ASM_MODULE_H */

arch/arm64/kernel/ftrace.c

@@ -107,8 +107,7 @@ int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
 		trampoline = get_plt_entry(addr, mod->arch.ftrace_trampoline);
 		if (!plt_entries_equal(mod->arch.ftrace_trampoline,
 				       &trampoline)) {
-			if (!plt_entries_equal(mod->arch.ftrace_trampoline,
-					       &(struct plt_entry){})) {
+			if (plt_entry_is_initialized(mod->arch.ftrace_trampoline)) {
 				pr_err("ftrace: far branches to multiple entry points unsupported inside a single module\n");
 				return -EINVAL;
 			}