arm64: ftrace: fix module PLTs with mcount

In the Linux kernel, the following vulnerability has been resolved:

arm64: ftrace: fix module PLTs with mcount

Li Huafei reports that mcount-based ftrace with module PLTs was broken

by commit:

a6253579977e4c6f ("arm64: ftrace: consistently handle PLTs.")

When a module PLTs are used and a module is loaded sufficiently far away

from the kernel, we'll create PLTs for any branches which are

out-of-range. These are separate from the special ftrace trampoline

PLTs, which the module PLT code doesn't directly manipulate.

When mcount is in use this is a problem, as each mcount callsite in a

module will be initialized to point to a module PLT, but since commit

a6253579977e4c6f ftrace_make_nop() will assume that the callsite has

been initialized to point to the special ftrace trampoline PLT, and

ftrace_find_callable_addr() rejects other cases.

This means that when ftrace tries to initialize a callsite via

ftrace_make_nop(), the call to ftrace_find_callable_addr() will find

that the `_mcount` stub is out-of-range and is not handled by the ftrace

PLT, resulting in a splat:

| ftrace_test: loading out-of-tree module taints kernel.

| ftrace: no module PLT for _mcount

| ------------[ ftrace bug ]------------

| ftrace failed to modify

| [<ffff800029180014>] 0xffff800029180014

| actual: 44:00:00:94

| Initializing ftrace call sites

| ftrace record flags: 2000000

| (0)

| expected tramp: ffff80000802eb3c

| ------------[ cut here ]------------

| WARNING: CPU: 3 PID: 157 at kernel/trace/ftrace.c:2120 ftrace_bug+0x94/0x270

| Modules linked in:

| CPU: 3 PID: 157 Comm: insmod Tainted: G O 6.0.0-rc6-00151-gcd722513a189-dirty #22

| Hardware name: linux,dummy-virt (DT)

| pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)

| pc : ftrace_bug+0x94/0x270

| lr : ftrace_bug+0x21c/0x270

| sp : ffff80000b2bbaf0

| x29: ffff80000b2bbaf0 x28: 0000000000000000 x27: ffff0000c4d38000

| x26: 0000000000000001 x25: ffff800009d7e000 x24: ffff0000c4d86e00

| x23: 0000000002000000 x22: ffff80000a62b000 x21: ffff8000098ebea8

| x20: ffff0000c4d38000 x19: ffff80000aa24158 x18: ffffffffffffffff

| x17: 0000000000000000 x16: 0a0d2d2d2d2d2d2d x15: ffff800009aa9118

| x14: 0000000000000000 x13: 6333626532303830 x12: 3030303866666666

| x11: 203a706d61727420 x10: 6465746365707865 x9 : 3362653230383030

| x8 : c0000000ffffefff x7 : 0000000000017fe8 x6 : 000000000000bff4

| x5 : 0000000000057fa8 x4 : 0000000000000000 x3 : 0000000000000001

| x2 : ad2cb14bb5438900 x1 : 0000000000000000 x0 : 0000000000000022

| Call trace:

| ftrace_bug+0x94/0x270

| ftrace_process_locs+0x308/0x430

| ftrace_module_init+0x44/0x60

| load_module+0x15b4/0x1ce8

| __do_sys_init_module+0x1ec/0x238

| __arm64_sys_init_module+0x24/0x30

| invoke_syscall+0x54/0x118

| el0_svc_common.constprop.4+0x84/0x100

| do_el0_svc+0x3c/0xd0

| el0_svc+0x1c/0x50

| el0t_64_sync_handler+0x90/0xb8

| el0t_64_sync+0x15c/0x160

| ---[ end trace 0000000000000000 ]---

| ---------test_init-----------

Fix this by reverting to the old behaviour of ignoring the old

instruction when initialising an mcount callsite in a module, which was

the behaviour prior to commit a6253579977e4c6f.