[PATCH] lib/test_ubsan.c: Fix panic from test_ubsan_out_of_bounds

[Mostafa Saleh <smostafa@google.com>]

Running lib_ubsan.ko on arm64 (without CONFIG_UBSAN_TRAP) panics the
kernel

[   31.616546] Kernel panic - not syncing: stack-protector: Kernel stack is corrupted in: test_ubsan_out_of_bounds+0x158/0x158 [test_ubsan]
[   31.646817] CPU: 3 UID: 0 PID: 179 Comm: insmod Not tainted 6.15.0-rc2 #1 PREEMPT
[   31.648153] Hardware name: linux,dummy-virt (DT)
[   31.648970] Call trace:
[   31.649345]  show_stack+0x18/0x24 (C)
[   31.650960]  dump_stack_lvl+0x40/0x84
[   31.651559]  dump_stack+0x18/0x24
[   31.652264]  panic+0x138/0x3b4
[   31.652812]  __ktime_get_real_seconds+0x0/0x10
[   31.653540]  test_ubsan_load_invalid_value+0x0/0xa8 [test_ubsan]
[   31.654388]  init_module+0x24/0xff4 [test_ubsan]
[   31.655077]  do_one_initcall+0xd4/0x280
[   31.655680]  do_init_module+0x58/0x2b4

That happens because the test corrupts other data in the stack:
400:   d5384108        mrs     x8, sp_el0
404:   f9426d08        ldr     x8, [x8, #1240]
408:   f85f83a9        ldur    x9, [x29, #-8]
40c:   eb09011f        cmp     x8, x9
410:   54000301        b.ne    470 <test_ubsan_out_of_bounds+0x154>  // b.any

As there is no guarantee the compiler will order the local variables
as declared in the module:
	volatile char above[4] = { }; /* Protect surrounding memory. */
	volatile int arr[4];
	volatile char below[4] = { }; /* Protect surrounding memory. */

So, instead of writing out-of-bound, we can read out-of-bound which
still triggers UBSAN but doesn't corrupt the stack.

Signed-off-by: Mostafa Saleh <smostafa@google.com>
---
 lib/test_ubsan.c | 11 ++++-------
 1 file changed, 4 insertions(+), 7 deletions(-)

diff --git a/lib/test_ubsan.c b/lib/test_ubsan.c
index 8772e5edaa4f..0e5c18b32b2d 100644
--- a/lib/test_ubsan.c
+++ b/lib/test_ubsan.c
@@ -77,18 +77,15 @@ static void test_ubsan_shift_out_of_bounds(void)
 
 static void test_ubsan_out_of_bounds(void)
 {
-	volatile int i = 4, j = 5, k = -1;
-	volatile char above[4] = { }; /* Protect surrounding memory. */
+	volatile int j = 5, k = -1;
+	volatile int scratch[4] = { };
 	volatile int arr[4];
-	volatile char below[4] = { }; /* Protect surrounding memory. */
-
-	above[0] = below[0];
 
 	UBSAN_TEST(CONFIG_UBSAN_BOUNDS, "above");
-	arr[j] = i;
+	scratch[1] = arr[j];
 
 	UBSAN_TEST(CONFIG_UBSAN_BOUNDS, "below");
-	arr[k] = i;
+	scratch[2] = arr[k];
 }
 
 enum ubsan_test_enum {
-- 
2.49.0.604.gff1f9ca942-goog