Fix a segfault issue due to the incorrect irq_stack_size on ARM64

See the following stack trace:
(gdb) bt
 #0  0x00005635ac2b166b in arm64_unwind_frame (frame=0x7ffdaf35cb70,
     bt=0x7ffdaf35d430) at arm64.c:2821
 crash-utility#1  arm64_back_trace_cmd (bt=0x7ffdaf35d430) at arm64.c:3306
 crash-utility#2  0x00005635ac27b108 in back_trace (bt=bt@entry=0x7ffdaf35d430) at
     kernel.c:3239
 crash-utility#3  0x00005635ac2880ae in cmd_bt () at kernel.c:2863
 crash-utility#4  0x00005635ac1f16dc in exec_command () at main.c:893
 crash-utility#5  0x00005635ac1f192a in main_loop () at main.c:840
 crash-utility#6  0x00005635ac50df81 in captured_main (data=<optimized out>) at main.c:1284
 crash-utility#7  gdb_main (args=<optimized out>) at main.c:1313
 crash-utility#8  0x00005635ac50e000 in gdb_main_entry (argc=<optimized out>,
     argv=<optimized out>) at main.c:1338
 crash-utility#9  0x00005635ac1ea2a5 in main (argc=5, argv=0x7ffdaf35dde8) at main.c:721

The issue may be encountered when thread_union symbol not found in vmlinux
due to compiling optimization.

This patch will try the following 2 methods to get the irq_stack_size
when thread_union symbol unavailable:

1. change the thread_shift when KASAN is enabled and with vmcoreinfo.
   In arm64/include/asm/memory.h:

   #if defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KASAN_SW_TAGS)
   ...
   #define IRQ_STACK_SIZE               THREAD_SIZE

   Since enabling the KASAN will affect the final value,
   this patch reset IRQ_STACK_SIZE according to the calculation process in
   kernel code.

2. Try getting the value from kernel code disassembly, to get
   THREAD_SHIFT directly from tbnz instruction.

   In arch/arm64/kernel/entry.S:
   .macro kernel_ventry, el:req, ht:req, regsize:req, label:req
   ...
         add     sp, sp, x0
         sub     x0, sp, x0
         tbnz    x0, #THREAD_SHIFT, 0f

   $ gdb vmlinux
   (gdb) disass vectors
   Dump of assembler code for function vectors:
      ...
      0xffff800080010804 <+4>:     add     sp, sp, x0
      0xffff800080010808 <+8>:     sub     x0, sp, x0
      0xffff80008001080c <+12>:    tbnz    w0, crash-utility#16, 0xffff80008001081c <vectors+28>

Signed-off-by: yeping.zheng <[email protected]>
Improved-by: Tao Liu <[email protected]>

arm64.c

@@ -95,6 +95,7 @@ static int arm64_is_uvaddr(ulong, struct task_context *);
 static void arm64_calc_KERNELPACMASK(void);
 static void arm64_recalc_KERNELPACMASK(void);
 static int arm64_get_vmcoreinfo(unsigned long *vaddr, const char *label, int base);
+static ulong arm64_set_irq_stack_size(void);
 
 struct kernel_range {
 	unsigned long modules_vaddr, modules_end;
@@ -2355,8 +2356,10 @@ arm64_irq_stack_init(void)
 		if (MEMBER_EXISTS("thread_union", "stack")) { 
 			if ((sz = MEMBER_SIZE("thread_union", "stack")) > 0)
 				ms->irq_stack_size = sz;
-		} else
-			ms->irq_stack_size = ARM64_IRQ_STACK_SIZE;
+		} else {
+			ulong res = arm64_set_irq_stack_size();
+			ms->irq_stack_size = (res > 0) ? res : ARM64_IRQ_STACK_SIZE;
+		}
 
 		machdep->flags |= IRQ_STACKS;
 
@@ -5073,6 +5076,57 @@ static void arm64_recalc_KERNELPACMASK(void){
 	}
 }
 
+static ulong arm64_set_irq_stack_size(void)
+{
+	int min_thread_shift = 14;
+	ulong thread_shift = 0;
+	char buf1[BUFSIZE];
+	char *pos1, *pos2;
+	int errflag = 0;
+
+	if (kernel_symbol_exists("vmcoreinfo_data") &&
+		kernel_symbol_exists("vmcoreinfo_size")) {
+		/*
+		 * Referring to arch/arm64/include/asm/memory.h
+		 */
+		if (kernel_symbol_exists("kasan_enable_current"))
+			min_thread_shift += 1;
+
+		if (MEMBER_EXISTS("task_struct", "stack_vm_area") &&
+			(min_thread_shift < machdep->pageshift))
+			thread_shift = machdep->pageshift;
+		else
+			thread_shift = min_thread_shift;
+	} else {
+		sprintf(buf1, "x/32i vectors");
+		open_tmpfile();
+		if (!gdb_pass_through(buf1, pc->tmpfile, GNU_RETURN_ON_ERROR))
+			goto out;
+
+		rewind(pc->tmpfile);
+		while (fgets(buf1, BUFSIZE, pc->tmpfile)) {
+			if ((pos1 = strstr(buf1, "tbnz"))) {
+				if ((pos2 = strchr(pos1, '#'))) {
+					pos2 += 1;
+					for (pos1 = pos2; *pos2 != '\0' && *pos2 != ','; pos2++);
+					*pos2 = '\0';
+					thread_shift = stol(pos1, RETURN_ON_ERROR|QUIET, &errflag);
+					if (errflag)
+						thread_shift = 0;
+					break;
+				}
+			}
+		}
+out:
+		close_tmpfile();
+	}
+
+	if (thread_shift)
+		return ((1UL) << thread_shift);
+
+	return 0;
+}
+
 #endif  /* ARM64 */