scripts/gdb: add lx_current support for arm64

arm64 uses SP_EL0 to save the current task_struct address.  While running
in EL0, SP_EL0 is clobbered by userspace.  So if the upper bit is not 1
(not TTBR1), the current address is invalid.  This patch checks the upper
bit of SP_EL0, if the upper bit is 1, lx_current() of arm64 will return
the derefrence of current task.  Otherwise, lx_current() will tell users
they are running in userspace(EL0).

While arm64 is running in EL0, it is actually pointless to print current
task as the memory of kernel space is not accessible in EL0.

Link: https://lkml.kernel.org/r/20210314203444.15188-3-song.bao.hua@hisilicon.com
Signed-off-by: Barry Song <song.bao.hua@hisilicon.com>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kieran Bingham <kbingham@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/dev-tools/gdb-kernel-debugging.rst

@@ -114,7 +114,7 @@ Examples of using the Linux-provided gdb helpers
     [     0.000000] BIOS-e820: [mem 0x000000000009fc00-0x000000000009ffff] reserved
     ....
 
-- Examine fields of the current task struct(supported by x86 only)::
+- Examine fields of the current task struct(supported by x86 and arm64 only)::
 
     (gdb) p $lx_current().pid
     $1 = 4998

scripts/gdb/linux/cpus.py

@@ -16,6 +16,9 @@ import gdb
 from linux import tasks, utils
 
 
+task_type = utils.CachedType("struct task_struct")
+
+
 MAX_CPUS = 4096
 
 
@@ -157,9 +160,19 @@ Note that VAR has to be quoted as string."""
 PerCpu()
 
 def get_current_task(cpu):
+    task_ptr_type = task_type.get_type().pointer()
+
     if utils.is_target_arch("x86"):
          var_ptr = gdb.parse_and_eval("&current_task")
          return per_cpu(var_ptr, cpu).dereference()
+    elif utils.is_target_arch("aarch64"):
+         current_task_addr = gdb.parse_and_eval("$SP_EL0")
+         if((current_task_addr >> 63) != 0):
+             current_task = current_task_addr.cast(task_ptr_type)
+             return current_task.dereference()
+         else:
+             raise gdb.GdbError("Sorry, obtaining the current task is not allowed "
+                                "while running in userspace(EL0)")
     else:
         raise gdb.GdbError("Sorry, obtaining the current task is not yet "
                            "supported with this arch")