scripts/gdb: support getting current task struct in UML

A running x86 UML kernel reports with architecture "i386:x86-64" as it is
a sub-architecture.  However, a difference with bare-metal x86 kernels is
in how it manages tasks and the current task struct.  To identify that the
inferior is a UML kernel and not bare-metal, check for the existence of
the UML specific symbol "cpu_tasks" which contains the current task
struct.

Link: https://lkml.kernel.org/r/b839d611e2906ccef2725c34d8e353fab35fe75e.1677469905.git.development@efficientek.com
Signed-off-by: Glenn Washburn <[email protected]>
Reviewed-by: Jan Kiszka <[email protected]>
Cc: Anton Ivanov <[email protected]>
Cc: Johannes Berg <[email protected]>
Cc: Kieran Bingham <[email protected]>
Cc: Richard Weinberger <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

scripts/gdb/linux/cpus.py

@@ -163,8 +163,14 @@ 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("&pcpu_hot.current_task")
-        return per_cpu(var_ptr, cpu).dereference()
+        if gdb.lookup_global_symbol("cpu_tasks"):
+            # This is a UML kernel, which stores the current task
+            # differently than other x86 sub architectures
+            var_ptr = gdb.parse_and_eval("(struct task_struct *)cpu_tasks[0].task")
+            return var_ptr.dereference()
+        else:
+            var_ptr = gdb.parse_and_eval("&pcpu_hot.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: