Enable crash to change gdb thread context

Previously we can only view the stack unwinding for the tasks which are
running on each CPUs. This patch will enable the ability to view
arbitrary tasks stack unwinding.

After crash get initialized, "info threads" will output like the
following:

crash> info threads
  Id   Target Id         Frame
  1    CPU 0             native_safe_halt () at arch/x86/include/asm/irqflags.h:54
...
* 8    CPU 7             blk_mq_rq_timed_out (req=0xffff880fdb246000, reserved=reserved@entry=false) at block/blk-mq.c:640
...
  13   CPU 12            <unavailable> in ?? ()
  14   CPU 13            native_safe_halt () at arch/x86/include/asm/irqflags.h:54
...

crash> ps
      PID    PPID  CPU       TASK        ST  %MEM      VSZ      RSS  COMM
>       0       0   0  ffffffff819f9480  RU   0.0        0        0  [swapper/0]
>       0       0   1  ffff880169411fa0  RU   0.0        0        0  [swapper/1]
...
	0       0  23  ffff8801694e0000  RU   0.0        0        0  [swapper/23]
	1       0  13  ffff880169b30000  IN   0.0   193052     4180  systemd

"info threads" show the tasks which are currently running on each CPU. If we'd
like to view systemd task's stack unwinding, which is inactive status, we
do the following:

crash> set 1
or
crash> set ffff880169b30000

Then the register cache of systemd will be swapped into CPU 13:

crash> info threads
  Id   Target Id         Frame
  1    CPU 0             native_safe_halt () at arch/x86/include/asm/irqflags.h:54
...
  8    CPU 7             blk_mq_rq_timed_out (req=0xffff880fdb246000, reserved=reserved@entry=false) at block/blk-mq.c:640
...
  13   CPU 12            <unavailable> in ?? ()
* 14   CPU 13            0xffffffff816a8f65 in context_switch (rq=0x0, next=0x0, prev=0xffff880169b30000) at kernel/sched/core.c:2527
...

And we can view the stack unwinding of systemd:

crash> bt
PID: 1        TASK: ffff880169b30000  CPU: 13   COMMAND: "systemd"
 #0 [ffff880169b3bd58] __schedule at ffffffff816a8f65
 crash-utility#1 [ffff880169b3bdc0] schedule at ffffffff816a94e9
 crash-utility#2 [ffff880169b3bdd0] schedule_hrtimeout_range_clock at ffffffff816a86fd
 crash-utility#3 [ffff880169b3be68] schedule_hrtimeout_range at ffffffff816a8733
 crash-utility#4 [ffff880169b3be78] ep_poll at ffffffff8124bb7e
 crash-utility#5 [ffff880169b3bf30] sys_epoll_wait at ffffffff8124d00d
 crash-utility#6 [ffff880169b3bf80] system_call_fastpath at ffffffff816b5009
    RIP: 00007f0449407923  RSP: 00007ffc35a3c378  RFLAGS: 00010246
    RAX: 00000000000000e8  RBX: ffffffff816b5009  RCX: 0000000000000071
    RDX: 000000000000001d  RSI: 00007ffc35a3d5a0  RDI: 0000000000000004
    RBP: 00007ffc35a3d810   R8: 0000000000000000   R9: 0000000000000000
    R10: 00000000ffffffff  R11: 0000000000000293  R12: 0000563ca2ebe980
    R13: 0000000000000003  R14: ffffffffffffffff  R15: 0000000000000001
    ORIG_RAX: 00000000000000e8  CS: 0033  SS: 002b
crash> gdb bt
 #0  0xffffffff816a8f65 in context_switch (rq=0x0, next=0x0, prev=0xffff880169b30000) at kernel/sched/core.c:2527
 crash-utility#1  __schedule () at kernel/sched/core.c:3540
 crash-utility#2  0xffffffff816a94e9 in schedule () at kernel/sched/core.c:3577
 crash-utility#3  0xffffffff816a86fd in schedule_hrtimeout_range_clock (expires=expires@entry=0x0, delta=delta@entry=0, mode=mode@entry=HRTIMER_MODE_ABS, clock=clock@entry=1) at kernel/hrtimer.c:1724
 crash-utility#4  0xffffffff816a8733 in schedule_hrtimeout_range (expires=expires@entry=0x0, delta=delta@entry=0, mode=mode@entry=HRTIMER_MODE_ABS) at kernel/hrtimer.c:1778
 crash-utility#5  0xffffffff8124bb7e in ep_poll (ep=0xffff880fd861f8c0, events=events@entry=0x7ffc35a3d5a0, maxevents=maxevents@entry=29, timeout=timeout@entry=-1) at fs/eventpoll.c:1669
 crash-utility#6  0xffffffff8124d00d in SYSC_epoll_wait (timeout=<optimized out>, maxevents=29, events=<optimized out>, epfd=<optimized out>) at fs/eventpoll.c:2043
 crash-utility#7  SyS_epoll_wait (epfd=<optimized out>, events=140721208415648, maxevents=29, timeout=4294967295) at fs/eventpoll.c:2008
 crash-utility#8  <signal handler called>
 crash-utility#9  0x00007f0449407923 in ?? ()

Signed-off-by: Tao Liu <[email protected]>
Signed-off-by: Aditya Gupta <[email protected]>

crash_target.c

@@ -29,9 +29,9 @@ extern "C" int gdb_readmem_callback(unsigned long, void *, int, int);
 extern "C" int crash_get_nr_cpus(void);
 extern "C" int crash_get_cpu_reg (int cpu, int regno, const char *regname,
                                   int regsize, void *val);
-extern "C" int gdb_change_cpu_context (unsigned int cpu);
 extern "C" int set_cpu(int cpu, int print_context);
-
+extern "C" int crash_set_thread(ulong);
+extern "C" int gdb_change_thread_context (ulong);
 
 /* The crash target.  */
 
@@ -109,11 +109,13 @@ crash_target::xfer_partial (enum target_object object, const char *annex,
 
 #define CRASH_INFERIOR_PID 1
 
+crash_target *target = NULL;
+
 void
 crash_target_init (void)
 {
   int nr_cpus = crash_get_nr_cpus();
-  crash_target *target = new crash_target ();
+  target = new crash_target ();
 
   /* Own the target until it is successfully pushed.  */
   target_ops_up target_holder (target);
@@ -136,24 +138,33 @@ crash_target_init (void)
   reinit_frame_cache ();
 }
 
-/*
- * Change gdb's thread context to the thread on given CPU
- **/
 extern "C" int
-gdb_change_cpu_context(unsigned int cpu)
+gdb_change_thread_context (ulong task)
 {
-  ptid_t ptid = ptid_t(CRASH_INFERIOR_PID, 0, cpu);
-  inferior *inf = current_inferior ();
-  thread_info *tp = find_thread_ptid (inf, ptid);
-
-  if (tp == nullptr)
+  int tried = 0;
+  inferior* inf = current_inferior();
+  int cpu = crash_set_thread(task);
+  if (cpu < 0)
     return FALSE;
 
-  /* Making sure that crash's context is same */
-  set_cpu(cpu, FALSE);
-
-  /* Switch to the thread */
-  switch_to_thread(tp);
+  ptid_t ptid = ptid_t(CRASH_INFERIOR_PID, 0, cpu);
 
-  return TRUE;
+retry:
+   thread_info *tp = find_thread_ptid (inf, ptid);
+   if (tp == nullptr && !tried) {
+     thread_info *thread = add_thread_silent(target,
+				ptid_t(CRASH_INFERIOR_PID, 0, cpu));
+     tried++;
+     if (thread) {
+       goto retry;
+     }
+   }
+
+   if (tp == nullptr && tried)
+     return FALSE;
+
+   target_fetch_registers(get_thread_regcache(tp), -1);
+   switch_to_thread(tp);
+   reinit_frame_cache ();
+   return TRUE;
 }

defs.h

@@ -6068,6 +6068,7 @@ void sort_tgid_array(void);
 int sort_by_tgid(const void *, const void *);
 int in_irq_ctx(ulonglong, int, ulong);
 void check_stack_overflow(void);
+int gdb_change_thread_context (ulong);
 
 /*
  *  extensions.c
@@ -6107,6 +6108,7 @@ void dump_kernel_table(int);
 void dump_bt_info(struct bt_info *, char *where);
 void dump_log(int);
 void parse_kernel_version(char *);
+int crash_set_thread(ulong);
 
 #define LOG_LEVEL(v) ((v) & 0x07)
 #define SHOW_LOG_LEVEL    (0x1)

kernel.c

@@ -6510,6 +6510,28 @@ set_cpu(int cpu, int print_context)
 		show_context(CURRENT_CONTEXT());
 }
 
+int
+crash_set_thread(ulong task)
+{
+	bool found = FALSE;
+	struct task_context *tc = FIRST_CONTEXT();
+
+	for (int i = 0; i < RUNNING_TASKS(); i++, tc++) {
+		if (tc->task == task) {
+			found = TRUE;
+			break;
+		}
+	}
+
+	if (!found)
+		return -1;
+
+	if (CURRENT_TASK() == tc->task)
+		return tc->processor;
+
+	set_context(tc->task, NO_PID, TRUE);
+	return tc->processor;
+}
 
 /*
  *  Collect the irq_desc[] entry along with its associated handler and

task.c

@@ -5304,7 +5304,7 @@ set_context(ulong task, ulong pid, uint update_gdb_thread)
 
 		/* change the selected thread in gdb, according to current context */
 		if (update_gdb_thread)
-			return gdb_change_cpu_context(tc->processor);
+			return gdb_change_thread_context(tc->task);
 		else
 			return TRUE;
 	} else {