threads/2015: DECR_PC_AFTER_BREAK mishandled in threaded debugging

[schwab at suse dot de]

>Number:         2015
>Category:       threads
>Synopsis:       DECR_PC_AFTER_BREAK mishandled in threaded debugging
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat Sep 24 10:48:01 UTC 2005
>Closed-Date:
>Last-Modified:
>Originator:     schwab@suse.de
>Release:        6.3.50.20050923-cvs
>Organization:
>Environment:
i386-linux, x86_64-linux, s390-linux
>Description:
During single stepping a threaded process a spurious SIGTRAP is reported instead of treating it as a breakpoint hit.  That can also result in the process receiving a SIGSEGV because the pc isn't properly backed up over the breakpoint before continuing.  This only happens on targets that set DECR_PC_AFTER_BREAK to non-zero.
>How-To-Repeat:
$ g++ -g thread4.cc -o thread4 -lpthread
$ gdb/gdb thread4
GNU gdb 6.3.50.20050923-cvs
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "x86_64-linux"...Using host libthread_db library "/lib64/tls/libthread_db.so.1".

(gdb) b 93
Breakpoint 1 at 0x401095: file thread4.cc, line 93.
(gdb) r 5 10
Starting program: /tmp/thread4 5 10
[Thread debugging using libthread_db enabled]
[New Thread 46912505087168 (LWP 31048)]
[New Thread 1075837280 (LWP 31051)]
[Switching to Thread 1075837280 (LWP 31051)]

Breakpoint 1, consumerThread (data=0x7fffffd4dd20) at thread4.cc:93
93            int data = tq->consume ();
(gdb) n
[New Thread 1077934432 (LWP 31052)]
[Switching to Thread 1077934432 (LWP 31052)]

Breakpoint 1, consumerThread (data=0x7fffffd4dd20) at thread4.cc:93
93            int data = tq->consume ();
(gdb) n
[New Thread 1080031584 (LWP 31053)]
[New Thread 1082128736 (LWP 31054)]
[Switching to Thread 1080031584 (LWP 31053)]

Breakpoint 1, consumerThread (data=0x7fffffd4dd20) at thread4.cc:93
93            int data = tq->consume ();
(gdb) 
[New Thread 1084225888 (LWP 31055)]
[Switching to Thread 1082128736 (LWP 31054)]

Breakpoint 1, consumerThread (data=0x7fffffd4dd20) at thread4.cc:93
93            int data = tq->consume ();
(gdb) 
[Switching to Thread 1077934432 (LWP 31052)]
94            if (data == -1)
(gdb) 
97            tq->process (data);
(gdb) s
[Switching to Thread 1080031584 (LWP 31053)]
94            if (data == -1)
(gdb) n
97            tq->process (data);
(gdb) s
[Switching to Thread 1075837280 (LWP 31051)]
94            if (data == -1)
(gdb) n
97            tq->process (data);
(gdb) s
TestQueue::process (this=0x7fffffd4dd20, data=2) at thread4.cc:66
66          pthread_mutex_lock (&queueLock_);
(gdb) n
[Switching to Thread 1084225888 (LWP 31055)]

Breakpoint 1, consumerThread (data=0x7fffffd4dd20) at thread4.cc:93
93            int data = tq->consume ();
(gdb) 
[Switching to Thread 1075837280 (LWP 31051)]
TestQueue::process (this=0x7fffffd4dd20, data=2) at thread4.cc:67
67          cout << "Thread t@" << pthread_self ()
(gdb) 
Thread t@1075837280 consuming [2]
69          pthread_mutex_unlock (&queueLock_);
(gdb) 
70        }
(gdb) 
Thread t@1080031584 consuming [1]
Thread t@1082128736 consuming [3]
Thread t@1077934432 consuming [0]
consumerThread (data=0x7fffffd4dd20) at thread4.cc:93
93            int data = tq->consume ();
(gdb) 

Program received signal SIGTRAP, Trace/breakpoint trap.
[Switching to Thread 1080031584 (LWP 31053)]
0x00000000004010b6 in consumerThread (data=0x7fffffd4dd20) at thread4.cc:97
97            tq->process (data);
(gdb) 
>Fix:
A partial fix is to call context_switch in adjust_pc_after_break so that prev_pc contains the value connected to the stopped thread, but that does not fix the spurious SIGSEGV.

--- infrun.c	01 Aug 2005 11:52:13 +0200	1.203
+++ infrun.c	24 Sep 2005 12:38:39 +0200	
@@ -1174,6 +1174,21 @@ adjust_pc_after_break (struct execution_
   if (ecs->ws.value.sig != TARGET_SIGNAL_TRAP)
     return;
 
+  /* Switch context now so that we access the state of the stopped
+     thread.  */
+  if (!ptid_equal (ecs->ptid, inferior_ptid))
+    {
+      if (debug_infrun)
+	fprintf_unfiltered (gdb_stdlog, "infrun: context switch\n");
+
+      context_switch (ecs);
+
+      if (deprecated_context_hook)
+	deprecated_context_hook (pid_to_thread_id (ecs->ptid));
+
+      flush_cached_frames ();
+    }
+
   /* Find the location where (if we've hit a breakpoint) the
      breakpoint would be.  */
   breakpoint_pc = read_pc_pid (ecs->ptid) - DECR_PC_AFTER_BREAK;
>Release-Note:
>Audit-Trail:
>Unformatted:
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