[binutils-gdb] Workaround a FreeBSD ptrace() bug with clearing thread events.

[John Baldwin <jhb at sourceware dot org>]

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=f169cfdc08761a3d9fcd587ad8661102672403ec

commit f169cfdc08761a3d9fcd587ad8661102672403ec
Author: John Baldwin <jhb@FreeBSD.org>
Date:   Tue Oct 24 21:06:00 2017 -0700

    Workaround a FreeBSD ptrace() bug with clearing thread events.
    
    When multiple threads within a process wish to report STOPPED events
    from wait(), the kernel picks one thread event as the thread event to
    report.  The chosen thread event is retrieved via PT_LWPINFO by
    passing the process ID as the request pid.  If multiple events are
    pending, then the subsequent wait() after resuming a process will
    report another STOPPED event after resuming the process to handle the
    next thread event and so on.
    
    A single thread event is cleared as a side effect of resuming the
    process with PT_CONTINUE, PT_STEP, etc.  In older kernels, however,
    the request pid was used to select which thread's event was cleared
    rather than always clearing the event that was just reported.  To
    avoid clearing the event of the wrong LWP, always pass the process ID
    instead of an LWP ID to PT_CONTINUE or PT_SYSCALL.
    
    In the case of stepping, the process ID cannot be used with PT_STEP
    since it would step the thread that reported an event which may not be
    the thread indicated by PTID.  For stepping, use PT_SETSTEP to enable
    stepping on the desired thread before resuming the process via
    PT_CONTINUE instead of using PT_STEP.
    
    This manifested as a failure in the
    gdb.threads/continue-pending-status.exp test.  Specifically, if thread
    2 reported a breakpoint and the test thus switched to thread 3 before
    continuing, thread 3's event (if any) was discarded and thread 2's
    breakpoint remained pending and was reported a second time as a
    duplicate event.  As a result, the PC was decremented twice for the
    same breakpoint resulting in an illegal instruction fault on x86.
    
    gdb/ChangeLog:
    
    	* fbsd-nat.c (fbsd_resume): Use PT_SETSTEP for stepping and a
    	wildcard process pid for super_resume for kernels with a
    	specific bug.

Diff:
---
 gdb/ChangeLog  |  6 ++++++
 gdb/fbsd-nat.c | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 38 insertions(+)

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 9b899e5..8ab6381 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,9 @@
+2018-02-28  John Baldwin  <jhb@FreeBSD.org>
+
+	* fbsd-nat.c (fbsd_resume): Use PT_SETSTEP for stepping and a
+	wildcard process pid for super_resume for kernels with a
+	specific bug.
+
 2018-02-27  Phil Muldoon  <pmuldoon@redhat.com>
 
 	* compile/compile.c (get_args): Add additional comments
diff --git a/gdb/fbsd-nat.c b/gdb/fbsd-nat.c
index b352418..3a216ab 100644
--- a/gdb/fbsd-nat.c
+++ b/gdb/fbsd-nat.c
@@ -1143,6 +1143,38 @@ fbsd_resume (struct target_ops *ops,
 	}
       ptid = inferior_ptid;
     }
+
+#if __FreeBSD_version < 1200052
+  /* When multiple threads within a process wish to report STOPPED
+     events from wait(), the kernel picks one thread event as the
+     thread event to report.  The chosen thread event is retrieved via
+     PT_LWPINFO by passing the process ID as the request pid.  If
+     multiple events are pending, then the subsequent wait() after
+     resuming a process will report another STOPPED event after
+     resuming the process to handle the next thread event and so on.
+
+     A single thread event is cleared as a side effect of resuming the
+     process with PT_CONTINUE, PT_STEP, etc.  In older kernels,
+     however, the request pid was used to select which thread's event
+     was cleared rather than always clearing the event that was just
+     reported.  To avoid clearing the event of the wrong LWP, always
+     pass the process ID instead of an LWP ID to PT_CONTINUE or
+     PT_SYSCALL.
+
+     In the case of stepping, the process ID cannot be used with
+     PT_STEP since it would step the thread that reported an event
+     which may not be the thread indicated by PTID.  For stepping, use
+     PT_SETSTEP to enable stepping on the desired thread before
+     resuming the process via PT_CONTINUE instead of using
+     PT_STEP.  */
+  if (step)
+    {
+      if (ptrace (PT_SETSTEP, get_ptrace_pid (ptid), NULL, 0) == -1)
+	perror_with_name (("ptrace"));
+      step = 0;
+    }
+  ptid = ptid_t (ptid.pid ());
+#endif
   super_resume (ops, ptid, step, signo);
 }