Fix gdb.base/watch-vfork.exp: Watchpoint triggers after vfork (sw) (timeout) with Linux 2.6.32 and older version

https://sourceware.org/ml/gdb-patches/2014-04/msg00047.html

Got gdb.base/watch-vfork.exp: Watchpoint triggers after vfork (sw)
(timeout) with Linux 2.6.32 and older version.

The rootcause is after the test use "set can-use-hw-watchpoints 0" let GDB
doesn't use hardware breakpoint and set a watchpoint on "global", GDB
continue will keep single step inside function "vfork".
The Linux 2.6.32 and older version doesn't have commit
6580807da14c423f0d0a708108e6df6ebc8bc83d (get more info please goto
http://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/commit/?id=6580807da14c423f0d0a708108e6df6ebc8bc83d).
When the function "vfork" do syscall, the single step flag TIF_SINGLESTEP
will copy to child process.
Then GDB detach it, child process and parent process will be hanged.

So I make a patch that do a single step before detach.  Then TIF_SINGLESTEP
of child process in old Linux kernel will be cleared before detach.
Child process in new Linux kernel will not be affected by this single step.

2014-06-08  Hui Zhu  <hui@codesourcery.com>

	* common/linux-ptrace.c (linux_disable_event_reporting): New
	function.
	* common/linux-ptrace.h (linux_disable_event_reporting): New
	declaration.
	* linux-nat.c (linux_child_follow_fork): Do a single step before
	detach.

1 files changed, 24 insertions, 1 deletions

diff --git a/gdb/linux-nat.c b/gdb/linux-nat.c
index 6ca5c7e..3b117b5 100644
--- a/gdb/linux-nat.c
+++ b/gdb/linux-nat.c
@@ -414,6 +414,7 @@ holding the child stopped.  Try \"set detach-on-fork\" or \
       if (detach_fork)
 	{
 	  struct cleanup *old_chain;
+	  int status = 0;
 
 	  /* Before detaching from the child, remove all breakpoints
 	     from it.  If we forked, then this has already been taken
@@ -447,7 +448,29 @@ holding the child stopped.  Try \"set detach-on-fork\" or \
 
 	  if (linux_nat_prepare_to_resume != NULL)
 	    linux_nat_prepare_to_resume (child_lp);
-	  ptrace (PTRACE_DETACH, child_pid, 0, 0);
+
+	  /* When debugging an inferior in an architecture that supports
+	     hardware single stepping on a kernel without commit
+	     6580807da14c423f0d0a708108e6df6ebc8bc83d, the vfork child
+	     process starts with the TIF_SINGLESTEP/X86_EFLAGS_TF bits
+	     set if the parent process had them set.
+	     To work around this, single step the child process
+	     once before detaching to clear the flags.  */
+
+	  if (!gdbarch_software_single_step_p (target_thread_architecture
+						   (child_lp->ptid)))
+	    {
+	      int status;
+
+	      linux_disable_event_reporting (child_pid);
+	      if (ptrace (PTRACE_SINGLESTEP, child_pid, 0, 0) < 0)
+		perror_with_name (_("Couldn't do single step"));
+	      if (my_waitpid (child_pid, &status, 0) < 0)
+		perror_with_name (_("Couldn't wait vfork process"));
+	    }
+
+	  if (WIFSTOPPED (status))
+	    ptrace (PTRACE_DETACH, child_pid, 0, WSTOPSIG (status));
 
 	  do_cleanups (old_chain);
 	}