[PowerPC] Always clear watchpoint with PTRACE_SET_DEBUGREG

This patches changes low_prepare_to_resume in the ppc linux native target
to always clear the watchpoint when the old PTRACE_SET_DEBUGREG interface
is used, even if another watchpoint GDB requested to the target is
written right after using the same call.

The reason for this is that there were some older kernel versions for
which overwriting a watchpoint with PTRACE_SET_DEBUGREG would not
re-activate the watchpoint if it was previouly disabled following a hit.
This happened when the kernel was configured with CONFIG_HW_BREAKPOINT on
and uses perf events to install watchpoints.

Previously, the ppc linux native target would immediately remove or
insert watchpoints following a request from the upper layers.  This was
changed in commit 227c0bf4b3dd0cf65dceb58e729e9da81b38b5a7 to fix other
issues, which caused watchpoint requests to be applied to the inferior
only in low_prepare_to_resume, right before the inferior is resumed.

Usually, but maybe not always, after a hit, GDB will remove the
watchpoint, resume the inferior for a single-step, possibly report the
watchpoint hit to the user, and then re-insert the watchpoint before the
inferior is next resumed.  In this case there would be no problems, but
since I can't guarantee that there aren't other paths in GDB that allow
the user to set a new watchpoint after the first one hit, and after its
deletion by GDB, but before the inferior is resumed, there is a chance
that PTRACE_SET_DEBUGREG could be called directly without the watchpoint
first having been cleared, which could cause a false negative with the
older kernel versions.

This issue would affect kernel versions starting from this commit:

5aae8a53708025d4e718f0d2e7c2f766779ddc71

Up to the fix in this commit:

a53fd61ac2f411745471c1c877d5e072fbbf0e5c

gdb/ChangeLog:

	PR breakpoints/26385
	* ppc-linux-nat.c (ppc_linux_nat_target::low_prepare_to_resume):
	Always clear watchpoint with PTRACE_SET_DEBUGREG.

2 files changed, 18 insertions, 9 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 5fbc887..5106b28 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,11 @@
 2020-08-14  Pedro Franco de Carvalho  <pedromfc@linux.ibm.com>
 
+	PR breakpoints/26385
+	* ppc-linux-nat.c (ppc_linux_nat_target::low_prepare_to_resume):
+	Always clear watchpoint with PTRACE_SET_DEBUGREG.
+
+2020-08-14  Pedro Franco de Carvalho  <pedromfc@linux.ibm.com>
+
 	* ppc-linux-nat.c (ppc_linux_dreg_interface::detect)
 	(ppc_linux_nat_target::low_prepare_to_resume): Use ptrace () < 0
 	and >= to check return value instead of == -1 and != -1.
diff --git a/gdb/ppc-linux-nat.c b/gdb/ppc-linux-nat.c
index 89efdae..5f823d7 100644
--- a/gdb/ppc-linux-nat.c
+++ b/gdb/ppc-linux-nat.c
@@ -2909,20 +2909,23 @@ ppc_linux_nat_target::low_prepare_to_resume (struct lwp_info *lp)
     {
       gdb_assert (m_dreg_interface.debugreg_p ());
 
-      /* Passing 0 to PTRACE_SET_DEBUGREG will clear the
-	 watchpoint.  */
-      long wp = 0;
+      /* Passing 0 to PTRACE_SET_DEBUGREG will clear the watchpoint.  We
+	 always clear the watchpoint instead of just overwriting it, in
+	 case there is a request for a new watchpoint, because on some
+	 older kernel versions and configurations simply overwriting the
+	 watchpoint after it was hit would not re-enable it.  */
+      if (ptrace (PTRACE_SET_DEBUGREG, lp->ptid.lwp (), 0, 0) < 0)
+	perror_with_name (_("Error clearing hardware watchpoint"));
 
       /* GDB requested a watchpoint to be installed.  */
       if (process_it != m_process_info.end ()
 	  && process_it->second.requested_wp_val.has_value ())
-	wp = *(process_it->second.requested_wp_val);
-
-      long ret = ptrace (PTRACE_SET_DEBUGREG, lp->ptid.lwp (),
-			 0, wp);
+	{
+	  long wp = *(process_it->second.requested_wp_val);
 
-      if (ret < 0)
-	perror_with_name (_("Error setting hardware watchpoint"));
+	  if (ptrace (PTRACE_SET_DEBUGREG, lp->ptid.lwp (), 0, wp) < 0)
+	    perror_with_name (_("Error setting hardware watchpoint"));
+	}
     }
 
   lp_arch_info->debug_regs_stale = false;