[PATCH 2/3] gdb/breakpoint: set the condition exp after parsing the condition successfully

[Tankut Baris Aktemur <tankut.baris.aktemur@intel.com>]

In 'set_breakpoint_condition', GDB resets the condition expressions
before parsing the condition input by the user.  This leads to the
problem of losing the condition expressions if the new condition
does not parse successfully and is thus rejected.

For instance:

  $ gdb ./test
  Reading symbols from ./test...
  (gdb) start
  Temporary breakpoint 1 at 0x114e: file test.c, line 4.
  Starting program: test

  Temporary breakpoint 1, main () at test.c:4
  4         int a = 10;
  (gdb) break 5
  Breakpoint 2 at 0x555555555155: file test.c, line 5.

Now define a condition that would evaluate to false.  Next, attempt
to overwrite that with an invalid condition:

  (gdb) cond 2 a == 999
  (gdb) cond 2 gibberish
  No symbol "gibberish" in current context.
  (gdb) info breakpoints
  Num     Type           Disp Enb Address            What
  2       breakpoint     keep y   0x0000555555555155 in main at test.c:5
          stop only if a == 999

It appears as if the bad condition is successfully rejected.  But if we
resume the program, we see that we hit the breakpoint although the condition
would evaluate to false.

  (gdb) continue
  Continuing.

  Breakpoint 2, main () at test.c:5
  5         a = a + 1; /* break-here */

Fix the problem by not resetting the condition expressions before
parsing the condition input.

Suppose the fix is applied.  A similar problem could occur if the
condition is valid, but has "junk" at the end.  In this case, parsing
succeeds, but an error is raised immediately after.  It is too late,
though; the condition expression is already updated.

For instance:

  $ gdb ./test
  Reading symbols from ./test...
  (gdb) start
  Temporary breakpoint 1 at 0x114e: file test.c, line 4.
  Starting program: test

  Temporary breakpoint 1, main () at test.c:4
  4         int a = 10;
  (gdb) break 5
  Breakpoint 2 at 0x555555555155: file test.c, line 5.
  (gdb) cond 2 a == 999
  (gdb) cond 2 a == 10 if
  Junk at end of expression
  (gdb) info breakpoints
  Num     Type           Disp Enb Address            What
  2       breakpoint     keep y   0x0000555555555155 in main at test.c:5
          stop only if a == 999
  (gdb) c
  Continuing.

  Breakpoint 2, main () at test.c:5
  5         a = a + 1; /* break-here */
  (gdb)

We should not have hit the breakpoint because the condition would
evaluate to false.

Fix this problem by updating the condition expression of the breakpoint
after parsing the input successfully and checking that there is no
remaining junk.

gdb/ChangeLog:
2020-06-29  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* breakpoint.c (set_breakpoint_condition): Update the condition
	expressions after checking that the input condition string parses
	successfully and does not contain junk at the end.

gdb/testsuite/ChangeLog:
2020-06-29  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* gdb.base/condbreak-bad.exp: Extend the test with scenarios
	that attempt to overwrite and existing condition with a condition
	that fails parsing and also with a condition that parses fine
	but contains junk at the end.
---
 gdb/breakpoint.c                         | 46 +++++++------
 gdb/testsuite/gdb.base/condbreak-bad.exp | 88 ++++++++++++++++++++++++
 2 files changed, 112 insertions(+), 22 deletions(-)

diff --git a/gdb/breakpoint.c b/gdb/breakpoint.c
index 1fc2d1b8966..abda470fe21 100644
--- a/gdb/breakpoint.c
+++ b/gdb/breakpoint.c
@@ -834,30 +834,30 @@ void
 set_breakpoint_condition (struct breakpoint *b, const char *exp,
 			  int from_tty)
 {
-  if (is_watchpoint (b))
-    {
-      struct watchpoint *w = (struct watchpoint *) b;
-
-      w->cond_exp.reset ();
-    }
-  else
+  if (*exp == 0)
     {
-      struct bp_location *loc;
+      xfree (b->cond_string);
+      b->cond_string = nullptr;
 
-      for (loc = b->loc; loc; loc = loc->next)
+      if (is_watchpoint (b))
 	{
-	  loc->cond.reset ();
+	  struct watchpoint *w = (struct watchpoint *) b;
 
-	  /* No need to free the condition agent expression
-	     bytecode (if we have one).  We will handle this
-	     when we go through update_global_location_list.  */
+	  w->cond_exp.reset ();
 	}
-    }
+      else
+	{
+	  struct bp_location *loc;
 
-  if (*exp == 0)
-    {
-      xfree (b->cond_string);
-      b->cond_string = nullptr;
+	  for (loc = b->loc; loc; loc = loc->next)
+	    {
+	      loc->cond.reset ();
+
+	      /* No need to free the condition agent expression
+		 bytecode (if we have one).  We will handle this
+		 when we go through update_global_location_list.  */
+	    }
+	}
 
       if (from_tty)
 	printf_filtered (_("Breakpoint %d now unconditional.\n"), b->number);
@@ -872,9 +872,10 @@ set_breakpoint_condition (struct breakpoint *b, const char *exp,
 
 	  innermost_block_tracker tracker;
 	  arg = exp;
-	  w->cond_exp = parse_exp_1 (&arg, 0, 0, 0, &tracker);
+	  expression_up new_exp = parse_exp_1 (&arg, 0, 0, 0, &tracker);
 	  if (*arg)
 	    error (_("Junk at end of expression"));
+	  w->cond_exp = std::move (new_exp);
 	  w->cond_exp_valid_block = tracker.block ();
 	}
       else
@@ -884,11 +885,12 @@ set_breakpoint_condition (struct breakpoint *b, const char *exp,
 	  for (loc = b->loc; loc; loc = loc->next)
 	    {
 	      arg = exp;
-	      loc->cond =
-		parse_exp_1 (&arg, loc->address,
-			     block_for_pc (loc->address), 0);
+	      expression_up new_exp
+		= parse_exp_1 (&arg, loc->address,
+			       block_for_pc (loc->address), 0);
 	      if (*arg)
 		error (_("Junk at end of expression"));
+	      loc->cond = std::move (new_exp);
 	    }
 	}
 
diff --git a/gdb/testsuite/gdb.base/condbreak-bad.exp b/gdb/testsuite/gdb.base/condbreak-bad.exp
index a01ba2a9340..84d32a0f15d 100644
--- a/gdb/testsuite/gdb.base/condbreak-bad.exp
+++ b/gdb/testsuite/gdb.base/condbreak-bad.exp
@@ -38,3 +38,91 @@ gdb_test "info break" \
 	 "${decimal}${fill}breakpoint${fill}keep y${fill}:${bp_location}"] \
     "breakpoint is unconditional"
 
+# Now define a valid condition.  Attempt to override that with a 'bad'
+# condition again.  The condition should be preserved.
+with_test_prefix "with run" {
+    gdb_test_no_output "cond $bpnum a == 10"
+
+    gdb_test "cond $bpnum gibberish" \
+	"No symbol \"gibberish\" in current context." \
+	"attempt a bad condition"
+
+    gdb_test "info break" \
+	[multi_line \
+	     "Num${fill}What" \
+	     "${decimal}${fill}breakpoint${fill}keep y${fill}:${bp_location}" \
+	     "${fill}stop only if a == 10${fill}"] \
+	"breakpoint condition is preserved"
+
+    # Run the code.  We should hit the breakpoint, because the
+    # condition evaluates to true.
+
+    gdb_run_cmd
+    gdb_test "" ".*reakpoint .*, main .*${srcfile}.*" "run to the bp"
+}
+
+# Restart.  Repeat the test above after the program has started.
+# This is needed to check a scenario where the breakpoints are no
+# longer re-inserted due to solib events.  Note that runto_main
+# deletes the breakpoints.
+with_test_prefix "with continue 1" {
+    if {![runto_main]} {
+	fail "could not run to main"
+	return -1
+    }
+
+    gdb_breakpoint "$bp_location"
+    set bpnum [get_integer_valueof "\$bpnum" 0 "get bpnum"]
+
+    gdb_test_no_output "cond $bpnum a == 10"
+
+    gdb_test "cond $bpnum gibberish" \
+	"No symbol \"gibberish\" in current context." \
+	"attempt a bad condition"
+
+    # Resume.  We should hit the breakpoint, because the
+    # condition evaluates to true.
+    gdb_continue_to_breakpoint "${srcfile}:${bp_location}"
+}
+
+# Repeat with a condition that evaluates to false.
+with_test_prefix "with continue 2" {
+    if {![runto_main]} {
+	fail "could not run to main"
+	return -1
+    }
+
+    gdb_breakpoint "$bp_location"
+    set bpnum [get_integer_valueof "\$bpnum" 0 "get bpnum"]
+
+    gdb_test_no_output "cond $bpnum a == 999"
+    
+    gdb_test "cond $bpnum gibberish" \
+	"No symbol \"gibberish\" in current context." \
+	"attempt a bad condition"
+
+    # Resume.  We should *not* hit the breakpoint, because the
+    # condition evaluates to false.
+    gdb_continue_to_end
+}
+
+# Repeat with a condition that contains junk at the end.
+with_test_prefix "with junk" {
+    if {![runto_main]} {
+	fail "could not run to main"
+	return -1
+    }
+
+    gdb_breakpoint "$bp_location"
+    set bpnum [get_integer_valueof "\$bpnum" 0 "get bpnum"]
+
+    gdb_test_no_output "cond $bpnum a == 999"
+
+    gdb_test "cond $bpnum a == 10 if" \
+	"Junk at end of expression" \
+	"attempt a bad condition"
+
+    # Resume.  We should *not* hit the breakpoint, because the
+    # condition evaluates to false.
+    gdb_continue_to_end
+}
-- 
2.17.1