Symptom:

Using the test program gdb.base/foll-fork.c, with follow-fork-mode set to
"child" and detach-on-fork set to "off", stepping or running past the fork
call results in the child process running to completion, when it should
just finish the single step.  In addition, the breakpoint is not removed
from the parent process, so if it is resumed it receives a SIGTRAP.

Cause:

No matter what the setting for detach-on-fork, when stepping past a fork,
the single-step breakpoint (step_resume_breakpoint) is not handled
correctly in the parent.  The SR breakpoint is cloned for the child
process, but before the clone is associated with the child it is treated as
a duplicate of the original, associated wth the parent.  This results in
the insertion state of the original SR breakpoint and the clone being
"swapped" by breakpoint.c:update_global_location_list, so that the clone is
marked as inserted.

In the case where the parent is not detached, the two breakpoints remain in
that state.  The breakpoint is never inserted in the child, because
although the cloned SR breakpoint is associated with the child, it is
marked as inserted.  When the child is resumed, it runs to completion.  The
breakpoint is never removed from the parent, so that if it is resumed after
the child exits, it gets a SIGTRAP.

Here is the sequence of events:

1) handle_inferior_event: FORK event is recognized.

2) handle_inferior_event: detach_breakpoints removes all breakpoints
from the child.

3) follow_fork: the parent SR breakpoint is cloned.  Part of this procedure
is to call update_global_location_list, which swaps the insertion state of
the original and cloned SR breakpoints as part of ensuring that duplicate
breakpoints are only inserted once.  At this point the original SR
breakpoint is not marked as inserted, and the clone is.  The breakpoint is
actually inserted in the parent but not the child.

4) follow_fork: the original breakpoint is deleted by calling
delete_step_resume_breakpoint.  Since the original is not marked as
inserted, the actual breakpoint remains in the parent process.
update_global_location_list is called again as part of the deletion.  The
clone is still associated with the parent, but since it is marked as
enabled and inserted, the breakpoint is left in the parent.

5) follow_fork: if detach-on-fork is 'on', the actual breakpoint will be
removed from the parent in target_detach, based on the cloned breakpoint
still associated with the parent.  Then the clone is no longer marked as
inserted.  In follow_inferior_reset_breakpoints the clone is associated
with the child, and can be inserted.

If detach-on-fork is 'off', the actual breakpoint in the parent is never
removed (although the breakpoint had been deleted from the list).  Since
the clone continues to be marked 'inserted', the SR breakpoint is never
inserted in the child.

Fix:

Set the cloned breakpoint as disabled from the moment it is created.  This
is done by modifying clone_momentary_breakpoint to take an additional
argument, LOC_ENABLED, which is used as the value of the
bp_location->enabled member.  The clone must be disabled at that point
because clone_momentary_breakpoint calls update_global_location_list, which
will swap treat the clone as a duplicate of the original breakpoint if it
is enabled.

All the calls to clone_momentary_breakpoint had to be modified to pass '1'
or '0'.  I looked at implementing an enum for the enabled member, but
concluded that readability would suffer because there are so many places it
is used as a boolean, e.g. "if (bl->enabled)".

In follow_inferior_reset_breakpoints the clone is set to enabled once it
has been associated with the child process.  With this, the bp_location
'inserted' member is maintained correctly throughout the follow-fork
procedure and the behavior is as expected.

The same treatment is given to the exception_resume_breakpoint when
following a fork.

Testing:

Ran 'make check' on Linux x64.

Along with the fix above, the coverage of the follow-fork test
gdb.base/foll-fork.exp was expanded to:

1) cover all the combinations of values for
   follow-fork-mode and detach-on-fork

2) make sure that both user breakpoints and
   single-step breakpoints are propagated
   correctly to the child

3) check that the inferior list has the
   expected contents after following the fork.

4) check that unfollowed, undetached inferiors
   can be resumed.

gdb/

2014-06-18  Don Breazeal  <donb@codesourcery.com>

	* breakpoint.c (set_longjmp_breakpoint): Call
	momentary_breakpoint_from_master with additional argument.
	(set_longjmp_breakpoint_for_call_dummy): Call
	momentary_breakpoint_from_master with additional argument.
	(set_std_terminate_breakpoint): Call
	momentary_breakpoint_from_master with additional argument.
	(momentary_breakpoint_from_master): Add argument to function
	definition and use it to initialize structure member flag.
	(clone_momentary_breakpoint): Call
	momentary_breakpoint_from_master with additional argument.
	* infrun.c (follow_inferior_reset_breakpoints): Clear structure
	member flags set in momentary_breakpoint_from_master.

gdb/testsuite/

2014-06-18  Don Breazeal  <donb@codesourcery.com>

	* gdb.base/foll-fork.exp (default_fork_parent_follow):
	Deleted procedure.
	(explicit_fork_parent_follow): Deleted procedure.
	(explicit_fork_child_follow): Deleted procedure.
	(test_follow_fork): New procedure.
	(do_fork_tests): Replace calls to deleted procedures with
	calls to test_follow_fork and reset GDB for subsequent
	procedure calls.

0 files changed, 0 insertions, 0 deletions