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gdb: fix handling of vfork by multi-threaded program (follow-fork-mode=parent, detach-on-fork=on)

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There is a problem with how GDB handles a vfork happening in a
multi-threaded program.  This problem was reported to me by somebody not
using vfork directly, but using system(3) in a multi-threaded program,
which may be implemented using vfork.

This patch only deals about the follow-fork-mode=parent,
detach-on-fork=on case, because it would be too much to chew at once to
fix the bugs in the other cases as well (I tried).

The problem
-----------

When a program vforks, the parent thread is suspended by the kernel
until the child process exits or execs.  Specifically, in a
multi-threaded program, only the thread that called vfork is suspended,
other threads keep running freely. This is documented in the vfork(2)
man page ("Caveats" section).

Let's suppose GDB is handling a vfork and the user's desire is to detach
from the child. Before detaching the child, GDB must remove the software
breakpoints inserted in the shared parent/child address space, in case
there's a breakpoint in the path the child is going to take before
exec'ing or exit'ing (unlikely, but possible). Otherwise the child could
hit a breakpoint instruction while running outside the control of GDB,
which would make it crash.  GDB must also avoid re-inserting breakpoints
in the parent as long as it didn't receive the "vfork done" event (that
is, when the child has exited or execed): since the address space is
shared with the child, that would re-insert breakpoints in the child
process also. So what GDB does is:

  1. Receive "vfork" event for the parent
  2. Remove breakpoints from the (shared) address space and set
     program_space::breakpoints_not_allowed to avoid re-inserting them
  3. Detach from the child thread
  4. Resume the parent
  5. Wait for and receive "vfork done" event for the parent
  6. Clean program_space::breakpoints_not_allowed and re-insert
     breakpoints
  7. Resume the parent

Resuming the parent at step 4 is necessary in order for the kernel to
report the "vfork done" event.  The kernel won't report a ptrace event
for a thread that is ptrace-stopped.  But the theory behind this is that
between steps 4 and 5, the parent won't actually do any progress even
though it is ptrace-resumed, because the kernel keeps it suspended,
waiting for the child to exec or exit.  So it doesn't matter for that
thread if breakpoints are not inserted.

The problem is when the program is multi-threaded.  In step 4, GDB
resumes all threads of the parent. The thread that did the vfork stays
suspended by the kernel, so that's fine. But other threads are running
freely while breakpoints are removed, which is a problem because they
could miss a breakpoint that they should have hit.

The problem is present with all-stop and non-stop targets.  The only
difference is that with an all-stop targets, the other threads are
stopped by the target when it reports the vfork event and are resumed by
the target when GDB resumes the parent.  With a non-stop target, the
other threads are simply never stopped.

The fix
-------

There many combinations of settings to consider (all-stop/non-stop,
target-non-stop on/off, follow-fork-mode parent/child, detach-on-fork
on/off, schedule-multiple on/off), but for this patch I restrict the
scope to follow-fork-mode=parent, detach-on-fork=on.  That's the
"default" case, where we detach the child and keep debugging the
parent.  I tried to fix them all, but it's just too much to do at once.
The code paths and behaviors for when we don't detach the child are
completely different.

The guiding principle for this patch is that all threads of the vforking
inferior should be stopped as long as breakpoints are removed.  This is
similar to handling in-line step-overs, in a way.

For non-stop targets (the default on Linux native), this is what
happens:

 - In follow_fork, we call stop_all_threads to stop all threads of the
   inferior
 - In follow_fork_inferior, we record the vfork parent thread in
   inferior::thread_waiting_for_vfork_done
 - Back in handle_inferior_event, we call keep_going, which resumes only
   the event thread (this is already the case, with a non-stop target).
   This is the thread that will be waiting for vfork-done.
 - When we get the vfork-done event, we go in the (new) handle_vfork_done
   function to restart the previously stopped threads.

In the same scenario, but with an all-stop target:

 - In follow_fork, no need to stop all threads of the inferior, the
   target has stopped all threads of all its inferiors before returning
   the event.
 - In follow_fork_inferior, we record the vfork parent thread in
   inferior::thread_waiting_for_vfork_done.
 - Back in handle_inferior_event, we also call keep_going.  However, we
   only want to resume the event thread here, not all inferior threads.
   In internal_resume_ptid (called by resume_1), we therefore now check
   whether one of the inferiors we are about to resume has
   thread_waiting_for_vfork_done set.  If so, we only resume that
   thread.

   Note that when resuming multiple inferiors, one vforking and one not
   non-vforking, we could resume the vforking thread from the vforking
   inferior plus all threads from the non-vforking inferior.  However,
   this is not implemented, it would require more work.
 - When we get the vfork-done event, the existing call to keep_going
   naturally resumes all threads.

Testing-wise, add a test that tries to make the main thread hit a
breakpoint while a secondary thread calls vfork.  Without the fix, the
main thread keeps going while breakpoints are removed, resulting in a
missed breakpoint and the program exiting.

Change-Id: I20eb78e17ca91f93c19c2b89a7e12c382ee814a1
This commit is contained in:
Simon Marchi 2022-01-14 15:40:59 -05:00 • committed by Simon Marchi
parent 05d65a7a6c
commit d8bbae6ea0
3 changed files with 311 additions and 8 deletions
Download patch file Download diff file Expand all files Collapse all files

135
gdb/infrun.c
View file

@ -96,6 +96,11 @@ static void resume (gdb_signal sig);
static void wait_for_inferior (inferior *inf);
static void restart_threads (struct thread_info *event_thread,
inferior *inf = nullptr);
static bool start_step_over (void);
/* Asynchronous signal handler registered as event loop source for
when we have pending events ready to be passed to the core. */
static struct async_event_handler *infrun_async_inferior_event_token;
@ -432,6 +437,8 @@ holding the child stopped. Try \"set detach-on-fork\" or \
inferior *parent_inf = current_inferior ();
inferior *child_inf = nullptr;
gdb_assert (parent_inf->thread_waiting_for_vfork_done == nullptr);
if (!follow_child)
{
/* Detach new forked process? */
@ -641,7 +648,6 @@ holding the child stopped. Try \"set detach-on-fork\" or \
child_inf->pending_detach = 0;
parent_inf->vfork_child = child_inf;
parent_inf->pending_detach = detach_fork;
parent_inf->thread_waiting_for_vfork_done = nullptr;
}
else if (detach_fork)
{
@ -773,6 +779,12 @@ follow_fork ()
parent = inferior_ptid;
child = tp->pending_follow.child_ptid ();
/* If handling a vfork, stop all the inferior's threads, they will be
restarted when the vfork shared region is complete. */
if (tp->pending_follow.kind () == TARGET_WAITKIND_VFORKED
&& target_is_non_stop_p ())
stop_all_threads ("handling vfork", tp->inf);
process_stratum_target *parent_targ = tp->inf->process_target ();
/* Set up inferior(s) as specified by the caller, and tell the
target to do whatever is necessary to follow either parent
@ -1034,6 +1046,53 @@ handle_vfork_child_exec_or_exit (int exec)
}
}
/* Handle TARGET_WAITKIND_VFORK_DONE. */
static void
handle_vfork_done (thread_info *event_thread)
{
/* We only care about this event if inferior::thread_waiting_for_vfork_done is
set, that is if we are waiting for a vfork child not under our control
(because we detached it) to exec or exit.
If an inferior has vforked and we are debugging the child, we don't use
the vfork-done event to get notified about the end of the shared address
space window. We rely instead on the child's exec or exit event, and the
inferior::vfork_{parent,child} fields are used instead. See
handle_vfork_child_exec_or_exit for that. */
if (event_thread->inf->thread_waiting_for_vfork_done == nullptr)
{
infrun_debug_printf ("not waiting for a vfork-done event");
return;
}
INFRUN_SCOPED_DEBUG_ENTER_EXIT;
/* We stopped all threads (other than the vforking thread) of the inferior in
follow_fork and kept them stopped until now. It should therefore not be
possible for another thread to have reported a vfork during that window.
If THREAD_WAITING_FOR_VFORK_DONE is set, it has to be the same thread whose
vfork-done we are handling right now. */
gdb_assert (event_thread->inf->thread_waiting_for_vfork_done == event_thread);
event_thread->inf->thread_waiting_for_vfork_done = nullptr;
event_thread->inf->pspace->breakpoints_not_allowed = 0;
/* On non-stop targets, we stopped all the inferior's threads in follow_fork,
resume them now. On all-stop targets, everything that needs to be resumed
will be when we resume the event thread. */
if (target_is_non_stop_p ())
{
/* restart_threads and start_step_over may change the current thread, make
sure we leave the event thread as the current thread. */
scoped_restore_current_thread restore_thread;
insert_breakpoints ();
restart_threads (event_thread, event_thread->inf);
start_step_over ();
}
}
/* Enum strings for "set|show follow-exec-mode". */
static const char follow_exec_mode_new[] = "new";
@ -1908,6 +1967,16 @@ start_step_over (void)
continue;
}
if (tp->inf->thread_waiting_for_vfork_done != nullptr)
{
/* When we stop all threads, handling a vfork, any thread in the step
over chain remains there. A user could also try to continue a
thread stopped at a breakpoint while another thread is waiting for
a vfork-done event. In any case, we don't want to start a step
over right now. */
continue;
}
/* Remove thread from the THREADS_TO_STEP chain. If anything goes wrong
while we try to prepare the displaced step, we don't add it back to
the global step over chain. This is to avoid a thread staying in the
@ -2143,8 +2212,41 @@ internal_resume_ptid (int user_step)
return a wildcard ptid. */
if (target_is_non_stop_p ())
return inferior_ptid;
else
return user_visible_resume_ptid (user_step);
/* The rest of the function assumes non-stop==off and
target-non-stop==off.
If a thread is waiting for a vfork-done event, it means breakpoints are out
for this inferior (well, program space in fact). We don't want to resume
any thread other than the one waiting for vfork done, otherwise these other
threads could miss breakpoints. So if a thread in the resumption set is
waiting for a vfork-done event, resume only that thread.
The resumption set width depends on whether schedule-multiple is on or off.
Note that if the target_resume interface was more flexible, we could be
smarter here when schedule-multiple is on. For example, imagine 3
inferiors with 2 threads each (1.1, 1.2, 2.1, 2.2, 3.1 and 3.2). Threads
2.1 and 3.2 are both waiting for a vfork-done event. Then we could ask the
target(s) to resume:
- All threads of inferior 1
- Thread 2.1
- Thread 3.2
Since we don't have that flexibility (we can only pass one ptid), just
resume the first thread waiting for a vfork-done event we find (e.g. thread
2.1). */
if (sched_multi)
{
for (inferior *inf : all_non_exited_inferiors ())
if (inf->thread_waiting_for_vfork_done != nullptr)
return inf->thread_waiting_for_vfork_done->ptid;
}
else if (current_inferior ()->thread_waiting_for_vfork_done != nullptr)
return current_inferior ()->thread_waiting_for_vfork_done->ptid;
return user_visible_resume_ptid (user_step);
}
/* Wrapper for target_resume, that handles infrun-specific
@ -3254,6 +3356,19 @@ proceed (CORE_ADDR addr, enum gdb_signal siggnal)
continue;
}
/* If a thread of that inferior is waiting for a vfork-done
(for a detached vfork child to exec or exit), breakpoints are
removed. We must not resume any thread of that inferior, other
than the one waiting for the vfork-done. */
if (tp->inf->thread_waiting_for_vfork_done != nullptr
&& tp != tp->inf->thread_waiting_for_vfork_done)
{
infrun_debug_printf ("[%s] another thread of this inferior is "
"waiting for vfork-done",
tp->ptid.to_string ().c_str ());
continue;
}
infrun_debug_printf ("resuming %s",
tp->ptid.to_string ().c_str ());
@ -3264,7 +3379,13 @@ proceed (CORE_ADDR addr, enum gdb_signal siggnal)
error (_("Command aborted."));
}
}
else if (!cur_thr->resumed () && !thread_is_in_step_over_chain (cur_thr))
else if (!cur_thr->resumed ()
&& !thread_is_in_step_over_chain (cur_thr)
/* In non-stop, forbid resuming a thread if some other thread of
that inferior is waiting for a vfork-done event (this means
breakpoints are out for this inferior). */
&& !(non_stop
&& cur_thr->inf->thread_waiting_for_vfork_done != nullptr))
{
/* The thread wasn't started, and isn't queued, run it now. */
reset_ecs (ecs, cur_thr);
@ -3748,8 +3869,6 @@ struct wait_one_event
};
static bool handle_one (const wait_one_event &event);
static void restart_threads (struct thread_info *event_thread,
inferior *inf = nullptr);
/* Prepare and stabilize the inferior for detaching it. E.g.,
detaching while a thread is displaced stepping is a recipe for
@ -5629,8 +5748,8 @@ handle_inferior_event (struct execution_control_state *ecs)
context_switch (ecs);
current_inferior ()->thread_waiting_for_vfork_done = nullptr;
current_inferior ()->pspace->breakpoints_not_allowed = 0;
handle_vfork_done (ecs->event_thread);
gdb_assert (inferior_thread () == ecs->event_thread);
if (handle_stop_requested (ecs))
return;