[PATCH 10/31] Thread options & clone events (Linux GDBserver)

Pedro Alves
2022-12-12T20:34:01Z

This patch teaches the Linux GDBserver backend to report clone events
to GDB, when GDB has requested them with the GDB_THREAD_OPTION_CLONE
thread option, via the new QThreadOptions packet.

This shuffles code in linux_process_target::handle_extended_wait
around to a more logical order when we now have to handle and
potentially report all of fork/vfork/clone.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=19675
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=27830
Change-Id: I3a19bc98801ec31e5c6fdbe1ebe17df855142bb2
---
gdbserver/linux-low.cc | 235 ++++++++++++++++++++++-------------------
gdbserver/linux-low.h | 2 +
2 files changed, 129 insertions(+), 108 deletions(-)

diff --git a/gdbserver/linux-low.cc b/gdbserver/linux-low.cc
index 6f96e16d6f0..d755cda0e44 100644
--- a/gdbserver/linux-low.cc
+++ b/gdbserver/linux-low.cc
@@ -491,7 +491,6 @@ linux_process_target::handle_extended_wait (lwp_info **orig_event_lwp,
struct lwp_info *event_lwp = *orig_event_lwp;
int event = linux_ptrace_get_extended_event (wstat);
struct thread_info *event_thr = get_lwp_thread (event_lwp);
- struct lwp_info *new_lwp;

gdb_assert (event_lwp->waitstatus.kind () == TARGET_WAITKIND_IGNORE);

@@ -503,7 +502,6 @@ linux_process_target::handle_extended_wait (lwp_info **orig_event_lwp,
if ((event == PTRACE_EVENT_FORK) || (event == PTRACE_EVENT_VFORK)
|| (event == PTRACE_EVENT_CLONE))
{
- ptid_t ptid;
unsigned long new_pid;
int ret, status;

@@ -527,61 +525,65 @@ linux_process_target::handle_extended_wait (lwp_info **orig_event_lwp,
warning ("wait returned unexpected status 0x%x", status);
}

- if (event == PTRACE_EVENT_FORK || event == PTRACE_EVENT_VFORK)
+ if (debug_threads)
{
- struct process_info *parent_proc;
- struct process_info *child_proc;
- struct lwp_info *child_lwp;
- struct thread_info *child_thr;
+ debug_printf ("HEW: Got %s event from LWP %ld, new child is %ld\n",
+ (event == PTRACE_EVENT_FORK ? "fork"
+ : event == PTRACE_EVENT_VFORK ? "vfork"
+ : event == PTRACE_EVENT_CLONE ? "clone"
+ : "???"),
+ ptid_of (event_thr).lwp (),
+ new_pid);
+ }
+
+ ptid_t child_ptid = (event != PTRACE_EVENT_CLONE
+ ? ptid_t (new_pid, new_pid)
+ : ptid_t (ptid_of (event_thr).pid (), new_pid));

- ptid = ptid_t (new_pid, new_pid);
+ lwp_info *child_lwp = add_lwp (child_ptid);
+ gdb_assert (child_lwp != NULL);
+ child_lwp->stopped = 1;
+ if (event != PTRACE_EVENT_CLONE)
+ child_lwp->must_set_ptrace_flags = 1;
+ child_lwp->status_pending_p = 0;

- threads_debug_printf ("Got fork event from LWP %ld, "
- "new child is %d",
- ptid_of (event_thr).lwp (),
- ptid.pid ());
+ thread_info *child_thr = get_lwp_thread (child_lwp);

+ /* If we're suspending all threads, leave this one suspended
+ too. If the fork/clone parent is stepping over a breakpoint,
+ all other threads have been suspended already. Leave the
+ child suspended too. */
+ if (stopping_threads == STOPPING_AND_SUSPENDING_THREADS
+ || event_lwp->bp_reinsert != 0)
+ {
+ threads_debug_printf ("leaving child suspended");
+ child_lwp->suspended = 1;
+ }
+
+ if (event_lwp->bp_reinsert != 0
+ && supports_software_single_step ()
+ && event == PTRACE_EVENT_VFORK)
+ {
+ /* If we leave single-step breakpoints there, child will
+ hit it, so uninsert single-step breakpoints from parent
+ (and child). Once vfork child is done, reinsert
+ them back to parent. */
+ uninsert_single_step_breakpoints (event_thr);
+ }
+
+ if (event != PTRACE_EVENT_CLONE)
+ {
/* Add the new process to the tables and clone the breakpoint
lists of the parent. We need to do this even if the new process
will be detached, since we will need the process object and the
breakpoints to remove any breakpoints from memory when we
detach, and the client side will access registers. */
- child_proc = add_linux_process (new_pid, 0);
+ process_info *child_proc = add_linux_process (new_pid, 0);
gdb_assert (child_proc != NULL);
- child_lwp = add_lwp (ptid);
- gdb_assert (child_lwp != NULL);
- child_lwp->stopped = 1;
- child_lwp->must_set_ptrace_flags = 1;
- child_lwp->status_pending_p = 0;
- child_thr = get_lwp_thread (child_lwp);
- child_thr->last_resume_kind = resume_stop;
- child_thr->last_status.set_stopped (GDB_SIGNAL_0);
-
- /* If we're suspending all threads, leave this one suspended
- too. If the fork/clone parent is stepping over a breakpoint,
- all other threads have been suspended already. Leave the
- child suspended too. */
- if (stopping_threads == STOPPING_AND_SUSPENDING_THREADS
- || event_lwp->bp_reinsert != 0)
- {
- threads_debug_printf ("leaving child suspended");
- child_lwp->suspended = 1;
- }

- parent_proc = get_thread_process (event_thr);
+ process_info *parent_proc = get_thread_process (event_thr);
child_proc->attached = parent_proc->attached;

- if (event_lwp->bp_reinsert != 0
- && supports_software_single_step ()
- && event == PTRACE_EVENT_VFORK)
- {
- /* If we leave single-step breakpoints there, child will
- hit it, so uninsert single-step breakpoints from parent
- (and child). Once vfork child is done, reinsert
- them back to parent. */
- uninsert_single_step_breakpoints (event_thr);
- }
-
clone_all_breakpoints (child_thr, event_thr);

target_desc_up tdesc = allocate_target_description ();
@@ -590,88 +592,97 @@ linux_process_target::handle_extended_wait (lwp_info **orig_event_lwp,

/* Clone arch-specific process data. */
low_new_fork (parent_proc, child_proc);
+ }

- /* Save fork info in the parent thread. */
- if (event == PTRACE_EVENT_FORK)
- event_lwp->waitstatus.set_forked (ptid);
- else if (event == PTRACE_EVENT_VFORK)
- event_lwp->waitstatus.set_vforked (ptid);
-
+ /* Save fork/clone info in the parent thread. */
+ if (event == PTRACE_EVENT_FORK)
+ event_lwp->waitstatus.set_forked (child_ptid);
+ else if (event == PTRACE_EVENT_VFORK)
+ event_lwp->waitstatus.set_vforked (child_ptid);
+ else if (event == PTRACE_EVENT_CLONE
+ && (event_thr->thread_options & GDB_THREAD_OPTION_CLONE) != 0)
+ event_lwp->waitstatus.set_thread_cloned (child_ptid);
+
+ if (event != PTRACE_EVENT_CLONE
+ || (event_thr->thread_options & GDB_THREAD_OPTION_CLONE) != 0)
+ {
/* The status_pending field contains bits denoting the
- extended event, so when the pending event is handled,
- the handler will look at lwp->waitstatus. */
+ extended event, so when the pending event is handled, the
+ handler will look at lwp->waitstatus. */
event_lwp->status_pending_p = 1;
event_lwp->status_pending = wstat;

- /* Link the threads until the parent event is passed on to
- higher layers. */
+ /* Link the threads until the parent's event is passed on to
+ GDB. */
event_lwp->fork_relative = child_lwp;
child_lwp->fork_relative = event_lwp;
-
- /* If the parent thread is doing step-over with single-step
- breakpoints, the list of single-step breakpoints are cloned
- from the parent's. Remove them from the child process.
- In case of vfork, we'll reinsert them back once vforked
- child is done. */
- if (event_lwp->bp_reinsert != 0
- && supports_software_single_step ())
- {
- /* The child process is forked and stopped, so it is safe
- to access its memory without stopping all other threads
- from other processes. */
- delete_single_step_breakpoints (child_thr);
-
- gdb_assert (has_single_step_breakpoints (event_thr));
- gdb_assert (!has_single_step_breakpoints (child_thr));
- }
-
- /* Report the event. */
- return 0;
}

- threads_debug_printf
- ("Got clone event from LWP %ld, new child is LWP %ld",
- lwpid_of (event_thr), new_pid);
-
- ptid = ptid_t (pid_of (event_thr), new_pid);
- new_lwp = add_lwp (ptid);
-
- /* Either we're going to immediately resume the new thread
- or leave it stopped. resume_one_lwp is a nop if it
- thinks the thread is currently running, so set this first
- before calling resume_one_lwp. */
- new_lwp->stopped = 1;
+ /* If the parent thread is doing step-over with single-step
+ breakpoints, the list of single-step breakpoints are cloned
+ from the parent's. Remove them from the child process.
+ In case of vfork, we'll reinsert them back once vforked
+ child is done. */
+ if (event_lwp->bp_reinsert != 0
+ && supports_software_single_step ())
+ {
+ /* The child process is forked and stopped, so it is safe
+ to access its memory without stopping all other threads
+ from other processes. */
+ delete_single_step_breakpoints (child_thr);

- /* If we're suspending all threads, leave this one suspended
- too. If the fork/clone parent is stepping over a breakpoint,
- all other threads have been suspended already. Leave the
- child suspended too. */
- if (stopping_threads == STOPPING_AND_SUSPENDING_THREADS
- || event_lwp->bp_reinsert != 0)
- new_lwp->suspended = 1;
+ gdb_assert (has_single_step_breakpoints (event_thr));
+ gdb_assert (!has_single_step_breakpoints (child_thr));
+ }

/* Normally we will get the pending SIGSTOP. But in some cases
we might get another signal delivered to the group first.
If we do get another signal, be sure not to lose it. */
if (WSTOPSIG (status) != SIGSTOP)
{
- new_lwp->stop_expected = 1;
- new_lwp->status_pending_p = 1;
- new_lwp->status_pending = status;
+ child_lwp->stop_expected = 1;
+ child_lwp->status_pending_p = 1;
+ child_lwp->status_pending = status;
}
- else if (cs.report_thread_events)
+ else if (event == PTRACE_EVENT_CLONE && cs.report_thread_events)
{
- new_lwp->waitstatus.set_thread_created ();
- new_lwp->status_pending_p = 1;
- new_lwp->status_pending = status;
+ child_lwp->waitstatus.set_thread_created ();
+ child_lwp->status_pending_p = 1;
+ child_lwp->status_pending = status;
}

+ if (event == PTRACE_EVENT_CLONE)
+ {
#ifdef USE_THREAD_DB
- thread_db_notice_clone (event_thr, ptid);
+ thread_db_notice_clone (event_thr, child_ptid);
#endif
+ }

- /* Don't report the event. */
- return 1;
+ if (event == PTRACE_EVENT_CLONE
+ && (event_thr->thread_options & GDB_THREAD_OPTION_CLONE) == 0)
+ {
+ threads_debug_printf
+ ("not reporting clone event from LWP %ld, new child is %ld\n",
+ ptid_of (event_thr).lwp (),
+ new_pid);
+ return 1;
+ }
+
+ /* Leave the child stopped until GDB processes the parent
+ event. */
+ child_thr->last_resume_kind = resume_stop;
+ child_thr->last_status.set_stopped (GDB_SIGNAL_0);
+
+ /* Report the event. */
+ threads_debug_printf
+ ("reporting %s event from LWP %ld, new child is %ld\n",
+ (event == PTRACE_EVENT_FORK ? "fork"
+ : event == PTRACE_EVENT_VFORK ? "vfork"
+ : event == PTRACE_EVENT_CLONE ? "clone"
+ : "???"),
+ ptid_of (event_thr).lwp (),
+ new_pid);
+ return 0;
}
else if (event == PTRACE_EVENT_VFORK_DONE)
{
@@ -3527,7 +3538,8 @@ linux_process_target::wait_1 (ptid_t ptid, target_waitstatus *ourstatus,

/* Break the unreported fork relationship chain. */
if (event_child->waitstatus.kind () == TARGET_WAITKIND_FORKED
- || event_child->waitstatus.kind () == TARGET_WAITKIND_VFORKED)
+ || event_child->waitstatus.kind () == TARGET_WAITKIND_VFORKED
+ || event_child->waitstatus.kind () == TARGET_WAITKIND_THREAD_CLONED)
{
event_child->fork_relative->fork_relative = NULL;
event_child->fork_relative = NULL;
@@ -4263,15 +4275,14 @@ linux_set_resume_request (thread_info *thread, thread_resume *resume, size_t n)
continue;
}

- /* Don't let wildcard resumes resume fork children that GDB
- does not yet know are new fork children. */
+ /* Don't let wildcard resumes resume fork/vfork/clone
+ children that GDB does not yet know are new children. */
if (lwp->fork_relative != NULL)
{
struct lwp_info *rel = lwp->fork_relative;

if (rel->status_pending_p
- && (rel->waitstatus.kind () == TARGET_WAITKIND_FORKED
- || rel->waitstatus.kind () == TARGET_WAITKIND_VFORKED))
+ && is_new_child_status (rel->waitstatus.kind ()))
{
threads_debug_printf
("not resuming LWP %ld: has queued stop reply",
@@ -5894,6 +5905,14 @@ linux_process_target::supports_vfork_events ()
return true;
}

+/* Return the set of supported thread options. */
+
+gdb_thread_options
+linux_process_target::supported_thread_options ()
+{
+ return GDB_THREAD_OPTION_CLONE;
+}
+
/* Check if exec events are supported. */

bool
diff --git a/gdbserver/linux-low.h b/gdbserver/linux-low.h
index 1594f063f47..69a34fb96fc 100644
--- a/gdbserver/linux-low.h
+++ b/gdbserver/linux-low.h
@@ -234,6 +234,8 @@ class linux_process_target : public process_stratum_target

bool supports_vfork_events () override;

+ gdb_thread_options supported_thread_options () override;
+
bool supports_exec_events () override;

void handle_new_gdb_connection () override;
--
2.36.0