[PATCH 09/16 v2] Extended-remote fork catchpoints

Don Breazeal
2014-08-21T00:33:06Z

This patch implements catchpoints for fork events on extended-remote linux
targets.

Implementation appeared to be straightforward, requiring four new functions
in remote.c to implement insert/remove of fork/vfork catchpoints. These
functions are essentially stubs that just return 0 ('success'). If the
fork events were being reported, then catchpoints were set and hit.

However, there are some extra issues that arise with catchpoints.

1) Thread creation reporting -- fork catchpoints are hit before the
follow_fork has been completed. In the native implementation, the new
process is not 'reported' until after the follow is done. It doesn't
show up in the inferiors list or the threads list. However, in
gdbserver, an 'info threads' will retrieve the new thread info from the
target and add it to GDB's data structures. Because of this premature
report, things on the GDB side eventually get very confused.

So in gdbserver, in server.c:handle_qxfer_threads_worker, we check
'last_status' and if it shows a FORK event, we know that we are in an
unfollowed fork and we do not report the new (forked) thread to GDB.

2) Kill process before fork is followed -- on the native side in
linux-nat.c:linux_nat_kill, there is some code to handle the case where
a fork has occurred but follow_fork hasn't been called yet. It does
this by using the last status to determine if a follow is pending, and
if it is, to kill the child task. I implemented similar code in
linux-low.c:linux_kill.

3) One of the tests related to fork catchpoints,
gdb.threads/fork-thread-pending.exp, depended on the threads being
reported in a specific order. GDBserver reported the threads in a
different order, that is, 'info threads' showed the same threads, but in
a different order. The test used a hard-coded thread number to find a
threads that (a) was not the main thread (blocked in pthread_join), and
(b) was not the forking thread (stopped in fork).

I implemented a new proc, find_unforked_thread, that uses a pretty
brute-force method of finding a thread. I considered just hard-coding
another number (the native case used thread 2, which was the forking
thread in the remote case), but that didn't seem future-proof.
Suggestions on how to do this better would be welcome.

Tested on x64 Ubuntu Lucid, native, remote, extended-remote. Tested the
case of killing the forking process before the fork has been followed
manually. It wasn't clear to me how to check that a process had actually
been killed from a dejagnu test.

Thanks
--Don

gdb/
2014-08-20 Don Breazeal <donb@codesourcery.com>
* remote.c (extended_remote_insert_fork_catchpoint): New function.
(extended_remote_remove_fork_catchpoint): New function.
(extended_remote_insert_vfork_catchpoint): New function.
(extended_remote_remove_vfork_catchpoint): New function.
(init_extended_remote_ops): Initialize target vector with
new fork catchpoint functions.

gdb/gdbserver/
2014-08-20 Don Breazeal <donb@codesourcery.com>

* linux-low.c (handle_extended_wait): Fix braces in multi-line if.
(linux_kill): Kill forked child when between fork and follow_fork.
* server.c (handle_qxfer_threads_worker): Skip forked child thread
when between fork and follow_fork.
(get_last_target_status): New function.
* server.h (get_last_target_status): Declare new function.

gdb/testsuite/
2014-08-20 Don Breazeal <donb@codesourcery.com>
* gdb.threads/fork-thread-pending.exp (find_unforked_thread):
New proc.

---
gdb/gdbserver/linux-low.c | 28 +++++++++++--
gdb/gdbserver/server.c | 18 ++++++++
gdb/gdbserver/server.h | 2 +
gdb/remote.c | 44 +++++++++++++++++++++
gdb/testsuite/gdb.threads/fork-thread-pending.exp | 23 ++++++++++-
5 files changed, 110 insertions(+), 5 deletions(-)

diff --git a/gdb/gdbserver/linux-low.c b/gdb/gdbserver/linux-low.c
index 5617f5e..2d9bd4a 100644
--- a/gdb/gdbserver/linux-low.c
+++ b/gdb/gdbserver/linux-low.c
@@ -415,10 +415,12 @@ handle_extended_wait (struct lwp_info *event_child, int wstat)
ptid = ptid_build (new_pid, new_pid, 0);

if (debug_threads)
- debug_printf ("HEW: Got fork event "
- "from LWP %ld, new child is %d\n",
- ptid_get_lwp (ptid_of (event_thr)),
- ptid_get_pid (ptid));
+ {
+ debug_printf ("HEW: Got fork event "
+ "from LWP %ld, new child is %d\n",
+ ptid_get_lwp (ptid_of (event_thr)),
+ ptid_get_pid (ptid));
+ }

/* 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
@@ -1066,6 +1068,24 @@ linux_kill (int pid)
{
struct process_info *process;
struct lwp_info *lwp;
+ struct target_waitstatus last;
+ ptid_t last_ptid;
+
+ /* If we're stopped while forking and we haven't followed yet,
+ kill the child task. We need to do this first because the
+ parent will be sleeping if this is a vfork. */
+
+ get_last_target_status (&last_ptid, &last);
+
+ if (last.kind == TARGET_WAITKIND_FORKED
+ || last.kind == TARGET_WAITKIND_VFORKED)
+ {
+ lwp = find_lwp_pid (last.value.related_pid);
+ gdb_assert (lwp != NULL);
+ kill_wait_lwp (lwp);
+ process = find_process_pid (ptid_get_pid (last.value.related_pid));
+ the_target->mourn (process);
+ }

process = find_process_pid (pid);
if (process == NULL)
diff --git a/gdb/gdbserver/server.c b/gdb/gdbserver/server.c
index 4dbdeb8..888160d 100644
--- a/gdb/gdbserver/server.c
+++ b/gdb/gdbserver/server.c
@@ -1316,6 +1316,15 @@ handle_qxfer_threads_worker (struct inferior_list_entry *inf, void *arg)
int core = target_core_of_thread (ptid);
char core_s[21];

+ /* Skip new threads created as the result of a fork if we are not done
+ handling that fork event. We won't know whether to tell GDB about
+ the new thread until we are done following the fork. */
+ if ((last_status.kind == TARGET_WAITKIND_FORKED
+ || last_status.kind == TARGET_WAITKIND_VFORKED)
+ && (ptid_get_pid (last_status.value.related_pid)
+ == ptid_get_pid (ptid)))
+ return;
+
write_ptid (ptid_s, ptid);

if (core != -1)
@@ -4047,3 +4056,12 @@ using_extended_protocol (void)
{
return extended_protocol;
}
+
+/* Retrieve the last waitstatus reported to GDB. */
+
+void
+get_last_target_status (ptid_t *ptid, struct target_waitstatus *last)
+{
+ *ptid = last_ptid;
+ *last = last_status;
+}
diff --git a/gdb/gdbserver/server.h b/gdb/gdbserver/server.h
index 2c5377b..17491bc 100644
--- a/gdb/gdbserver/server.h
+++ b/gdb/gdbserver/server.h
@@ -115,6 +115,8 @@ typedef int gdb_fildes_t;
extern int handle_serial_event (int err, gdb_client_data client_data);
extern int handle_target_event (int err, gdb_client_data client_data);
extern int using_extended_protocol (void);
+extern void get_last_target_status (ptid_t *ptid,
+ struct target_waitstatus *last);

#include "remote-utils.h"

diff --git a/gdb/remote.c b/gdb/remote.c
index 6939eb5..6694ca2 100644
--- a/gdb/remote.c
+++ b/gdb/remote.c
@@ -7855,6 +7855,42 @@ extended_remote_kill (struct target_ops *ops)
target_mourn_inferior ();
}

+/* Insert fork catchpoint target routine. If fork events are enabled
+ then return success, nothing more to do. */
+
+static int
+extended_remote_insert_fork_catchpoint (struct target_ops *ops, int pid)
+{
+ return !extended_remote_feature_supported (FORK_EVENT);
+}
+
+/* Remove fork catchpoint target routine. Nothing to do, just
+ return success. */
+
+static int
+extended_remote_remove_fork_catchpoint (struct target_ops *ops, int pid)
+{
+ return 0;
+}
+
+/* Insert vfork catchpoint target routine. If vfork events are enabled
+ then return success, nothing more to do. */
+
+static int
+extended_remote_insert_vfork_catchpoint (struct target_ops *ops, int pid)
+{
+ return !extended_remote_feature_supported (VFORK_EVENT);
+}
+
+/* Remove vfork catchpoint target routine. Nothing to do, just
+ return success. */
+
+static int
+extended_remote_remove_vfork_catchpoint (struct target_ops *ops, int pid)
+{
+ return 0;
+}
+
/* Target routine for follow-fork. */

static int
@@ -11717,6 +11753,14 @@ init_extended_remote_ops (void)
Specify the serial device it is connected to (e.g. /dev/ttya).";
extended_remote_ops.to_open = extended_remote_open;
extended_remote_ops.to_create_inferior = extended_remote_create_inferior;
+ extended_remote_ops.to_insert_fork_catchpoint
+ = extended_remote_insert_fork_catchpoint;
+ extended_remote_ops.to_remove_fork_catchpoint
+ = extended_remote_remove_fork_catchpoint;
+ extended_remote_ops.to_insert_vfork_catchpoint
+ = extended_remote_insert_vfork_catchpoint;
+ extended_remote_ops.to_remove_vfork_catchpoint
+ = extended_remote_remove_vfork_catchpoint;
extended_remote_ops.to_follow_fork = extended_remote_follow_fork;
extended_remote_ops.to_mourn_inferior = extended_remote_mourn;
extended_remote_ops.to_detach = extended_remote_detach;
diff --git a/gdb/testsuite/gdb.threads/fork-thread-pending.exp b/gdb/testsuite/gdb.threads/fork-thread-pending.exp
index 57e45c9..e8c4830 100644
--- a/gdb/testsuite/gdb.threads/fork-thread-pending.exp
+++ b/gdb/testsuite/gdb.threads/fork-thread-pending.exp
@@ -31,6 +31,26 @@ if {[gdb_compile_pthreads "${srcdir}/${subdir}/${srcfile}" "${binfile}" executab
return -1
}

+# Find a thread that did not fork and is not the main thread and
+# return its thread number. We can't just hard-code the thread
+# number since we have no guarantee as to the ordering of the threads
+# in gdb. We know that the main thread is in pthread_join and the
+# forking thread is in fork, so we use this rather ungainly regexp
+# to capture an entry from 'info threads' that doesn't show one of
+# those routines, then extract the thread number.
+
+proc find_unforked_thread { } {
+ gdb_test_multiple "info threads" "find unforked thread" {
+ -re "(\[^\r]*Thread\[^\r]* in \[^fp]\[^ot]\[^rh]\[^kr]\[^e]\[^a]\[^d]\[^_]\[^j]\[^\r]*\r\n)" {
+ regexp "(\[ ]*)(\[0-9]*)(\[ ]*Thread\[^\r]*\r\n)" $expect_out(0,string) ignore lead_spc threadnum rest
+ }
+ timeout {
+ set threadnum -1
+ }
+ }
+ return $threadnum
+}
+
clean_restart ${binfile}

if ![runto_main] then {
@@ -46,7 +66,8 @@ gdb_test "continue" "Catchpoint.*" "1, get to the fork event"

gdb_test "info threads" " Thread .* Thread .* Thread .* Thread .*" "1, multiple threads found"

-gdb_test "thread 2" ".*" "1, switched away from event thread"
+set threadnum [find_unforked_thread]
+gdb_test "thread $threadnum" ".*" "1, switched away from event thread to thread $threadnum"

gdb_test "continue" "Not resuming.*" "1, refused to resume"

--
1.7.0.4