Re: [PATCH 04/16 v3] Determine supported extended-remote features

Don Breazeal
2015-01-12T22:36:37Z

> As I mentioned, you'll need to make old GDB against new
> GDBserver work correctly, that is, make gdbserver not send these new
> events if debugging with old GDB, which is not expecting them.
> That can be handled by making GDB send a "fork-events+" in its
> own qSupported packet, so that GDBserver knows its talking to a
> new GDB. See how the "multiprocess+" feature is handled in both
> GDB and GDBserver. For a while, GDB didn't
> broadcast "multiprocess+" when connecting with "target remote".

Hi Pedro,
This patch implements fork-events+ and vfork-events+ in the
qSupported packet as you described.

> One of the reasons I wanted that on "remote" was exactly to
> make it possible to have fork events without caring for
> remote vs extended-remote.

I share this goal.

>
> If there are bigger problems, we can make GDB not broadcast
> the support with plain remote for starters. But I'd be curious
> to hear what the problems are.

My previous response included two patches that "almost worked" for
target remote. Please disregard those. As I went through the remaining
issues, it became clear to me that there are enough underlying changes
needed in the target remote multiprocess support that they deserve a
separate patch. I would like to proceed with the extended-remote support
and address target remote issue in a subsequent patch.

The issues that I ran into with target remote and follow fork
were mostly related to detach and kill in making decisions about
when and where to mourn an inferior and keeping things in sync
between GDB and gdbserver. The issues aren't insurmountable, but
I'm proposing that try to make progress by taking things in smaller
bites: first the extended-remote support, then the underlying target
remote multiprocess additions, then target remote follow fork.

Note that the remote vs. extended-remote issue is moot for this
patch except in the construction of the qSupported packet.

Let me know what you think.
Thanks,
--Don

This patch implements a mechanism for GDB to determine whether fork
events are supported in gdbserver. This is a preparatory patch for
remote fork and exec event support.

Two new RSP packets are defined for fork and vfork events. Other
events like vfork-done may be added later in the patch series if
needed.

These packets are used just like PACKET_multiprocess_feature to denote
whether the corresponding event is supported by inquiring about packet
support in the qSupported packet, then enabling or disabling the packet
based on the qSupported response. Target functions used to query for
support are included for each new packet.

In order for gdbserver to know whether the events are supported at the
point where the qSupported packet arrives, the code in nat/linux-ptrace.c
had to be reorganized. Previously it would test for fork/exec event
support, then enable the events using the pid of the inferior. When the
qSupported packet arrives there may not be an inferior. So the mechanism
was split into two parts: a function that checks whether the events are
supported, called when gdbserver starts up, and another that enables the
events when the inferior stops for the first time.

Another gdbserver change was to add some global variables similar to
multi_process, one per new packet. These are used to control whether
the events are reported. If GDB does not inquire about the event
support in the qSupported packet, then gdbserver will not set these
"report the event" flags. If the flags are not set, the events are
ignored like they were in the past.

There is an #ifdef that have been added temporarily to allow the code for
managing the events to be included in this patch without enabling the
events. See PTRACE_EVENT_SUPPORT. This #ifdef will be removed later in
the patch series as the events are enabled.

Tested on Ubuntu x64, native/remote/extended-remote, and as part of
subsequent patches in the series.

gdb/gdbserver/
2015-01-12 Don Breazeal <donb@codesourcery.com>

* linux-low.c (linux_supports_fork_events): New function.
(linux_supports_vfork_events): New function.
(linux_target_ops): Initialize new structure members.
(initialize_low): Call linux_ptrace_set_additional_flags
and linux_test_for_event_reporting.
* lynx-low.c (lynx_target_ops): Initialize new structure
members.
* server.c (report_fork_events, report_vfork_events,
report_exec_events): New global flags.
(handle_query): Add new features to qSupported packet.
(captured_main): Initialize new global variables.
* target.h (struct target_ops) <supports_fork_events>:
New member.
<supports_vfork_events>: New member.
(target_supports_fork_events): New macro.
(target_supports_vfork_events): New macro.
* win32-low.c (win32_target_ops): Initialize new structure
members.

gdb/
2015-01-12 Don Breazeal <donb@codesourcery.com>

* nat/linux-ptrace.c (linux_test_for_event_reporting): Rename
from linux_check_ptrace_features and make it extern.
(linux_test_for_tracefork): Reformat code.
(linux_enable_event_reporting): Change name of called function
to linux_check_ptrace_features.
* nat/linux-ptrace.h: Declare linux_test_for_event_reporting.
* remote.c (anonymous enum) <PACKET_fork_event_feature,
PACKET_vfork_event_feature>: New enumeration constants.
* remote.c (remote_fork_event_p): New function.
(remote_vfork_event_p): New function.
(remote_query_supported): Add new feature queries to qSupported
packet.
(remote_protocol_features): Add table entries for new packets.
(_initialize_remote): Exempt new packets from the requirement
to have 'set remote' commands.

---
gdb/gdbserver/linux-low.c | 22 ++++++++++++++++++++++
gdb/gdbserver/lynx-low.c | 2 ++
gdb/gdbserver/server.c | 22 ++++++++++++++++++++++
gdb/gdbserver/target.h | 14 ++++++++++++++
gdb/gdbserver/win32-low.c | 2 ++
gdb/nat/linux-ptrace.c | 13 +++++++------
gdb/nat/linux-ptrace.h | 1 +
gdb/remote.c | 41 +++++++++++++++++++++++++++++++++++++++--
8 files changed, 109 insertions(+), 8 deletions(-)

diff --git a/gdb/gdbserver/linux-low.c b/gdb/gdbserver/linux-low.c
index 65f72a2..b1201b3 100644
--- a/gdb/gdbserver/linux-low.c
+++ b/gdb/gdbserver/linux-low.c
@@ -5129,6 +5129,22 @@ linux_supports_multi_process (void)
return 1;
}

+/* Check if fork events are supported. */
+
+static int
+linux_supports_fork_events (void)
+{
+ return linux_supports_tracefork ();
+}
+
+/* Check if vfork events are supported. */
+
+static int
+linux_supports_vfork_events (void)
+{
+ return linux_supports_tracefork ();
+}
+
static int
linux_supports_disable_randomization (void)
{
@@ -6040,6 +6056,8 @@ static struct target_ops linux_target_ops = {
linux_async,
linux_start_non_stop,
linux_supports_multi_process,
+ linux_supports_fork_events,
+ linux_supports_vfork_events,
#ifdef USE_THREAD_DB
thread_db_handle_monitor_command,
#else
@@ -6115,4 +6133,8 @@ initialize_low (void)
sigaction (SIGCHLD, &sigchld_action, NULL);

initialize_low_arch ();
+
+ /* Enable any extended ptrace events that are supported. */
+ linux_ptrace_set_additional_flags (0);
+ linux_test_for_event_reporting ();
}
diff --git a/gdb/gdbserver/lynx-low.c b/gdb/gdbserver/lynx-low.c
index 3b83669..e3fb788 100644
--- a/gdb/gdbserver/lynx-low.c
+++ b/gdb/gdbserver/lynx-low.c
@@ -754,6 +754,8 @@ static struct target_ops lynx_target_ops = {
NULL, /* async */
NULL, /* start_non_stop */
NULL, /* supports_multi_process */
+ NULL, /* supports_fork_events */
+ NULL, /* supports_vfork_events */
NULL, /* handle_monitor_command */
};

diff --git a/gdb/gdbserver/server.c b/gdb/gdbserver/server.c
index 173a22e..b092020 100644
--- a/gdb/gdbserver/server.c
+++ b/gdb/gdbserver/server.c
@@ -57,6 +57,8 @@ static int exit_requested;
int run_once;

int multi_process;
+int report_fork_events;
+int report_vfork_events;
int non_stop;

/* Whether we should attempt to disable the operating system's address
@@ -1841,6 +1843,18 @@ handle_query (char *own_buf, int packet_len, int *new_packet_len_p)
/* GDB supports relocate instruction requests. */
gdb_supports_qRelocInsn = 1;
}
+ if (strcmp (p, "fork-events+") == 0)
+ {
+ /* GDB supports and wants fork events if possible. */
+ if (target_supports_fork_events ())
+ report_fork_events = 1;
+ }
+ if (strcmp (p, "vfork-events+") == 0)
+ {
+ /* GDB supports and wants vfork events if possible. */
+ if (target_supports_vfork_events ())
+ report_vfork_events = 1;
+ }
else
target_process_qsupported (p);

@@ -1891,6 +1905,12 @@ handle_query (char *own_buf, int packet_len, int *new_packet_len_p)
if (target_supports_multi_process ())
strcat (own_buf, ";multiprocess+");

+ if (target_supports_fork_events ())
+ strcat (own_buf, ";fork-events+");
+
+ if (target_supports_vfork_events ())
+ strcat (own_buf, ";vfork-events+");
+
if (target_supports_non_stop ())
strcat (own_buf, ";QNonStop+");

@@ -3242,6 +3262,8 @@ captured_main (int argc, char *argv[])

noack_mode = 0;
multi_process = 0;
+ report_fork_events = 0;
+ report_vfork_events = 0;
/* Be sure we're out of tfind mode. */
current_traceframe = -1;
cont_thread = null_ptid;
diff --git a/gdb/gdbserver/target.h b/gdb/gdbserver/target.h
index 5e29b7f..e25e5f8 100644
--- a/gdb/gdbserver/target.h
+++ b/gdb/gdbserver/target.h
@@ -262,6 +262,12 @@ struct target_ops
/* Returns true if the target supports multi-process debugging. */
int (*supports_multi_process) (void);

+ /* Returns true if fork events are supported. */
+ int (*supports_fork_events) (void);
+
+ /* Returns true if vfork events are supported. */
+ int (*supports_vfork_events) (void);
+
/* If not NULL, target-specific routine to process monitor command.
Returns 1 if handled, or 0 to perform default processing. */
int (*handle_monitor_command) (char *);
@@ -390,6 +396,14 @@ void set_target_ops (struct target_ops *);

int kill_inferior (int);

+#define target_supports_fork_events() \
+ (the_target->supports_fork_events ? \
+ (*the_target->supports_fork_events) () : 0)
+
+#define target_supports_vfork_events() \
+ (the_target->supports_vfork_events ? \
+ (*the_target->supports_vfork_events) () : 0)
+
#define detach_inferior(pid) \
(*the_target->detach) (pid)

diff --git a/gdb/gdbserver/win32-low.c b/gdb/gdbserver/win32-low.c
index e714933..a35ff91 100644
--- a/gdb/gdbserver/win32-low.c
+++ b/gdb/gdbserver/win32-low.c
@@ -1823,6 +1823,8 @@ static struct target_ops win32_target_ops = {
NULL, /* async */
NULL, /* start_non_stop */
NULL, /* supports_multi_process */
+ NULL, /* supports_fork_events */
+ NULL, /* supports_vfork_events */
NULL, /* handle_monitor_command */
NULL, /* core_of_thread */
NULL, /* read_loadmap */
diff --git a/gdb/nat/linux-ptrace.c b/gdb/nat/linux-ptrace.c
index a0e0c32..542e762 100644
--- a/gdb/nat/linux-ptrace.c
+++ b/gdb/nat/linux-ptrace.c
@@ -311,8 +311,8 @@ static void linux_test_for_exitkill (int child_pid);

/* Determine ptrace features available on this target. */

-static void
-linux_check_ptrace_features (void)
+void
+linux_test_for_event_reporting (void)
{
int child_pid, ret, status;

@@ -433,9 +433,10 @@ linux_test_for_tracefork (int child_pid)
/* We got the PID from the grandchild, which means fork
tracing is supported. */
current_ptrace_options |= PTRACE_O_TRACECLONE;
- current_ptrace_options |= (additional_flags & (PTRACE_O_TRACEFORK
- | PTRACE_O_TRACEVFORK
- | PTRACE_O_TRACEEXEC));
+ current_ptrace_options |= (additional_flags
+ & (PTRACE_O_TRACEFORK
+ | PTRACE_O_TRACEVFORK
+ | PTRACE_O_TRACEEXEC));

/* Do some cleanup and kill the grandchild. */
my_waitpid (second_pid, &second_status, 0);
@@ -477,7 +478,7 @@ linux_enable_event_reporting (pid_t pid, int attached)
/* Check if we have initialized the ptrace features for this
target. If not, do it now. */
if (current_ptrace_options == -1)
- linux_check_ptrace_features ();
+ linux_test_for_event_reporting ();

ptrace_options = current_ptrace_options;
if (attached)
diff --git a/gdb/nat/linux-ptrace.h b/gdb/nat/linux-ptrace.h
index 588d38a..edbacfd 100644
--- a/gdb/nat/linux-ptrace.h
+++ b/gdb/nat/linux-ptrace.h
@@ -90,6 +90,7 @@ struct buffer;

extern void linux_ptrace_attach_fail_reason (pid_t pid, struct buffer *buffer);
extern void linux_ptrace_init_warnings (void);
+extern void linux_test_for_event_reporting (void);
extern void linux_enable_event_reporting (pid_t pid, int attached);
extern void linux_disable_event_reporting (pid_t pid);
extern int linux_supports_tracefork (void);
diff --git a/gdb/remote.c b/gdb/remote.c
index 4b9b099..891329a 100644
--- a/gdb/remote.c
+++ b/gdb/remote.c
@@ -1327,6 +1327,12 @@ enum {
/* Support for qXfer:libraries-svr4:read with a non-empty annex. */
PACKET_augmented_libraries_svr4_read_feature,

+ /* Support for fork events. */
+ PACKET_fork_event_feature,
+
+ /* Support for vfork events. */
+ PACKET_vfork_event_feature,
+
PACKET_MAX
};

@@ -1432,6 +1438,24 @@ remote_multi_process_p (struct remote_state *rs)
return packet_support (PACKET_multiprocess_feature) == PACKET_ENABLE;
}

+#if PTRACE_FORK_EVENTS
+/* Returns true if fork events are supported. */
+
+static int
+remote_fork_event_p (struct remote_state *rs)
+{
+ return packet_support (PACKET_fork_event_feature) == PACKET_ENABLE;
+}
+
+/* Returns true if vfork events are supported. */
+
+static int
+remote_vfork_event_p (struct remote_state *rs)
+{
+ return packet_support (PACKET_vfork_event_feature) == PACKET_ENABLE;
+}
+#endif
+
/* Tokens for use by the asynchronous signal handlers for SIGINT. */
static struct async_signal_handler *async_sigint_remote_twice_token;
static struct async_signal_handler *async_sigint_remote_token;
@@ -4010,7 +4034,11 @@ static const struct protocol_feature remote_protocol_features[] = {
{ "Qbtrace:off", PACKET_DISABLE, remote_supported_packet, PACKET_Qbtrace_off },
{ "Qbtrace:bts", PACKET_DISABLE, remote_supported_packet, PACKET_Qbtrace_bts },
{ "qXfer:btrace:read", PACKET_DISABLE, remote_supported_packet,
- PACKET_qXfer_btrace }
+ PACKET_qXfer_btrace },
+ { "fork-events", PACKET_DISABLE, remote_supported_packet,
+ PACKET_fork_event_feature },
+ { "vfork-events", PACKET_DISABLE, remote_supported_packet,
+ PACKET_vfork_event_feature },
};

static char *remote_support_xml;
@@ -4084,6 +4112,12 @@ remote_query_supported (void)

q = remote_query_supported_append (q, "qRelocInsn+");

+ if (rs->extended)
+ {
+ q = remote_query_supported_append (q, "fork-events+");
+ q = remote_query_supported_append (q, "vfork-events+");
+ }
+
q = reconcat (q, "qSupported:", q, (char *) NULL);
putpkt (q);

@@ -12191,7 +12225,8 @@ Show the maximum size of the address (in bits) in a memory packet."), NULL,
add_packet_config_cmd (&remote_protocol_packets[PACKET_qXfer_btrace],
"qXfer:btrace", "read-btrace", 0);

- /* Assert that we've registered commands for all packet configs. */
+ /* Assert that we've registered "set remote foo-packet" commands
+ for all packet configs. */
{
int i;

@@ -12210,6 +12245,8 @@ Show the maximum size of the address (in bits) in a memory packet."), NULL,
case PACKET_DisconnectedTracing_feature:
case PACKET_augmented_libraries_svr4_read_feature:
case PACKET_qCRC:
+ case PACKET_fork_event_feature:
+ case PACKET_vfork_event_feature:
/* Additions to this list need to be well justified:
pre-existing packets are OK; new packets are not. */
excepted = 1;
--
1.7.0.4