[PATCH 28/31] Document remote clone events, and QThreadOptions packet

Pedro Alves
2022-12-12T20:34:38Z

This commit documents in both manual and NEWS:

- the new remote clone event stop reply,
- the new QThreadOptions packet and its current defined options,
- the associated "set/show remote thread-events-packet" command,
- and the associated QThreadOptions qSupported feature.

Approved-By: Eli Zaretskii <eliz@gnu.org>
Change-Id: Ic1c8de1fefba95729bbd242969284265de42427e
---
gdb/NEWS | 20 +++++++
gdb/doc/gdb.texinfo | 127 ++++++++++++++++++++++++++++++++++++++++++--
2 files changed, 144 insertions(+), 3 deletions(-)

diff --git a/gdb/NEWS b/gdb/NEWS
index 0aa153b83da..b1d3dd7e7d9 100644
--- a/gdb/NEWS
+++ b/gdb/NEWS
@@ -160,6 +160,10 @@ set style tui-current-position [on|off]
Whether to style the source and assembly code highlighted by the
TUI's current position indicator. The default is off.

+set remote thread-options-packet
+show remote thread-options-packet
+ Set/show the use of the thread options packet.
+
* Changed commands

document user-defined
@@ -285,6 +289,22 @@ GNU/Linux/CSKY (gdbserver) csky*-*linux*

GDB now supports floating-point on LoongArch GNU/Linux.

+* New remote packets
+
+New stop reason: clone
+ Indicates that a clone system call was executed.
+
+QThreadOptions
+ Enable/disable optional event reporting, on a per-thread basis.
+ Currently supported options are GDB_THREAD_OPTION_CLONE, to enable
+ clone event reporting, and GDB_THREAD_OPTION_EXIT to enable thread
+ exit event reporting.
+
+QThreadOptions in qSupported
+ The qSupported packet allows GDB to inform the stub it supports the
+ QThreadOptions packet, and the qSupported response can contain the
+ set of thread options the remote stub supports.
+
*** Changes in GDB 12

* DBX mode is deprecated, and will be removed in GDB 13
diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index 5e75f32e0cd..ef62fac366f 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -24079,6 +24079,10 @@ are:
@tab @code{QThreadEvents}
@tab Tracking thread lifetime.

+@item @code{thread-options}
+@tab @code{QThreadOptions}
+@tab Set thread event reporting options.
+
@item @code{no-resumed-stop-reply}
@tab @code{no resumed thread left stop reply}
@tab Tracking thread lifetime.
@@ -42110,6 +42114,17 @@ appropriate @samp{qSupported} feature (@pxref{qSupported}). The
remote stub must also supply the appropriate @samp{qSupported} feature
indicating support.

+@cindex thread clone events, remote reply
+@anchor{thread clone event}
+@item clone
+The packet indicates that @code{clone} was called, and @var{r} is the
+thread ID of the new child thread, as specified in @ref{thread-id
+syntax}. This packet is only applicable to targets that support clone
+events.
+
+This packet should not be sent by default; @value{GDBN} requests it
+with the @ref{QThreadOptions} packet.
+
@cindex thread create event, remote reply
@anchor{thread create event}
@item create
@@ -42148,9 +42163,10 @@ hex strings.
@item w @var{AA} ; @var{tid}

The thread exited, and @var{AA} is the exit status. This response
-should not be sent by default; @value{GDBN} requests it with the
-@ref{QThreadEvents} packet. See also @ref{thread create event} above.
-@var{AA} is formatted as a big-endian hex string.
+should not be sent by default; @value{GDBN} requests it with either
+the @ref{QThreadEvents} or @ref{QThreadOptions} packets. See also
+@ref{thread create event} above. @var{AA} is formatted as a
+big-endian hex string.

@item N
There are no resumed threads left in the target. In other words, even
@@ -42875,6 +42891,11 @@ same thread. @value{GDBN} does not enable this feature unless the
stub reports that it supports it by including @samp{QThreadEvents+} in
its @samp{qSupported} reply.

+This packet always enables/disables event reporting for all threads of
+all processes under control of the remote stub. For per-thread
+control of optional event reporting, see the @ref{QThreadOptions}
+packet.
+
Reply:
@table @samp
@item OK
@@ -42891,6 +42912,94 @@ the stub.
Use of this packet is controlled by the @code{set remote thread-events}
command (@pxref{Remote Configuration, set remote thread-events}).

+@anchor{QThreadOptions}
+@item QThreadOptions@r{[};@var{options}@r{[}:@var{thread-id}@r{]]}@dots{}
+@cindex thread options, remote request
+@cindex @samp{QThreadOptions} packet
+
+For each inferior thread, the last @var{options} in the list with a
+matching @var{thread-id} are applied. Any options previously set on a
+thread are discarded and replaced by the new options specified.
+Threads that do not match any @var{thread-id} retain their
+previously-set options. Thread IDs are specified using the syntax
+described in @ref{thread-id syntax}. If multiprocess extensions
+(@pxref{multiprocess extensions}) are supported, options can be
+specified to apply to all threads of a process by using the
+@samp{p@var{pid}.-1} form of @var{thread-id}. Options with no
+@var{thread-id} apply to all threads. Specifying no options is an
+error.
+
+@var{options} is the bitwise @code{OR} of the following values. All
+values are given in hexadecimal representation.
+
+@table @code
+@item GDB_THREAD_OPTION_CLONE (0x1)
+Report thread clone events (@pxref{thread clone event}). This is only
+meaningful for targets that support clone events (e.g., GNU/Linux
+systems).
+
+@item GDB_THREAD_OPTION_EXIT (0x2)
+Report thread exit events (@pxref{thread exit event}).
+@end table
+
+@noindent
+
+For example, @value{GDBN} enables the @code{GDB_THREAD_OPTION_EXIT}
+and @code{GDB_THREAD_OPTION_CLONE} options when single-stepping a
+thread past a breakpoint, for the following reasons:
+
+@itemize @bullet
+@item
+If the single-stepped thread exits (e.g., it executes a thread exit
+system call), enabling @code{GDB_THREAD_OPTION_EXIT} prevents
+@value{GDBN} from waiting forever, not knowing that it should no
+longer expect a stop for that same thread, and blocking other threads
+from progressing.
+
+@item
+If the single-stepped thread spawns a new clone child (i.e., it
+executes a clone system call), enabling @code{GDB_THREAD_OPTION_CLONE}
+halts the cloned thread before it executes any instructions, and thus
+prevents the following problematic situations:
+
+@itemize @minus
+@item
+If the breakpoint is stepped-over in-line, the spawned thread would
+incorrectly run free while the breakpoint being stepped over is not
+inserted, and thus the cloned thread may potentially run past the
+breakpoint without stopping for it;
+
+@item
+If displaced (out-of-line) stepping is used, the cloned thread starts
+running at the out-of-line PC, leading to undefined behavior, usually
+crashing or corrupting data.
+@end itemize
+
+@end itemize
+
+New threads start with thread options cleared.
+
+@value{GDBN} does not enable this feature unless the stub reports that
+it supports it by including
+@samp{QThreadOptions=@var{supported_options}} in its @samp{qSupported}
+reply.
+
+Reply:
+@table @samp
+@item OK
+The request succeeded.
+
+@item E @var{nn}
+An error occurred. The error number @var{nn} is given as hex digits.
+
+@item @w{}
+An empty reply indicates that @samp{QThreadOptions} is not supported by
+the stub.
+@end table
+
+Use of this packet is controlled by the @code{set remote thread-options}
+command (@pxref{Remote Configuration, set remote thread-options}).
+
@item qRcmd,@var{command}
@cindex execute remote command, remote request
@cindex @samp{qRcmd} packet
@@ -43336,6 +43445,11 @@ These are the currently defined stub features and their properties:
@tab @samp{-}
@tab No

+@item @samp{QThreadOptions}
+@tab Yes
+@tab @samp{-}
+@tab No
+
@item @samp{no-resumed}
@tab No
@tab @samp{-}
@@ -43557,6 +43671,13 @@ The remote stub reports the supported actions in the reply to
@item QThreadEvents
The remote stub understands the @samp{QThreadEvents} packet.

+@item QThreadOptions=@var{supported_options}
+The remote stub understands the @samp{QThreadOptions} packet.
+@var{supported_options} indicates the set of thread options the remote
+stub supports. @var{supported_options} has the same format as the
+@var{options} parameter of the @code{QThreadOptions} packet, described
+at @ref{QThreadOptions}.
+
@item no-resumed
The remote stub reports the @samp{N} stop reply.

--
2.36.0