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

Andrew Burgess via Gdb-patches
2023-06-05T15:53:42Z

Pedro Alves <pedro@palves.net> writes:

> 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.

I think it is implied, but would be must better explicitly stated, that
it is valid to send 0 in order to clear all options.

Thanks,
Andrew

> +
> +@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