Re: [PATCH v4 8/8] Forced quit cases handled by resetting sync_quit_force_run

Pedro Alves
2023-01-30T19:02:40Z

On 2023-01-12 1:56 a.m., Kevin Buettner wrote:
> During my audit of the use of gdb_exception with regard to QUIT
> processing, I found a try/catch in the scoped_switch_fork_info
> destructor.
>
> Static analysis found this call path from the destructor to
> maybe_quit():
>
> scoped_switch_fork_info::~scoped_switch_fork_info()
> -> remove_breakpoints()
> -> remove_breakpoint(bp_location*)
> -> remove_breakpoint_1(bp_location*, remove_bp_reason)
> -> memory_validate_breakpoint(gdbarch*, bp_target_info*)
> -> target_read_memory(unsigned long, unsigned char*, long)
> -> target_read(target_ops*, target_object, char const*, unsigned char*, unsigned long, long)
> -> maybe_quit()
>
> Since it's not safe to do a 'throw' from a destructor, we simply
> call set_quit_flag and, for gdb_exception_forced_quit, also
> set sync_quit_force_run. This will cause the appropriate
> exception to be rethrown at the next QUIT check.
>
> Another case is the try / catch in tui_getc() in tui-io.c. The
> existing catch swallows the exception. I've added a catch for
> 'gdb_exception_forced_quit', which also swallows the exception,
> but also sets sync_quit_force_run and calls set_quit_flag in
> order to restart forced quit processing at the next QUIT check.
> This is required because it isn't safe to throw into/through
> readline.
>
> Thanks to Pedro Alves for suggesting this idea.
>
> Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=26761
> ---
> gdb/linux-fork.c | 13 +++++++++++++
> gdb/tui/tui-io.c | 9 +++++++++
> 2 files changed, 22 insertions(+)
>
> diff --git a/gdb/linux-fork.c b/gdb/linux-fork.c
> index 61545b859ea..fc2f00d0766 100644
> --- a/gdb/linux-fork.c
> +++ b/gdb/linux-fork.c
> @@ -430,6 +430,19 @@ class scoped_switch_fork_info
> fork_load_infrun_state (m_oldfp);
> insert_breakpoints ();
> }
> + catch (const gdb_exception_quit &ex)
> + {
> + /* We can't throw from a destructor, so re-set the quit flag
> + for later QUIT checking. */
> + set_quit_flag ();
> + }
> + catch (const gdb_exception_forced_quit &ex)
> + {
> + /* Like above, but (eventually) cause GDB to terminate by
> + setting sync_quit_force_run. */
> + sync_quit_force_run = 1;
> + set_quit_flag ();

I think it'd be nice if we had a function that did both of these for us.
Something like "set_force_quit_flag()".

> + }
> catch (const gdb_exception &ex)
> {
> warning (_("Couldn't restore checkpoint state in %s: %s"),
> diff --git a/gdb/tui/tui-io.c b/gdb/tui/tui-io.c
> index 2f39e34df2f..ac3c7296499 100644
> --- a/gdb/tui/tui-io.c
> +++ b/gdb/tui/tui-io.c
> @@ -1275,6 +1275,15 @@ tui_getc (FILE *fp)
> {
> return tui_getc_1 (fp);
> }
> + catch (const gdb_exception_forced_quit &ex)
> + {
> + /* As noted below, it's not safe to let an exception escape
> + to newline, so, for this case, reset the quit flag for

Spurious double space before "reset".

> + later QUIT checking. */
> + sync_quit_force_run = 1;
> + set_quit_flag ();
> + return 0;
> + }
> catch (const gdb_exception &ex)
> {
> /* Just in case, don't ever let an exception escape to readline.
>