Re: [PATCH] [gdb] Fix assert in delete_breakpoint

[Simon Marchi <simark@simark.ca>]

On 11/13/23 10:26, Tom de Vries wrote:
> On a pinebook (aarch64 with 64-bit kernel and 32-bit userland) with test-case
> gdb.base/gdb-sigterm.exp I run into:
> ...
> intrusive_list.h:458: internal-error: erase_element: \
>   Assertion `elem_node->prev != INTRUSIVE_LIST_UNLINKED_VALUE' failed.^M
> ...
> 
> Fix this similar to:
> - commit c0afd99439f ("[gdb/tui] Fix assert in ~gdbpy_tui_window_maker"), and
> - commit 995a34b1772 ("Guard against frame.c destructors running before
>   frame-info.c's"
> by checking is_linked () before attempting to remove from breakpoint_chain.
> 
> Tested on the pinebook and x86_64-linux.
> 
> PR gdb/31061
> Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31061

Hi Tom,

Did you dig a little bit to understand why this happened specifically
with this setup, and that it's indeed expected that the breakpoint is
not in the breakpoint chain?  The stack trace you showed contains (with
names demangled):

    0xaae1d42b delete_breakpoint(breakpoint*)^M
            /home/rock/gdb/src/gdb/breakpoint.c:12636^M
    0xab1ed045 delete_thread_breakpoint^M
            /home/rock/gdb/src/gdb/thread.c:98^M
    0xab1ed0ab delete_single_step_breakpoints(thread_info*)^M
            /home/rock/gdb/src/gdb/thread.c:123^M
    0xab021e81 delete_thread_infrun_breakpoints^M
            /home/rock/gdb/src/gdb/infrun.c:3733^M
    0xab021edd for_each_just_stopped_thread^M
            /home/rock/gdb/src/gdb/infrun.c:3752^M
    0xab021f79 delete_just_stopped_threads_infrun_breakpoints^M
            /home/rock/gdb/src/gdb/infrun.c:3768^M

It looks like this is specific to architectures that use software
single stepping?  Does "32-bit userland on 64-bit kernel on aarch64" use
software single stepping, like 32-bit ARM?

Software single step breakpoints are inserted with:

  if (tp->control.single_step_breakpoints == NULL)
    {
      std::unique_ptr<breakpoint> b
	(new momentary_breakpoint (gdbarch, bp_single_step,
				   current_program_space,
				   null_frame_id,
				   tp->global_num));

      tp->control.single_step_breakpoints
	= add_to_breakpoint_chain (std::move (b));
    }

They are added to the global breakpoint_chain by
add_to_breakpoint_chain.  So how come the breakpoint is not linked when
comes the time to delete it?

Simon