Re: [PATCH v4 03/13] arm64: debug: call software breakpoint handlers statically

[Will Deacon <will@kernel.org>]

On Fri, Jun 20, 2025 at 10:11:57PM +0100, Ada Couprie Diaz wrote:
> Software breakpoints pass an immediate value in ESR ("comment") that can
> be used to call a specialized handler (KGDB, KASAN...).
> We do so in two different ways :
>  - During early boot, `early_brk64` statically checks against known
>    immediates and calls the corresponding handler,
>  - During init, handlers are dynamically registered into a list. When
>    called, the generic software breakpoint handler will iterate over
>    the list to find the appropriate handler.
> 
> The dynamic registration does not provide any benefit here as it is not
> exported and all its uses are within the arm64 tree. It also depends on an
> RCU list, whose safe access currently relies on the non-preemptible state
> of `do_debug_exception`.
> 
> Replace the list iteration logic in `call_break_hooks` to call
> the breakpoint handlers statically if they are enabled, like in
> `early_brk64`.
> Expose the handlers in their respective headers to be reachable from
> `arch/arm64/kernel/debug-monitors.c` at link time.
> 
> Unify the naming of the software breakpoint handlers to XXX_brk_handler(),
> making it clear they are related and to differentiate from the
> hardware breakpoints.
> 
> Signed-off-by: Ada Couprie Diaz <ada.coupriediaz@arm.com>
> Tested-by: Luis Claudio R. Goncalves <lgoncalv@redhat.com>
> ---
>  arch/arm64/include/asm/kgdb.h                 |  3 +
>  arch/arm64/include/asm/kprobes.h              |  8 +++
>  arch/arm64/include/asm/traps.h                |  6 ++
>  arch/arm64/include/asm/uprobes.h              |  2 +
>  arch/arm64/kernel/debug-monitors.c            | 54 +++++++++++++----
>  arch/arm64/kernel/kgdb.c                      | 22 ++-----
>  arch/arm64/kernel/probes/kprobes.c            | 31 ++--------
>  arch/arm64/kernel/probes/kprobes_trampoline.S |  2 +-
>  arch/arm64/kernel/probes/uprobes.c            |  9 +--
>  arch/arm64/kernel/traps.c                     | 59 ++++---------------
>  10 files changed, 83 insertions(+), 113 deletions(-)

Reviewed-by: Will Deacon <will@kernel.org>

Will