Re: [PATCH v4 11/13] arm64: debug: split brk64 exception entry

[Will Deacon <will@kernel.org>]

On Fri, Jun 20, 2025 at 10:12:05PM +0100, Ada Couprie Diaz wrote:
> Currently all debug exceptions share common entry code and are routed
> to `do_debug_exception()`, which calls dynamically-registered
> handlers for each specific debug exception. This is unfortunate as
> different debug exceptions have different entry handling requirements,
> and it would be better to handle these distinct requirements earlier.
> 
> The BRK64 instruction can only be triggered by a BRK instruction. Thus,
> we know that the PC is a legitimate address and isn't being used to train
> a branch predictor with a bogus address : we don't need to call
> `arm64_apply_bp_hardening()`.
> 
> We do not need to handle the Cortex-A76 erratum #1463225 either, as it
> only relevant for single stepping at EL1.
> BRK64 does not write FAR_EL1 either, as only hardware watchpoints do so.
> 
> Split the BRK64 exception entry, adjust the function signature, and its
> behaviour to match the lack of needed mitigations.
> Further, as the EL0 and EL1 code paths are cleanly separated, we can split
> `do_brk64()` into `do_el0_brk64()` and `do_el1_brk64()`, and call them
> directly from the relevant entry paths.
> Use `die()` directly for the EL1 error path, as in `do_el1_bti()` and
> `do_el1_undef()`.
> We can also remove `NOKRPOBE_SYMBOL` for the EL0 path, as it cannot
> lead to a kprobe recursion.
> 
> When taking a BRK64 exception from EL0, the exception handling is safely
> preemptible : the only possible handler is `uprobe_brk_handler()`.
> It only operates on task-local data and properly checks its validity,
> then raises a Thread Information Flag, processed before returning
> to userspace in `do_notify_resume()`, which is already preemptible.
> Thus we can safely unmask interrupts and enable preemption before
> handling the break itself, fixing a PREEMPT_RT issue where the handler
> could call a sleeping function with preemption disabled.
> 
> Given that the break hook registration is handled statically in
> `call_break_hook` since
> (arm64: debug: call software break handlers statically)
> and that we now bypass the exception handler registration, this change
> renders `early_brk64` redundant : its functionality is now handled through
> the post-init path.
> 
> This also removes the last usage of `el1_dbg()`.
> 
> Signed-off-by: Ada Couprie Diaz <ada.coupriediaz@arm.com>
> Tested-by: Luis Claudio R. Goncalves <lgoncalv@redhat.com>
> ---
>  arch/arm64/include/asm/exception.h |  2 ++
>  arch/arm64/kernel/debug-monitors.c | 48 ++++++++++++++----------------
>  arch/arm64/kernel/entry-common.c   | 19 ++++++++----
>  3 files changed, 38 insertions(+), 31 deletions(-)

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

Will