arm64: debug: split brk64 exception entry

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()`.

This also removes the last usage of `el0_dbg()` without `CONFIG_COMPAT`.
Mark it `__maybe_unused`, to prevent a warning when building this patch
without `CONFIG_COMPAT`, as the following patch removes `el0_dbg()`.

Signed-off-by: Ada Couprie Diaz <ada.coupriediaz@arm.com>
Tested-by: Luis Claudio R. Goncalves <lgoncalv@redhat.com>
Reviewed-by: Will Deacon <will@kernel.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20250707114109.35672-12-ada.coupriediaz@arm.com
Signed-off-by: Will Deacon <will@kernel.org>

0 files changed, 0 insertions, 0 deletions