On Mon, 1 Sep 2025 18:23:44 +0800
Jinchao Wang [off-list ref] wrote:

On 9/1/25 15:06, Masami Hiramatsu (Google) wrote:

quoted

Hi Jinchao,

Hi Masami,

quoted

On Mon, 18 Aug 2025 20:26:07 +0800
Jinchao Wang [off-list ref] wrote:

quoted

Add arch_reinstall_hw_breakpoint() to enable atomic context modification
of hardware breakpoint parameters without deallocating and reallocating
the breakpoint slot.

The existing arch_install_hw_breakpoint() allocates a new debug register
slot, while arch_uninstall_hw_breakpoint() deallocates it. However, some
use cases require modifying breakpoint parameters (address, length, type)
atomically without losing the allocated slot, particularly when operating
in atomic contexts where allocation might fail or be unavailable.

This is particularly useful for debugging tools like kstackwatch that
need to dynamically update breakpoint targets in atomic contexts while
maintaining consistent hardware state.

I'm also trying to find this interface for my wprobe. So the idea is good.
But this looks hacky and only for x86. I think the interface should be
more generic and do not use this arch internal function directly.

I agree with your point about the architectural dependency. I have been
considering this problem not only for the hardware breakpoint 
reinstallation,
but also for other related parts of the series, such as canary finding and
stack address resolving. These parts also rely on arch-specific code.

Yes, even though, the hw-breakpoint is an independent feature.
Directly using arch_*() functions (which are expected to be used
internally) introduces a hidden dependency between these two
components and looses maintainability.

quoted

It seems that the slot is allocated by "type", thus, if this reinstall
hwbp without deallocate/allocate slot, it must NOT change the type.
See __modify_bp_slot. Also, provide CONFIG_HAVE_... option for checking
whether the architecture support that interface.

Regarding the slot allocation, I would like to clarify my point. I 
believe the
event->attr.type should not be changed when reinstalling a hardware
breakpoint, as this defines the fundamental nature of the event. The type
must always be PERF_TYPE_BREAKPOINT.

The event->attr.bp_type, however, can be changed. For example, from a
HW_BREAKPOINT_W to a HW_BREAKPOINT_RW without needing to deallocate and
reallocate the slot. This is useful for future applications, even though the
current use case for KStackWatch only requires HW_BREAKPOINT_W.

I understand your point, so it also needs another wrapper which checks
the type is compatible on the architecture.

By the way, I have sent an updated series.
https://lore.kernel.org/all/20250828073311.1116593-1-wangjinchao600@gmail.com/ (local)

Yeah, OK, let me review the series. Thanks for update!

Thank you again for your valuable review.
-- 
Best regards,
Jinchao

-- 
Masami Hiramatsu (Google) [off-list ref]