Re: [RFC PATCH 02/13] x86/HWBP: Add arch_reinstall_hw_breakpoint() for atomic updates

[Jinchao Wang <wangjinchao600@gmail.com>]

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

> On Mon, 18 Aug 2025 20:26:07 +0800
> Jinchao Wang <wangjinchao600@gmail.com> wrote:
> 
>> 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.


> 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.

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

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