Restriction on setting debug registers (HW breakpoint) at kernel space

Restriction on setting debug registers (HW breakpoint) at kernel space

[Probir Roy]

Hi,

I am patching the perf tool of linux to set HW breakpoints at kernel
space. I found that there is an interface for user-space
(modify_user_hw_breakpoint) but no interface for system wide
breakpoint modification. Older version of linux had this support by K.
Prasad. I was wondering why linux remove this support. Moreover, when
I patch the perf tool for kernel space breakpoint following steps from
"modify_user_hw_breakpoint", kernel panics but nothing is written on
log file. What could be the issue? Is it a race condition while
modifying the address or is it because kernel restricts it to set HW
breakpoint at some kernel address(if so, where is it written)?

Regards
-- 
Probir Roy

Re: Restriction on setting debug registers (HW breakpoint) at kernel space

[Thomas Gleixner]

On Fri, 13 Oct 2017, Probir Roy wrote:
> I am patching the perf tool of linux to set HW breakpoints at kernel
> space. I found that there is an interface for user-space
> (modify_user_hw_breakpoint) but no interface for system wide
> breakpoint modification. Older version of linux had this support by K.
> Prasad. I was wondering why linux remove this support. Moreover, when
> I patch the perf tool for kernel space breakpoint following steps from
> "modify_user_hw_breakpoint", kernel panics but nothing is written on
> log file. What could be the issue? Is it a race condition while
> modifying the address or is it because kernel restricts it to set HW
> breakpoint at some kernel address(if so, where is it written)?

Can you please provide the patch you use and the perf command line which
causes the kernel to panic?

Also telling which kernel version and hardware system you are using would
be helpful.

Thanks,

	tglx