Hi James,

On 18/03/2016:06:12:20 PM, James Morse wrote:

Hi Pratyush,

On 18/03/16 14:43, Pratyush Anand wrote:

quoted

On 18/03/2016:02:02:49 PM, James Morse wrote:

quoted

In kernel/entry.S when entered from EL0 we test for TIF_SINGLESTEP in the
thread_info flags, and use disable_step_tsk/enable_step_tsk to save/restore the
single-step state.

Could we do this regardless of which EL we came from?

Thanks for another idea. I think, we can not do this as it is, because
TIF_SINGLESTEP will not be set for kprobe events.

Hmmm, I see kernel_enable_single_step() doesn't set it, but setup_singlestep()
in patch 5 could...

There is probably a good reason its never set for a kernel thread, I will have a
look at where else it is used.

quoted

But, we can introduce a
variant disable_step_kernel and enable_step_kernel, which can be called in
el1_da. 

What about sp/pc misalignment, or undefined instructions?
Or worse... an irq occurs during your el1_da call (el1_da may re-enable irqs).
el1_irq doesn't know you were careful not to unmask debug exceptions, it blindly
turns them back on.

The problem is the 'single step me' bit is still set, save/restoring it will
save us having to consider every interaction, (and then missing some!).

It would also mean you don't have to disable interrupts while single stepping in
patch 5 (comment above kprobes_save_local_irqflag()).

I see.
kernel_enable_single_step() is called from watchpoint and kgdb handler. It seems
to me that, similar issue may arise there as well. So, it would be a good idea
to set TIF_SINGLESTEP in kernel_enable_single_step() and clear in
kernel_disable_single_step().

Meanwhile, I prepared a test case to reproduce the issue without this patch.
Instrumented a kprobe at an instruction of __copy_to_user() which stores in user
space memory. I can see a sea of messages "Unexpected kernel single-step
exception at EL1" within few seconds.  While with patch[1] applied, I do not see
any such messages. 

May be I can send [1] as RFC and seek feedback.

~Pratyush

[1] https://github.com/pratyushanand/linux/commit/7623c8099ac22eaa00e7e0f52430f7a4bd154652