Hi Pratyush, On 18/03/16 14:43, Pratyush Anand wrote: > On 18/03/2016:02:02:49 PM, James Morse wrote: >> 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. > 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()). Thanks, James
