Subject: Query: ARM64: Behavior of el1_dbg exception while executing el0_dbg

From: [email-masked] (Pratyush Anand)
Date: Thu, 08 Jan 2015 18:45:58 +0530

Hi All,

I am trying to test following scenario, which seems valid to me. But I 
am very new to ARM64 as well as to debugging tools, so seeking expert's 
comment here.

-- I have inserted a kprobe to the function uprobe_breakpoint_handler 
which is called from elo_dbg 
(el0_dbg->do_debug_exception->brk_handler->call_break_hook->uprobe_breakpoint_handler)

-- kprobe is enabled.

-- an uprobe is inserted into a test application and enabled.

So, when uprobe is enabled and test code execution reaches to probe 
instruction, it executes uprobe breakpoint instruction and el0_dbg 
exception is raised.

When control reaches to start of uprobe_breakpoint_handler and it 
executes first instruction (which has been replaced with a kprobe 
breakpoint instruction), el1_dbg exception is raised.

Further Call sequence goes like, 
el1_dbg->do_debug_exception->brk_handler->call_break_hook->kprobe_breakpoint_handler, 
and kprobe breakpoint handler does everything what it should have done.

After return from above (first) el1_dbg, second el1_dbg is raised for 
single steping of kprobe instruction, and instruction pointer does not 
matches with the kcb->ss_ctx.match_addr and so, kprobe_ss_hit fails, 
which is strange.

To debug it further, I examined ELR_EL1 value in el1_dbg after execution 
of first el1_dbg, and it was fffffdfffc000004.

So, my question is how can instruction pointer has a value 
fffffe0000092470(which is actually el1_inv + 0x4) when second el1_dbg is 
received?

Am I missing something or trying something which is not supported by ARM64?

I have put some printk in the code. You can have a detailed view of 
debug code and print log here:

https://github.com/pratyushanand/linux.git

branch: 
ml_arm64_uprobe_devel_debug_kprobe_insertion_at_uprobe_breakpoint_handler

~Pratyush