Re: [PATCH v6 0/6] arm64: Add kernel probes (kprobes) support

[Will Deacon]

On Tue, Apr 28, 2015 at 03:58:21AM +0100, William Cohen wrote:
> Hi All,

Hi Will,

> I have been experimenting with the patches for arm64 kprobes support.
> On occasion the kernel gets stuck in a loop printing output:
> 
>  Unexpected kernel single-step exception at EL1
> 
> This message by itself is not that enlighten.  I added the attached
> patch to get some additional information about register state when the
> warning is printed out.  Below is an example output:

Given that we've got the pt_regs in our hands at that point, I'm happy to
print something more useful if you like (e.g. the PC?).

> [14613.263536] Unexpected kernel single-step exception at EL1
> [14613.269001] kcb->ss_ctx.ss_status = 1
> [14613.272643] kcb->ss_ctx.match_addr = fffffdfffc001250 0xfffffdfffc001250
> [14613.279324] instruction_pointer(regs) = fffffe0000093358 el1_da+0x8/0x70
> [14613.286003] 
> [14613.287487] CPU: 3 PID: 621 Comm: irqbalance Tainted: G           OE   4.0.0u4+ #6
> [14613.295019] Hardware name: AppliedMicro Mustang/Mustang, BIOS 1.1.0-rh-0.15 Mar 13 2015
> [14613.302982] task: fffffe01d6806780 ti: fffffe01d68ac000 task.ti: fffffe01d68ac000
> [14613.310430] PC is at el1_da+0x8/0x70
> [14613.313990] LR is at trampoline_probe_handler+0x188/0x1ec

> The really odd thing is the address of the PC it is in el1_da the code
> to handle data aborts.  it looks like it is getting the unexpected
> single_step exception right after the enable_debug in el1_da.  I think
> what might be happening is:
> 
> -an instruction is instrumented with kprobe
> -the instruction is copied to a buffer
> -a breakpoint replaces the instruction
> -the kprobe fires when the breakpoint is encountered
> -the instruction in the buffer is set to single step
> -a single step of the instruction is attempted
> -a data abort exception is raised
> -el1_da is called

So that's the bit that I find weird. Can you take a look at what we're doing
in trampoline_probe_handler, please? It could be that we're doing something
like get_user and aborting on a faulting userspace address, but I think
kprobes should handle that rather than us trying to get the generic
single-step code to deal with it.

> It looks like commit 1059c6bf8534acda249e7e65c81e7696fb074dc1 from Mon
> Sep 22   "arm64: debug: don't re-enable debug exceptions on return from el1_dbg"
> was trying to address a similar problem for the el1_dbg
> function.  Should el1_da and other el1_* functions have the enable_dbg
> removed?

I don't think so. The current behaviour of the low-level debug handler is to
step into traps, which is more flexible than trying to step over them (which
could lead to us stepping over interrupts, or preemption points). It should
be up to the higher-level debugger (e.g. kprobes, kgdb) to distinguish
between the traps it does and does not care about.

An equivalent userspace example would be GDB stepping into single handlers,
I suppose.

Will
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