Re: [RFC PATCH 2/4] powerpc: kprobe: add arch specific blacklist

On 2016/11/18 04:48PM, Michael Ellerman wrote:

"Naveen N. Rao" [off-list ref] writes:

quoted

Add symbol to mark end of entry_*.S and use the same to blacklist all
addresses from kernel start (_stext) to entry code from kprobes. Much of
this code is early exception handling where we can't really take a trap.

I'm not sure about this. entry_*.S is actually a bit of jumble,
especially the 64bit version. I've been wanting to split it up for a
long time.

Ok. Let me take a stab at that.

It doesn't actually contain any early exception handling. It does
contain the common syscall handler, and the exception return paths, some
of which should be black listed. And lots of other junk.

Also I'm not sure if it's guaranteed that there won't be other code
between _stext and the end of entry, it's not handled explicitly in the
linker script, it just tends to get linked early because it's in head-y.

I actually considered that. One of the issues in trying to get entry_* 
linked in early has to do with the exception common handlers - they 
start at 0x7000 or 0x8000 and are placed in .text *and* I think they 
need to be within 64k from the exception vectors. As such, placing 
entry_* in a separate section and linking it after HEAD_TEXT resulted in 
moving down the common exception handlers.

Regardless of the kprobe blacklist, does it make sense to put the common 
exception handlers into a separate section so as to separate them out 
from the rest of the code?

So I think it would be better if we had a clearer picture of exactly
what in this file we want to blacklist.

Agreed. As a first step, I wanted to get a coarser blacklist in place 
and fine tune it later. But, I can see why entry_* needs a smaller 
blacklist. I'll get back on this.


- Naveen