Re: [PATCH v3 4/6] powerpc/64s: Un-blacklist system_call() from kprobes

[Michael Ellerman <mpe@ellerman.id.au>]

Nicholas Piggin <npiggin@gmail.com> writes:

> On Thu, 22 Jun 2017 00:08:40 +0530
> "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com> wrote:
>
>> It is actually safe to probe system_call() in entry_64.S, but only till
>> we unset MSR_RI. To allow this, add a new symbol system_call_exit()
>> after the mtmsrd and blacklist that. Though the mtmsrd instruction
>> itself is now whitelisted, we won't be allowed to probe on it as we
>> don't allow probing on rfi and mtmsr instructions (checked for in
>> arch_prepare_kprobe()).
>
> Can you add a little comment to say probes aren't allowed, and it's
> located after the mtmsr in order to avoid contaminating traces?
>
> Also I wonder if a slightly different name would be more instructive?
> I don't normally care, but the system_call_common code isn't trivial
> to follow. system_call_exit might give the impression that it is the
> entire exit path (which would pair with system_call for entry).

It is the entire path in the happy case isn't it? I'm not sure I know
what you mean.

> Perhaps system_call_exit_notrace? No that sucks too :(

A bit :D

If you're tracing etc. then you'll be in syscall_exit_work, isn't that
sufficient to differentiate the two?

cheers