[PATCH v4 5/7] powerpc/64s: Un-blacklist system_call() from kprobes

[Naveen N. Rao]

On 2017/06/29 08:55PM, Nicholas Piggin wrote:
> On Thu, 29 Jun 2017 16:11:08 +0530
> "Naveen N. Rao" <naveen.n.rao at 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 perhaps add a small comment to explain the label
> (and why it's safe to have after the mtmsrd). It could be
> a bit confusing to read if you don't have that detail of
> the tracer in your mind.

Ah, I see. You did ask for this previously, but I thought you were just 
asking for a comment that this is blacklisted, which can be found out by 
looking for the _ASM_NOKPROBE_SYMBOL() further below.

I will add a comment.

Thanks,
Naveen

> 
> Other than that
> 
> Reviewed-by: Nicholas Piggin <npiggin at gmail.com>
> 
> 
> > 
> > Suggested-by: Michael Ellerman <mpe at ellerman.id.au>
> > Signed-off-by: Naveen N. Rao <naveen.n.rao at linux.vnet.ibm.com>
> > ---
> >  arch/powerpc/kernel/entry_64.S | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
> > index c39436706555..7a87427a67cd 100644
> > --- a/arch/powerpc/kernel/entry_64.S
> > +++ b/arch/powerpc/kernel/entry_64.S
> > @@ -205,6 +205,7 @@ system_call:			/* label this so stack traces look sane */
> >  	mtmsrd	r11,1
> >  #endif /* CONFIG_PPC_BOOK3E */
> >  
> > +system_call_exit:
> >  	ld	r9,TI_FLAGS(r12)
> >  	li	r11,-MAX_ERRNO
> >  	andi.	r0,r9,(_TIF_SYSCALL_DOTRACE|_TIF_SINGLESTEP|_TIF_USER_WORK_MASK|_TIF_PERSYSCALL_MASK)
> > @@ -413,7 +414,7 @@ END_FTR_SECTION_IFSET(CPU_FTR_HAS_PPR)
> >  	b	.	/* prevent speculative execution */
> >  #endif
> >  _ASM_NOKPROBE_SYMBOL(system_call_common);
> > -_ASM_NOKPROBE_SYMBOL(system_call);
> > +_ASM_NOKPROBE_SYMBOL(system_call_exit);
> >  
> >  /* Save non-volatile GPRs, if not already saved. */
> >  _GLOBAL(save_nvgprs)
>