Re: [PATCH 1/7] uprobes: Kill UPROBE_SKIP_SSTEP and can_skip_sstep()

[Srikar Dronamraju <srikar@linux.vnet.ibm.com>]

> UPROBE_COPY_INSN, UPROBE_SKIP_SSTEP, and uprobe->flags must die. This
> patch kills UPROBE_SKIP_SSTEP. I never understood why it was added;
> not only it doesn't help, it harms.
> 
> It can only help to avoid arch_uprobe_skip_sstep() if it was already
> called before and failed. But this is ugly, if we want to know whether
> we can emulate this instruction or not we should do this analysis in
> arch_uprobe_analyze_insn(), not when we hit this probe for the first
> time.
> 
> And in fact this logic is simply wrong. arch_uprobe_skip_sstep() can
> fail or not depending on the task/register state, if this insn can be
> emulated but, say, put_user() fails we need to xol it this time, but
> this doesn't mean we shouldn't try to emulate it when this or another
> thread hist this bp next time.
> 
> And this is the actual reason for this change. We need to emulate the
> "call" insn, but push(return-address) can obviously fail.
> 
> Per-arch notes:
> 
> 	x86: __skip_sstep() can only emulate "rep;nop". With this
> 	     change it will be called every time and most probably
> 	     for no reason.
> 
> 	     This will be fixed by the next changes. We need to
> 	     change this suboptimal code anyway.
> 
> 	arm: Should not be affected. It has its own "bool simulate"
> 	     flag checked in arch_uprobe_skip_sstep().
> 
> 	ppc: Looks like, it can emulate almost everything. Does it
> 	     actually needs to record the fact that emulate_step()
> 	     failed? Hopefully not. But if yes, it can add the ppc-
> 	     specific flag into arch_uprobe.
> 
> TODO: rename arch_uprobe_skip_sstep() to arch_uprobe_emulate_insn(),
> 
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>

Acked-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>

-- 
Thanks and Regards
Srikar Dronamraju