Re: [PATCH 3/5] uprobes: Fix UPROBE_SKIP_SSTEP checks in handle_swbp()

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

* Oleg Nesterov <oleg@redhat.com> [2012-09-14 19:15:57]:

> If handle_swbp()->add_utask() fails but UPROBE_SKIP_SSTEP is set,
> cleanup_ret: path do not restart the insn, this is wrong. Remove
> this check and add the additional label for can_skip_sstep() = T
> case.
> 
> Note also that UPROBE_SKIP_SSTEP can be false positive, we simply
> can not trust it unless arch_uprobe_skip_sstep() was already called.
> 
> Also, move another UPROBE_SKIP_SSTEP check before can_skip_sstep()
> into this helper, this looks more clean and understandable.
> 
> Note: probably we should rename "skip" to "emulate" and I think

yes we can rename can_skip_step to can_emulate_insn and
arch_uprobe_skip_step() to arch_uprobe_emulate_insn

Similarly UPROBE_SKIP_SSTEP can be renamed as UPROBE_EMULATE_INSN

> that "clear UPROBE_SKIP_SSTEP" should be moved to arch_can_skip.
> 

Currently struct uprobe is not exposed to arch specific code as
suggested by Ingo. Adding a flag in arch_uprobe just for this and
expecting all archs to define one is probably an overhead.
Hence I am not sure moving the clear flag to arch is a good idea.

> Signed-off-by: Oleg Nesterov <oleg@redhat.com>

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

> ---
>  kernel/events/uprobes.c |   31 +++++++++++++++----------------
>  1 files changed, 15 insertions(+), 16 deletions(-)
> 
> diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> index 9893cba..403d2e0 100644
> --- a/kernel/events/uprobes.c
> +++ b/kernel/events/uprobes.c
> @@ -1389,10 +1389,11 @@ bool uprobe_deny_signal(void)
>   */
>  static bool can_skip_sstep(struct uprobe *uprobe, struct pt_regs *regs)
>  {
> -	if (arch_uprobe_skip_sstep(&uprobe->arch, regs))
> -		return true;
> -
> -	uprobe->flags &= ~UPROBE_SKIP_SSTEP;
> +	if (uprobe->flags & UPROBE_SKIP_SSTEP) {
> +		if (arch_uprobe_skip_sstep(&uprobe->arch, regs))
> +			return true;
> +		uprobe->flags &= ~UPROBE_SKIP_SSTEP;
> +	}
>  	return false;
>  }
> 
> @@ -1494,12 +1495,12 @@ static void handle_swbp(struct pt_regs *regs)
>  		utask = add_utask();
>  		/* Cannot allocate; re-execute the instruction. */
>  		if (!utask)
> -			goto cleanup_ret;
> +			goto restart;
>  	}
> 
>  	handler_chain(uprobe, regs);
> -	if (uprobe->flags & UPROBE_SKIP_SSTEP && can_skip_sstep(uprobe, regs))
> -		goto cleanup_ret;
> +	if (can_skip_sstep(uprobe, regs))
> +		goto out;
> 
>  	if (!pre_ssout(uprobe, regs, bp_vaddr)) {
>  		arch_uprobe_enable_step(&uprobe->arch);
> @@ -1508,15 +1509,13 @@ static void handle_swbp(struct pt_regs *regs)
>  		return;
>  	}
> 
> -cleanup_ret:
> -	if (!(uprobe->flags & UPROBE_SKIP_SSTEP))
> -
> -		/*
> -		 * cannot singlestep; cannot skip instruction;
> -		 * re-execute the instruction.
> -		 */
> -		instruction_pointer_set(regs, bp_vaddr);
> -
> +restart:
> +	/*
> +	 * cannot singlestep; cannot skip instruction;
> +	 * re-execute the instruction.
> +	 */
> +	instruction_pointer_set(regs, bp_vaddr);
> +out:
>  	put_uprobe(uprobe);
>  }
> 
> -- 
> 1.5.5.1
>