linux-kernel - Re: [PATCH v2 9/9] uprobes/x86: Teach arch_uprobe_post

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Date:	Fri, 04 Apr 2014 14:56:47 -0700
From:	Jim Keniston <jkenisto@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Anton Arapov <aarapov@...hat.com>,
	David Long <dave.long@...aro.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Jonathan Lebon <jlebon@...hat.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 9/9] uprobes/x86: Teach arch_uprobe_post_xol() to
 restart if possible

On Fri, 2014-04-04 at 20:51 +0200, Oleg Nesterov wrote:
> SIGILL after the failed arch_uprobe_post_xol() should only be used as
> a last resort, we should try to restart the probed insn if possible.
> 
> Currently only adjust_ret_addr() can fail, and this can only happen if
> another thread unmapped our stack after we executed "call" out-of-line.
> Most probably the application if buggy, but even in this case it can
> have a handler for SIGSEGV/etc. And in theory it can be even correct
> and do something non-trivial with its memory.
> 
> Of course we can't restart unconditionally, so arch_uprobe_post_xol()
> does this only if ->post_xol() returns -ERESTART even if currently this
> is the only possible error.

When re-executing the call instruction, I'd think the stack pointer
would be wrong the second time around, unless you pop off the return
address from the first try.

> 
> Note: this is not "perfect", we do not want the extra handler_chain()
> after restart, but I think this is the best solution we can realistically
> do without too much uglifications.
> 
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>
> ---
>  arch/x86/kernel/uprobes.c |   15 +++++++++++----
>  1 files changed, 11 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
> index e72903e..b820668 100644
> --- a/arch/x86/kernel/uprobes.c
> +++ b/arch/x86/kernel/uprobes.c
> @@ -443,16 +443,17 @@ static int default_post_xol_op(struct arch_uprobe *auprobe, struct pt_regs *regs
>  {
>  	struct uprobe_task *utask = current->utask;
>  	long correction = (long)(utask->vaddr - utask->xol_vaddr);
> -	int ret = 0;
> 
>  	handle_riprel_post_xol(auprobe, regs, &correction);
>  	if (auprobe->fixups & UPROBE_FIX_IP)
>  		regs->ip += correction;
> 
> -	if (auprobe->fixups & UPROBE_FIX_CALL)
> -		ret = adjust_ret_addr(regs->sp, correction);
> +	if (auprobe->fixups & UPROBE_FIX_CALL) {
> +		if (adjust_ret_addr(regs->sp, correction))
> +			return -ERESTART;
> +	}
> 
> -	return ret;
> +	return 0;
>  }
> 
>  static struct uprobe_xol_ops default_xol_ops = {
> @@ -599,6 +600,12 @@ int arch_uprobe_post_xol(struct arch_uprobe *auprobe, struct pt_regs *regs)
>  		int err = auprobe->ops->post_xol(auprobe, regs);
>  		if (err) {
>  			arch_uprobe_abort_xol(auprobe, regs);
> +			/*
> +			 * Restart the probed insn. ->post_xol() must ensure
> +			 * this is really possible if it returns -ERESTART.
> +			 */
> +			if (err == -ERESTART)
> +				return 0;
>  			return err;
>  		}
>  	}


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