linux-kernel - Re: [PATCH v3 22/71] ARC: [Review] Prevent incorrect syscall restarts

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Date:	Mon, 28 Jan 2013 13:12:57 +0530
From:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:	Al Viro <viro@...IV.linux.org.uk>
CC:	<linux-arch@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<arnd@...db.de>
Subject: Re: [PATCH v3 22/71] ARC: [Review] Prevent incorrect syscall restarts

Hi Al,

On Thursday 24 January 2013 04:20 PM, Vineet Gupta wrote:
> Per Al Viro's "signals for dummies" https://lkml.org/lkml/2012/12/6/366
> there are 3 golden rules for (not) restarting syscalls:
> 
> "	What we need to guarantee is
> * restarts do not happen on signals caught in interrupts or exceptions
> * restarts do not happen on signals caught in sigreturn()
> * restart should happen only once, even if we get through do_signal()
>   many times."
> 
> ARC Port already handled #1, this patch fixes #2 and #3.
> 
> We use the additional state in pt_regs->orig_r8 to ckh if restarting
> has already been done once.
> 
> Thanks to Al Viro for spotting this.
> 
> Signed-off-by: Vineet Gupta <vgupta@...opsys.com>
> Cc: Al Viro <viro@...IV.linux.org.uk>
> ---
>  arch/arc/include/asm/ptrace.h |    3 +++
>  arch/arc/kernel/signal.c      |   12 ++++++++----
>  2 files changed, 11 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arc/include/asm/ptrace.h b/arch/arc/include/asm/ptrace.h
> index 1711d56..8bf7ff4 100644
> --- a/arch/arc/include/asm/ptrace.h
> +++ b/arch/arc/include/asm/ptrace.h
> @@ -135,6 +135,9 @@ struct user_regs_struct {
>  #define in_syscall(regs)    (regs->event & orig_r8_IS_SCALL)
>  #define in_brkpt_trap(regs) (regs->event & orig_r8_IS_BRKPT)
>  
> +#define syscall_wont_restart(regs) (regs->event |= orig_r8_IS_SCALL_RESTARTED)
> +#define syscall_restartable(regs) !(regs->event &  orig_r8_IS_SCALL_RESTARTED)
> +
>  #define current_pt_regs()					\
>  ({								\
>  	/* open-coded current_thread_info() */			\
> diff --git a/arch/arc/kernel/signal.c b/arch/arc/kernel/signal.c
> index 887a383..9a1ea2b 100644
> --- a/arch/arc/kernel/signal.c
> +++ b/arch/arc/kernel/signal.c
> @@ -131,6 +131,9 @@ SYSCALL_DEFINE0(rt_sigreturn)
>  		if (restore_altstack(&sf->uc.uc_stack))
>  			goto badframe;
>  
> +	/* Don't restart from sigreturn */
> +	syscall_wont_restart(regs);
> +
>  	return regs->r0;
>  
>  badframe:
> @@ -321,13 +324,13 @@ void do_signal(struct pt_regs *regs)
>  
>  	signr = get_signal_to_deliver(&info, &ka, regs, NULL);
>  
> -	/* Are we from a system call? */
> -	restart_scall = in_syscall(regs);
> +	restart_scall = in_syscall(regs) && syscall_restartable(regs);
>  
>  	if (signr > 0) {
> -		if (restart_scall)
> +		if (restart_scall) {
>  			arc_restart_syscall(&ka, regs);
> -
> +			syscall_wont_restart(regs);	/* No more restarts */
> +		}
>  		handle_signal(signr, &ka, &info, regs);
>  		return;
>  	}
> @@ -342,6 +345,7 @@ void do_signal(struct pt_regs *regs)
>  			regs->r8 = __NR_restart_syscall;
>  			regs->ret -= 4;
>  		}
> +		syscall_wont_restart(regs);	/* No more restarts */
>  	}
>  
>  	/* If there's no signal to deliver, restore the saved sigmask back */
> 

If this looks OK, can I get your ACK on this.

Thx,
-Vineet
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