linux-kernel - Re: [PATCH 1/5] Split wait_noreap

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Date:	Wed, 20 May 2009 17:21:06 +0200
From:	Vitaly Mayatskikh <v.mayatskih@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Roland McGrath <roland@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/5] Split wait_noreap_copyout()

At Mon, 11 May 2009 15:25:50 +0200, Vitaly Mayatskikh wrote:
> 
> Move getrusage() and put_user() code from wait_noreap_copyout()
> to copy_wait_opts_to_user(). The same code is spreaded across all
> wait_task_*() routines, it's better to reuse one copy.
> 
> Signed-off-by: Vitaly Mayatskikh <v.mayatskih@...il.com>
> ---
>  kernel/exit.c |   39 +++++++++++++++++++++++----------------
>  1 files changed, 23 insertions(+), 16 deletions(-)
> 
> diff --git a/kernel/exit.c b/kernel/exit.c
> index 25782da..9546362 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -1123,27 +1123,34 @@ static int eligible_child(struct wait_opts *wo, struct task_struct *p)
>  	return 1;
>  }
>  
> -static int wait_noreap_copyout(struct wait_opts *wo, struct task_struct *p,
> -				pid_t pid, uid_t uid, int why, int status)
> +static int copy_wait_opts_to_user(struct wait_opts *wo, struct task_struct *p,
> +				  pid_t pid, uid_t uid, int why, int status, int signal)
>  {
> -	struct siginfo __user *infop;
> +	struct siginfo __user *infop = wo->wo_info;
>  	int retval = wo->wo_rusage
>  		? getrusage(p, RUSAGE_BOTH, wo->wo_rusage) : 0;
>  
> +	if (!retval && infop) {
> +		retval = put_user(signal, &infop->si_signo);
...
> +static int wait_noreap_copyout(struct wait_opts *wo, struct task_struct *p,
> +				pid_t pid, uid_t uid, int why, int status)
> +{
> +	int retval = copy_wait_opts_to_user(wo, p, pid, uid, why, status, SIGCHLD);
>  	put_task_struct(p);
> -	infop = wo->wo_info;
> -	if (!retval)
> -		retval = put_user(SIGCHLD, &infop->si_signo);
...

Oleg has pointed me to broken behaviour here. Previously
wait_noreap_copyout was doing unconditional put_user and was returning
EFAULT when infop is NULL. Now it uses copy_wait_opts_to_user, which
checks infop and return NULL in the same case. This change is visible
from userspace in waitid() function.

There're 2 opportunities how to deal with new behaviour:

1. Assume wait_task_zombie had a bug previously, and let this patch go.
2. Fix copy_wait_opts_to_user to old behaviour by something like:

	if (!retval && (infop || WNOWAIT)) {

What's your opinion?

-- 
wbr, Vitaly
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