linux-kernel - Re: [PATCH] sunrpc: avoid warning in gss_key

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Date:	Fri, 9 Oct 2015 16:10:59 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Trond Myklebust <trond.myklebust@...marydata.com>,
	linux-nfs@...r.kernel.org,
	Anna Schumaker <anna.schumaker@...app.com>,
	Jeff Layton <jlayton@...chiereds.net>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] sunrpc: avoid warning in gss_key_timeout

On Fri, Oct 09, 2015 at 04:13:45PM +0200, Arnd Bergmann wrote:
> The gss_key_timeout() function causes a harmless warning in some
> configurations, e.g. ARM imx_v6_v7_defconfig with gcc-5.2, if the
> compiler cannot figure out the state of the 'expire' variable across
> an rcu_read_unlock():
> 
> net/sunrpc/auth_gss/auth_gss.c: In function 'gss_key_timeout':
> net/sunrpc/auth_gss/auth_gss.c:1422:211: warning: 'expire' may be used uninitialized in this function [-Wmaybe-uninitialized]
> 
> To avoid this warning without adding a bogus initialization, this
> rewrites the function so the comparison is done inside of the
> critical section. As a side-effect, it also becomes slightly
> easier to understand because the implementation now more closely
> resembles the comment above it.

Looks reasonable, thanks; applying for 4.4--b.

> 
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> Fixes: c5e6aecd034e7 ("sunrpc: fix RCU handling of gc_ctx field")
> 
> diff --git a/net/sunrpc/auth_gss/auth_gss.c b/net/sunrpc/auth_gss/auth_gss.c
> index dace13d7638e..799e65b944b9 100644
> --- a/net/sunrpc/auth_gss/auth_gss.c
> +++ b/net/sunrpc/auth_gss/auth_gss.c
> @@ -1411,17 +1411,16 @@ gss_key_timeout(struct rpc_cred *rc)
>  {
>  	struct gss_cred *gss_cred = container_of(rc, struct gss_cred, gc_base);
>  	struct gss_cl_ctx *ctx;
> -	unsigned long now = jiffies;
> -	unsigned long expire;
> +	unsigned long timeout = jiffies + (gss_key_expire_timeo * HZ);
> +	int ret = 0;
>  
>  	rcu_read_lock();
>  	ctx = rcu_dereference(gss_cred->gc_ctx);
> -	if (ctx)
> -		expire = ctx->gc_expiry - (gss_key_expire_timeo * HZ);
> +	if (!ctx || time_after(timeout, ctx->gc_expiry))
> +		ret = -EACCES;
>  	rcu_read_unlock();
> -	if (!ctx || time_after(now, expire))
> -		return -EACCES;
> -	return 0;
> +
> +	return ret;
>  }
>  
>  static int
--
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