linux-kernel - Re: [patch] tlb_uv: handle large snprintf() returns

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Date:	Mon, 27 Sep 2010 16:12:03 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Dan Carpenter <error27@...il.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	Cliff Wickman <cpw@....com>, Jack Steiner <steiner@....com>,
	Robin Holt <holt@....com>, linux-kernel@...r.kernel.org,
	kernel-janitors@...r.kernel.org
Subject: Re: [patch] tlb_uv: handle large snprintf() returns

On Mon, 16 Aug 2010 12:55:02 +0200
Dan Carpenter <error27@...il.com> wrote:

> snprintf() returns the number of bytes that *would* have been copied if
> the buffer was large enough, so it can be larger than the size of the
> buffer.  In this case it's ok, but let's put a cap on it anyway so it's
> easier to audit.
> 
> Signed-off-by: Dan Carpenter <error27@...il.com>
> 
> diff --git a/arch/x86/kernel/tlb_uv.c b/arch/x86/kernel/tlb_uv.c
> index 312ef02..5e88b3a 100644
> --- a/arch/x86/kernel/tlb_uv.c
> +++ b/arch/x86/kernel/tlb_uv.c
> @@ -1012,6 +1012,9 @@ static ssize_t tunables_read(struct file *file, char __user *userbuf,
>  		timeoutsb4reset, ipi_reset_limit, complete_threshold,
>  		congested_response_us, congested_reps, congested_period);
>  
> +	if (ret > 300)
> +		ret = 300;
> +
>  	return simple_read_from_buffer(userbuf, count, ppos, buf, ret);
>  }

That 300-byte local array is yuk.

Duplicating the "300" later in the function (instead of using sizeof)
is also yuk.

The code can be deyukked by using sprintf_to_user(), only we don't have
one.  But we do have kasprintf().

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