linux-kernel - Re: [PATCH] /dev/mem: Allow rewinding

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Date:	Tue, 6 Apr 2010 13:30:29 +0800
From:	Américo Wang <xiyou.wangcong@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Dave Jones <davej@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Wu Fengguang <fengguang.wu@...el.com>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] /dev/mem: Allow rewinding

On Tue, Apr 6, 2010 at 5:19 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> Le lundi 05 avril 2010 à 23:04 +0200, Eric Dumazet a écrit :
>> Andrew, commit dcefafb6ac90ece8d68a6c203105f3d313e52da4 seems to be the
>> problem.
>>
>> It breaks rewinds (negative offsets to lseek (... SEEK_CUR))
>>
>
> Here is a patch to make rewind working again on /dev/mem
>
> [PATCH] /dev/mem: Allow rewinding
>
> commit dcefafb6 (/dev/mem: dont allow seek to last page)
> inadvertently disabled rewinding on /dev/mem.
>
> This broke x86info for example.
>
> Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
> ---
> diff --git a/drivers/char/mem.c b/drivers/char/mem.c
> index 1f3215a..3973a1d 100644
> --- a/drivers/char/mem.c
> +++ b/drivers/char/mem.c
> @@ -710,11 +710,6 @@ static loff_t memory_lseek(struct file *file, loff_t offset, int orig)
>        switch (orig) {
>        case SEEK_CUR:
>                offset += file->f_pos;
> -               if ((unsigned long long)offset <
> -                   (unsigned long long)file->f_pos) {
> -                       ret = -EOVERFLOW;
> -                       break;
> -               }

Why completely dropping the overflow check? What you need to do is just
adding the 'offset < 0' case check.

Thanks.
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