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

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Date:	Tue, 6 Apr 2010 14:24:05 +0800
From:	Américo Wang <xiyou.wangcong@...il.com>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Dave Jones <davej@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] /dev/mem: Allow rewinding

On Tue, Apr 6, 2010 at 1:43 PM, Wu Fengguang <fengguang.wu@...el.com> wrote:
> On Tue, Apr 06, 2010 at 01:30:29PM +0800, Américo Wang wrote:
>> 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.
>
> It will fall through to the next EOVERFLOW check in the SEEK_SET case :)
>

Oh, right, I didn't read the code, merely the patch.

Thanks.
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