linux-kernel - Re: [PATCH] seq_file: always update file->f_pos in seq

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Date:	Fri, 25 Oct 2013 21:23:49 +0800
From:	Jiaxing Wang <hello.wjx@...il.com>
To:	Gu Zheng <guz.fnst@...fujitsu.com>,
	Al Viro <viro@...iv.linux.org.uk>
CC:	fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] seq_file: always update file->f_pos in seq_lseek()

On 10/25/2013 06:15 PM, Gu Zheng wrote:
> This issue was first pointed out by Jiaxing Wang several months ago, but no
> further comments:
> https://lkml.org/lkml/2013/6/29/41
> 
> As we know pread() does not change f_pos, so after pread(), file->f_pos
> and m->read_pos become different. And seq_lseek() does not update file->f_pos
> if offset equals to m->read_pos, so after pread() and seq_lseek()(lseek to
> m->read_pos), then a subsequent read may read from a wrong position, the
> following program produces the problem:
> 
>     char str1[32] = { 0 };
>     char str2[32] = { 0 };
>     int poffset = 10;
>     int count = 20;
> 
>     /*open any seq file*/
>     int fd = open("/proc/modules", O_RDONLY);
> 
>     pread(fd, str1, count, poffset);
>     printf("pread:%s\n", str1);
> 
>     /*seek to where m->read_pos is*/
>     lseek(fd, poffset+count, SEEK_SET);
> 
>     /*supposed to read from poffset+count, but this read from position 0*/
>     read(fd, str2, count);
>     printf("read:%s\n", str2);
> 
> out put:
> pread:
>  ck_netbios_ns 12665
> read:
>  nf_conntrack_netbios
> 
> /proc/modules:
> nf_conntrack_netbios_ns 12665 0 - Live 0xffffffffa038b000
> nf_conntrack_broadcast 12589 1 nf_conntrack_netbios_ns, Live 0xffffffffa0386000
> 
> So we always update file->f_pos to offset in seq_lseek() to fix this issue.
> 
> Signed-off-by: Jiaxing Wang <hello.wjx@...il.com>
> Signed-off-by: Gu Zheng <guz.fnst@...fujitsu.com>
> ---
>  fs/seq_file.c |    2 ++
>  1 files changed, 2 insertions(+), 0 deletions(-)
> 
> diff --git a/fs/seq_file.c b/fs/seq_file.c
> index 3135c25..a290157 100644
> --- a/fs/seq_file.c
> +++ b/fs/seq_file.c
> @@ -328,6 +328,8 @@ loff_t seq_lseek(struct file *file, loff_t offset, int whence)
>  				m->read_pos = offset;
>  				retval = file->f_pos = offset;
>  			}
> +		} else {
> +			file->f_pos = offset;
>  		}
>  	}
>  	file->f_version = m->version;
> 

Thanks Gu, I should have post this patch again.
And Viro, please take a few minutes to review this, thanks.
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