linux-kernel - Re: [PATCH] seq_file:update file->f_pos when lseek() to m->read

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Date:	Mon, 01 Jul 2013 08:41:30 -0400
From:	fangdong <fangdong@...ul.org>
To:	Jiaxing Wang <hello.wjx@...il.com>
CC:	viro@...iv.linux.org.uk, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] seq_file:update file->f_pos when lseek() to m->read_pos

On 06/29/2013 05:11 AM, Jiaxing Wang wrote:
> After pread(), file->f_pos and m->read_pos get different,
> and lseek() to m->read_pos did not update file->f_pos, then
> a subsequent read may read from a wrong position, the following
> program shows 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);
>
> Signed-off-by: Jiaxing Wang <hello.wjx@...il.com>
> ---
>   fs/seq_file.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/fs/seq_file.c b/fs/seq_file.c
> index 774c1eb..4b22b26 100644
> --- a/fs/seq_file.c
> +++ b/fs/seq_file.c
> @@ -328,7 +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;
>   	mutex_unlock(&m->lock);
>
This does not appear to be a problem, in linux man page, the behaver 
seems clearly defined:

DESCRIPTION
        pread() reads up to count bytes from file descriptor fd at 
offset  off-set  (from the start of the file) into the buffer starting 
at buf.  The file offset is not changed.

        pwrite() writes up to count bytes from the buffer starting at 
buf  to the  file  descriptor  fd  at  offset  offset.   The file offset 
is not changed.

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