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

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Date:	Tue, 02 Jul 2013 08:43:58 +0800
From:	Jiaxing Wang <lkml.wjx@...il.com>
To:	fangdong <fangdong@...ul.org>
CC:	Jiaxing Wang <hello.wjx@...il.com>, 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 07/01/2013 08:41 PM, fangdong wrote:
> 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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There's no problem that pread() don't change file->f_pos, but I think lseek() should have changed it.
Any comments from other people, and Alexander?
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