linux-kernel - Re: [PATCH 1/1] ftruncate: create FAN_MODIFY and IN

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Date:	Mon, 06 Oct 2014 22:09:12 +0200
From:	Heinrich Schuchardt <xypron.glpk@....de>
To:	Jan Kara <jack@...e.cz>
CC:	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Eric Paris <eparis@...isplace.org>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	John McCutchan <john@...nmccutchan.com>,
	Robert Love <rlove@...ve.org>
Subject: Re: [PATCH 1/1] ftruncate: create FAN_MODIFY and IN_MODIFY events

On 06.10.2014 15:24, Jan Kara wrote:
> On Fri 03-10-14 20:16:30, Heinrich Schuchardt wrote:
>> The fanotify and the inotify API can be used to monitor changes of the file
>> system.
>>
>> System call ftruncate modifies files. Hence it should trigger the corresponding
>> fanotify (FAN_MODIFY) and inotify (IN_MODIFY) events.
>    Hum, I would think that the appropriate event gets generated by
> fsnotify_change() called from notify_change()?
>
> 								Honza

Hello Jan,

thank you for reviewing.

fsnotify_change() calls
fsnotify(inode, mask, inode, FSNOTIFY_EVENT_INODE, NULL, 0);

fsnotify_modify() calls
fsnotify(inode, mask, path, FSNOTIFY_EVENT_PATH, NULL, 0);

Only with FSNOTIFY_EVENT_PATH a mount is determined in fsnotify().

So a FAN_MODIFY event for a mount cannot occur. But I also did not see a 
FAN_MODIFY event when watching a file either.

IN_MODIFY events are actually created.

So would it be better to do the change in do_truncate and replace the 
call to notify_change by a call to fsnotify_modify?

I just had a look at
http://lxr.free-electrons.com/ident?i=notify_change

There seem to be some other usages of notify_change that deserve 
consideration, e.g. in ecryptfs_truncate().

Best regards

Heinrich Schuchardt

>>
>> Signed-off-by: Heinrich Schuchardt <xypron.glpk@....de>
>> ---
>>   fs/open.c | 2 ++
>>   1 file changed, 2 insertions(+)
>>
>> diff --git a/fs/open.c b/fs/open.c
>> index d6fd3ac..e36f26e 100644
>> --- a/fs/open.c
>> +++ b/fs/open.c
>> @@ -189,6 +189,8 @@ static long do_sys_ftruncate(unsigned int fd, loff_t length, int small)
>>   		error = security_path_truncate(&f.file->f_path);
>>   	if (!error)
>>   		error = do_truncate(dentry, length, ATTR_MTIME|ATTR_CTIME, f.file);
>> +	if (!error)
>> +		fsnotify_modify(f.file);
>>   	sb_end_write(inode->i_sb);
>>   out_putf:
>>   	fdput(f);
>> --
>> 2.1.0
>>

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