linux-kernel - Re: [f2fs-dev] [PATCH] f2fs: add F2FS_IOC_SEC_TRIM

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Date:   Tue, 9 Jun 2020 20:15:32 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     Daeho Jeong <daeho43@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, kernel-team@...roid.com,
        Daeho Jeong <daehojeong@...gle.com>
Subject: Re: [f2fs-dev] [PATCH] f2fs: add F2FS_IOC_SEC_TRIM_FILE ioctl

On Wed, Jun 10, 2020 at 11:05:46AM +0900, Daeho Jeong wrote:
> > > Added a new ioctl to send discard commands or/and zero out
> > > to whole data area of a regular file for security reason.
> >
> > With this ioctl available, what is the exact procedure to write and then later
> > securely erase a file on f2fs?  In particular, how can the user prevent f2fs
> > from making multiple copies of file data blocks as part of garbage collection?
> >
> 
> To prevent the file data from garbage collecting, the user needs to
> use pinfile ioctl and fallocate system call after creating the file.
> The sequence is like below.
> 1. create an empty file
> 2. pinfile
> 3. fallocate()

Is that persistent?  So the file will never be moved afterwards?

Is there a place where this is (or should be) documented?

> > > +
> > > +     if (f2fs_readonly(sbi->sb))
> > > +             return -EROFS;
> >
> > Isn't this redundant with mnt_want_write_file()?
> >
> > Also, shouldn't write access to the file be required, i.e.
> > (filp->f_mode & FMODE_WRITE)?  Then the f2fs_readonly() and
> > mnt_want_write_file() checks would be unnecessary.
> >
> 
> Using FMODE_WRITE is more proper for this case, since we're going to
> modify the data. But I think mnt_want_write_file() is still required
> to prevent the filesystem from freezing or something else.

Right, the freezing check is actually still necessary.  But getting write access
to the mount is not necessary.  I think you should use file_start_write() and
file_end_write(), like vfs_write() does.

> >
> > > +
> > > +     if (get_user(flags, (u32 __user *)arg))
> > > +             return -EFAULT;
> > > +     if (!(flags & F2FS_TRIM_FILE_MASK))
> > > +             return -EINVAL;
> >
> > Need to reject unknown flags:
> >
> >         if (flags & ~F2FS_TRIM_FILE_MASK)
> >                 return -EINVAL;
> 
> I needed a different thing here. This was to check neither discard nor
> zeroing out are not here. But we still need to check unknown flags,
> too.
> The below might be better.
> if (!flags || flags & ~F2FS_TRIM_FILE_MASK)
>        return -EINVAL;

Sure, but please put parentheses around the second clause:

	if (flags == 0 || (flags & ~F2FS_TRIM_FILE_MASK))
		return -EINVAL;

- Eric