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From:   Daeho Jeong <daeho43@gmail.com>
Date:   Wed, 10 Jun 2020 12:55:40 +0900
Message-ID: <CACOAw_wErOPC=Kf3UU8nFGhWRy84ZnCeJbsyPhSCcXv51B_XxQ@mail.gmail.com>
Subject: Re: [f2fs-dev] [PATCH] f2fs: add F2FS_IOC_SEC_TRIM_FILE ioctl
To:     Eric Biggers <ebiggers@kernel.org>
Cc:     linux-kernel@vger.kernel.org,
        linux-f2fs-devel@lists.sourceforge.net, kernel-team@android.com,
        Daeho Jeong <daehojeong@google.com>
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> >
> > 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?

Yes, this is persistent. F2FS_IOC_SET_PIN_FILE ioctl is to prevent
file data from moving and being garbage collected, and further update
to the file will be handled in in-place update manner.
I don't see any document on this, but you can find the below in
Documentation/filesystems/f2fs.rst

However, once F2FS receives ioctl(fd, F2FS_IOC_SET_PIN_FILE) in prior to
fallocate(fd, DEFAULT_MODE), it allocates on-disk blocks addresses having
zero or random data, which is useful to the below scenario where:

 1. create(fd)
 2. ioctl(fd, F2FS_IOC_SET_PIN_FILE)
 3. fallocate(fd, 0, 0, size)
 4. address =3D fibmap(fd, offset)
 5. open(blkdev)
 6. write(blkdev, address)

> 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.

Yes, agreed.

2020=EB=85=84 6=EC=9B=94 10=EC=9D=BC (=EC=88=98) =EC=98=A4=ED=9B=84 12:15, =
Eric Biggers <ebiggers@kernel.org>=EB=8B=98=EC=9D=B4 =EC=9E=91=EC=84=B1:
>
> 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 t=
hen later
> > > securely erase a file on f2fs?  In particular, how can the user preve=
nt f2fs
> > > from making multiple copies of file data blocks as part of garbage co=
llection?
> > >
> >
> > 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 =3D=3D 0 || (flags & ~F2FS_TRIM_FILE_MASK))
>                 return -EINVAL;
>
> - Eric