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From:   harshad shirwadkar <harshadshirwadkar@gmail.com>
Date:   Fri, 26 Mar 2021 18:43:52 -0700
Message-ID: <CAD+ocbxp5s5QfOKheftMMyd69RaZtS9z8RBnjUqZ3siOCdfFbg@mail.gmail.com>
Subject: Re: [PATCH 1/2] ext4: wipe filename upon file deletion
To:     Andreas Dilger <adilger@dilger.ca>
Cc:     Leah Rumancik <leah.rumancik@gmail.com>,
        Ext4 Developers List <linux-ext4@vger.kernel.org>,
        "Theodore Y. Ts'o" <tytso@mit.edu>
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On Fri, Mar 26, 2021 at 4:08 PM Andreas Dilger <adilger@dilger.ca> wrote:
>
> On Mar 25, 2021, at 12:12 PM, Leah Rumancik <leah.rumancik@gmail.com> wrote:
> >
> > Zero out filename field when file is deleted. Also, add mount option
> > nowipe_filename to disable this filename wipeout if desired.
>
> I would personally be against "wipe out entries on delete" as the default
> behavior.  I think most users would prefer that their data is maximally
> recoverable, rather than the minimal security benefit of erasing deleted
> content by default.
I understand that persistence of filenames provides recoverability
that users might like but I feel like that may break sooner or later.
For example, if we get directory shrinking patches[1] merged in or if
we redesign the directory htree using generic btrees (which will
hopefully support shrinking), this kind of recovery will become very
hard.

Also, I was wondering if persistence of file names was by design? or
it was there due to the way we implemented directories?

[1] https://patchwork.ozlabs.org/project/linux-ext4/list/?series=166560

Thanks,
Harshad
>
> I think that Darrick made a good point that using the EXT4_SECRM_FL on
> the inode gives users a good option to enable/disable this on a per
> file or directory basis.
>
> > Signed-off-by: Leah Rumancik <leah.rumancik@gmail.com>
> > ---
> > fs/ext4/ext4.h  |  1 +
> > fs/ext4/namei.c |  4 ++++
> > fs/ext4/super.c | 11 ++++++++++-
> > 3 files changed, 15 insertions(+), 1 deletion(-)
> >
> > diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> > index 826a56e3bbd2..8011418176bc 100644
> > --- a/fs/ext4/ext4.h
> > +++ b/fs/ext4/ext4.h
> > @@ -1247,6 +1247,7 @@ struct ext4_inode_info {
> > #define EXT4_MOUNT2_JOURNAL_FAST_COMMIT       0x00000010 /* Journal fast commit */
> > #define EXT4_MOUNT2_DAX_NEVER         0x00000020 /* Do not allow Direct Access */
> > #define EXT4_MOUNT2_DAX_INODE         0x00000040 /* For printing options only */
> > +#define EXT4_MOUNT2_WIPE_FILENAME       0x00000080 /* Wipe filename on del entry */
> >
> >
> > #define clear_opt(sb, opt)            EXT4_SB(sb)->s_mount_opt &= \
> > diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
> > index 883e2a7cd4ab..ae6ecabd4d97 100644
> > --- a/fs/ext4/namei.c
> > +++ b/fs/ext4/namei.c
> > @@ -2492,6 +2492,10 @@ int ext4_generic_delete_entry(struct inode *dir,
> >                       else
> >                               de->inode = 0;
> >                       inode_inc_iversion(dir);
> > +
> > +                     if (test_opt2(dir->i_sb, WIPE_FILENAME))
> > +                             memset(de_del->name, 0, de_del->name_len);
> > +
> >                       return 0;
> >               }
> >               i += ext4_rec_len_from_disk(de->rec_len, blocksize);
> > diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> > index b9693680463a..5e8737b3f171 100644
> > --- a/fs/ext4/super.c
> > +++ b/fs/ext4/super.c
> > @@ -1688,7 +1688,7 @@ enum {
> >       Opt_dioread_nolock, Opt_dioread_lock,
> >       Opt_discard, Opt_nodiscard, Opt_init_itable, Opt_noinit_itable,
> >       Opt_max_dir_size_kb, Opt_nojournal_checksum, Opt_nombcache,
> > -     Opt_prefetch_block_bitmaps,
> > +     Opt_prefetch_block_bitmaps, Opt_nowipe_filename,
> > #ifdef CONFIG_EXT4_DEBUG
> >       Opt_fc_debug_max_replay, Opt_fc_debug_force
> > #endif
> > @@ -1787,6 +1787,7 @@ static const match_table_t tokens = {
> >       {Opt_test_dummy_encryption, "test_dummy_encryption"},
> >       {Opt_inlinecrypt, "inlinecrypt"},
> >       {Opt_nombcache, "nombcache"},
> > +     {Opt_nowipe_filename, "nowipe_filename"},
> >       {Opt_nombcache, "no_mbcache"},  /* for backward compatibility */
> >       {Opt_prefetch_block_bitmaps, "prefetch_block_bitmaps"},
> >       {Opt_removed, "check=none"},    /* mount option from ext2/3 */
> > @@ -2007,6 +2008,8 @@ static const struct mount_opts {
> >       {Opt_max_dir_size_kb, 0, MOPT_GTE0},
> >       {Opt_test_dummy_encryption, 0, MOPT_STRING},
> >       {Opt_nombcache, EXT4_MOUNT_NO_MBCACHE, MOPT_SET},
> > +     {Opt_nowipe_filename, EXT4_MOUNT2_WIPE_FILENAME, MOPT_CLEAR | MOPT_2 |
> > +             MOPT_EXT4_ONLY},
> >       {Opt_prefetch_block_bitmaps, EXT4_MOUNT_PREFETCH_BLOCK_BITMAPS,
> >        MOPT_SET},
> > #ifdef CONFIG_EXT4_DEBUG
> > @@ -2621,6 +2624,10 @@ static int _ext4_show_options(struct seq_file *seq, struct super_block *sb,
> >       } else if (test_opt2(sb, DAX_INODE)) {
> >               SEQ_OPTS_PUTS("dax=inode");
> >       }
> > +
> > +     if (!test_opt2(sb, WIPE_FILENAME))
> > +             SEQ_OPTS_PUTS("nowipe_filename");
> > +
> >       ext4_show_quota_options(seq, sb);
> >       return 0;
> > }
> > @@ -4161,6 +4168,8 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
> >       if (def_mount_opts & EXT4_DEFM_DISCARD)
> >               set_opt(sb, DISCARD);
> >
> > +     set_opt2(sb, WIPE_FILENAME);
> > +
> >       sbi->s_resuid = make_kuid(&init_user_ns, le16_to_cpu(es->s_def_resuid));
> >       sbi->s_resgid = make_kgid(&init_user_ns, le16_to_cpu(es->s_def_resgid));
> >       sbi->s_commit_interval = JBD2_DEFAULT_MAX_COMMIT_AGE * HZ;
> > --
> > 2.31.0.291.g576ba9dcdaf-goog
> >
>
>
> Cheers, Andreas
>
>
>
>
>