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Subject: Re: [RESEND PATCH v2] hfsplus: prevent negative dentries when
 casefolded
From:   Viacheslav Dubeyko <slava@dubeyko.com>
In-Reply-To: <20210716073635.1613671-1-cccheng@synology.com>
Date:   Fri, 16 Jul 2021 12:30:56 -0700
Cc:     Linux FS Devel <linux-fsdevel@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        gustavoars@kernel.org, gregkh@linuxfoundation.org,
        keescook@chromium.org, mszeredi@redhat.com
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Message-Id: <02B9566C-A78E-42FB-924B-A503E4BC6D2F@dubeyko.com>
References: <20210716073635.1613671-1-cccheng@synology.com>
To:     Chung-Chiang Cheng <cccheng@synology.com>
Precedence: bulk


> On Jul 16, 2021, at 12:36 AM, Chung-Chiang Cheng =
<cccheng@synology.com> wrote:
>=20
> hfsplus uses the case-insensitive filenames by default, but VFS =
negative
> dentries are incompatible with case-insensitive. For example, the
> following instructions will get a cached filename 'aaa' which isn't
> expected. There is no such problem in macOS.
>=20
>  touch aaa
>  rm aaa
>  touch AAA
>=20
> This patch takes the same approach to drop negative dentires as vfat =
does.
> The dentry is revalidated without blocking and storing to the dentry,
> and should be safe in rcu-walk.
>=20
> Signed-off-by: Chung-Chiang Cheng <cccheng@synology.com>
> ---
> fs/hfsplus/hfsplus_fs.h |  1 +
> fs/hfsplus/inode.c      |  1 +
> fs/hfsplus/unicode.c    | 32 ++++++++++++++++++++++++++++++++
> 3 files changed, 34 insertions(+)
>=20
> diff --git a/fs/hfsplus/hfsplus_fs.h b/fs/hfsplus/hfsplus_fs.h
> index 1798949f269b..4ae7f1ca1584 100644
> --- a/fs/hfsplus/hfsplus_fs.h
> +++ b/fs/hfsplus/hfsplus_fs.h
> @@ -520,6 +520,7 @@ int hfsplus_asc2uni(struct super_block *sb, struct =
hfsplus_unistr *ustr,
> int hfsplus_hash_dentry(const struct dentry *dentry, struct qstr =
*str);
> int hfsplus_compare_dentry(const struct dentry *dentry, unsigned int =
len,
> 			   const char *str, const struct qstr *name);
> +int hfsplus_revalidate_dentry(struct dentry *dentry, unsigned int =
flags);
>=20
> /* wrapper.c */
> int hfsplus_submit_bio(struct super_block *sb, sector_t sector, void =
*buf,
> diff --git a/fs/hfsplus/inode.c b/fs/hfsplus/inode.c
> index 6fef67c2a9f0..4188a0760118 100644
> --- a/fs/hfsplus/inode.c
> +++ b/fs/hfsplus/inode.c
> @@ -179,6 +179,7 @@ const struct address_space_operations hfsplus_aops =
=3D {
> const struct dentry_operations hfsplus_dentry_operations =3D {
> 	.d_hash       =3D hfsplus_hash_dentry,
> 	.d_compare    =3D hfsplus_compare_dentry,
> +	.d_revalidate =3D hfsplus_revalidate_dentry,
> };
>=20
> static void hfsplus_get_perms(struct inode *inode,
> diff --git a/fs/hfsplus/unicode.c b/fs/hfsplus/unicode.c
> index 73342c925a4b..e336631334eb 100644
> --- a/fs/hfsplus/unicode.c
> +++ b/fs/hfsplus/unicode.c
> @@ -10,6 +10,7 @@
>  */
>=20
> #include <linux/types.h>
> +#include <linux/namei.h>
> #include <linux/nls.h>
> #include "hfsplus_fs.h"
> #include "hfsplus_raw.h"
> @@ -518,3 +519,34 @@ int hfsplus_compare_dentry(const struct dentry =
*dentry,
> 		return 1;
> 	return 0;
> }
> +
> +int hfsplus_revalidate_dentry(struct dentry *dentry, unsigned int =
flags)
> +{

What=E2=80=99s about this code?

If (flags & LOOKUP_RCU)
   return -ECHILD;

Do we really need to miss it here?

Thanks,
Slava.


> +	/*
> +	 * dentries are always valid when disabling casefold.
> +	 */
> +	if (!test_bit(HFSPLUS_SB_CASEFOLD, =
&HFSPLUS_SB(dentry->d_sb)->flags))
> +		return 1;
> +
> +	/*
> +	 * Positive dentries are valid when enabling casefold.
> +	 *
> +	 * Note, rename() to existing directory entry will have =
->d_inode, and
> +	 * will use existing name which isn't specified name by user.
> +	 *
> +	 * We may be able to drop this positive dentry here. But =
dropping
> +	 * positive dentry isn't good idea. So it's unsupported like
> +	 * rename("filename", "FILENAME") for now.
> +	 */
> +	if (d_really_is_positive(dentry))
> +		return 1;
> +
> +	/*
> +	 * Drop the negative dentry, in order to make sure to use the =
case
> +	 * sensitive name which is specified by user if this is for =
creation.
> +	 */
> +	if (flags & (LOOKUP_CREATE | LOOKUP_RENAME_TARGET))
> +		return 0;
> +
> +	return 1;
> +}
> --=20
> 2.25.1
>=20