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From:   Andreas Dilger <adilger@dilger.ca>
Message-Id: <08A11E25-0208-4B4F-8759-75C1841E7017@dilger.ca>
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Subject: Re: [RFC PATCH] f*xattr: allow O_PATH descriptors
Date:   Wed, 8 Jun 2022 10:53:11 -0600
In-Reply-To: <20220607153139.35588-1-cgzones@googlemail.com>
Cc:     selinux@vger.kernel.org, Miklos Szeredi <mszeredi@redhat.com>,
        linux-api@vger.kernel.org, linux-man@vger.kernel.org,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
To:     =?utf-8?Q?Christian_G=C3=B6ttsche?= <cgzones@googlemail.com>
References: <20220607153139.35588-1-cgzones@googlemail.com>
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On Jun 7, 2022, at 9:31 AM, Christian G=C3=B6ttsche =
<cgzones@googlemail.com> wrote:
>=20
> From: Miklos Szeredi <mszeredi@redhat.com>
>=20
> Support file descriptors obtained via O_PATH for extended attribute
> operations.
>=20
> Extended attributes are for example used by SELinux for the security
> context of file objects. To avoid time-of-check-time-of-use issues =
while
> setting those contexts it is advisable to pin the file in question and
> operate on a file descriptor instead of the path name. This can be
> emulated in userspace via /proc/self/fd/NN [1] but requires a procfs,
> which might not be mounted e.g. inside of chroots, see[2].

Will this allow get/set xattrs directly on symlinks?  That is one =
problem
that we have with some of the xattrs that are inherited on symlinks, but
there is no way to change them.  Allowing setxattr directly on a symlink
would be very useful.

Cheers, Andreas

> [1]: =
https://github.com/SELinuxProject/selinux/commit/7e979b56fd2cee28f647376a7=
233d2ac2d12ca50
> [2]: =
https://github.com/SELinuxProject/selinux/commit/de285252a1801397306032e07=
0793889c9466845
>=20
> Original patch by Miklos Szeredi <mszeredi@redhat.com>
> =
https://patchwork.kernel.org/project/linux-fsdevel/patch/20200505095915.11=
275-6-mszeredi@redhat.com/
>=20
>> While this carries a minute risk of someone relying on the property =
of
>> xattr syscalls rejecting O_PATH descriptors, it saves the trouble of
>> introducing another set of syscalls.
>>=20
>> Only file->f_path and file->f_inode are accessed in these functions.
>>=20
>> Current versions return EBADF, hence easy to detect the presense of
>> this feature and fall back in case it's missing.
>=20
> CC: linux-api@vger.kernel.org
> CC: linux-man@vger.kernel.org
> Signed-off-by: Christian G=C3=B6ttsche <cgzones@googlemail.com>
> ---
> fs/xattr.c | 8 ++++----
> 1 file changed, 4 insertions(+), 4 deletions(-)
>=20
> diff --git a/fs/xattr.c b/fs/xattr.c
> index e8dd03e4561e..16360ac4eb1b 100644
> --- a/fs/xattr.c
> +++ b/fs/xattr.c
> @@ -656,7 +656,7 @@ SYSCALL_DEFINE5(lsetxattr, const char __user *, =
pathname,
> SYSCALL_DEFINE5(fsetxattr, int, fd, const char __user *, name,
> 		const void __user *,value, size_t, size, int, flags)
> {
> -	struct fd f =3D fdget(fd);
> +	struct fd f =3D fdget_raw(fd);
> 	int error =3D -EBADF;
>=20
> 	if (!f.file)
> @@ -768,7 +768,7 @@ SYSCALL_DEFINE4(lgetxattr, const char __user *, =
pathname,
> SYSCALL_DEFINE4(fgetxattr, int, fd, const char __user *, name,
> 		void __user *, value, size_t, size)
> {
> -	struct fd f =3D fdget(fd);
> +	struct fd f =3D fdget_raw(fd);
> 	ssize_t error =3D -EBADF;
>=20
> 	if (!f.file)
> @@ -844,7 +844,7 @@ SYSCALL_DEFINE3(llistxattr, const char __user *, =
pathname, char __user *, list,
>=20
> SYSCALL_DEFINE3(flistxattr, int, fd, char __user *, list, size_t, =
size)
> {
> -	struct fd f =3D fdget(fd);
> +	struct fd f =3D fdget_raw(fd);
> 	ssize_t error =3D -EBADF;
>=20
> 	if (!f.file)
> @@ -910,7 +910,7 @@ SYSCALL_DEFINE2(lremovexattr, const char __user *, =
pathname,
>=20
> SYSCALL_DEFINE2(fremovexattr, int, fd, const char __user *, name)
> {
> -	struct fd f =3D fdget(fd);
> +	struct fd f =3D fdget_raw(fd);
> 	int error =3D -EBADF;
>=20
> 	if (!f.file)
> --
> 2.36.1
>=20


Cheers, Andreas


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