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[80.187.86.54]) by smtp.gmail.com with ESMTPSA id 4fb4d7f45d1cf-6b7c23c672asm734155a12.15.2026.11.13.02.02.16 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 13 Nov 2026 02:02:17 -0800 (PST) Message-ID: Subject: Re: [PATCH RFC 04/41] fs: add SB_I_PSEUDO From: Alexander Mikhalitsyn To: Christian Brauner , linux-fsdevel@vger.kernel.org Cc: Jann Horn , Chris Mason , Josef Bacik , Christoph Hellwig , linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, linux-nvme@lists.infradead.org, Alexander Viro , Jan Kara , linux-scsi@vger.kernel.org, target-devel@vger.kernel.org, driver-core@lists.linux.dev, linux-mm@kvack.org, bpf@vger.kernel.org, linux-security-module@vger.kernel.org, selinux@vger.kernel.org, apparmor@lists.ubuntu.com, linux-nfs@vger.kernel.org, netdev@vger.kernel.org, fuse-devel@lists.linux.dev, linux-efi@vger.kernel.org, xen-devel@lists.xenproject.org, ocfs2-devel@lists.linux.dev, linux-s390@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-usb@vger.kernel.org, Amir Goldstein , linux-erofs@lists.ozlabs.org, linux-kselftest@vger.kernel.org Date: Fri, 13 Nov 2026 11:02:15 +0100 In-Reply-To: <20261110-work-loop-pseudo-v1-4-4c998976c6c7@kernel.org> References: <20261110-work-loop-pseudo-v1-0-4c998976c6c7@kernel.org> <20261110-work-loop-pseudo-v1-4-4c998976c6c7@kernel.org> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.56.2-0+deb13u1 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 On Tue, 2026-11-10 at 10:14 +0100, Christian Brauner wrote: > Currently, there's no way to tell whether the files of a filesystem > store data or whether they are kernel interfaces. Files on pseudo > filesystems such as procfs or sysfs generate their contents when > they're > read and writes to them are commands to the kernel. >=20 > But a few places in the kernel take an arbitrary file and use it as > storage: loop, zloop, the nvme target's file backend, the scsi > target's > fileio backend, fuse passthrough, the usb mass storage gadget and > erofs > file-backed mounts. At most they check the file type and whether the > file implements ->read_iter() and ->write_iter(). That's not enough. > kernfs provides both for every sysfs attribute for example. >=20 > Add SB_I_PSEUDO so a filesystem can state that its regular files are > kernel interfaces and must not be used as storage. >=20 > Pseudo filesystems also never hold executables or device nodes. But > currently each of them decides on its own whether to raise > SB_I_NOEXEC > and SB_I_NODEV. Some do, most don't. Add fc_init_pseudo() as the > common > initialization point for all pseudo filesystems that raises all three > flags and switch init_pseudo() over to it. The following patches > convert the mountable pseudo filesystems. >=20 > Nothing checks SB_I_PSEUDO yet so no functional changes. >=20 > Signed-off-by: Christian Brauner (Amutable) Reviewed-by: Alexander Mikhalitsyn > --- > =C2=A0fs/libfs.c=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 |=C2=A0 2 += - > =C2=A0include/linux/fs/super_types.h |=C2=A0 1 + > =C2=A0include/linux/pseudo_fs.h=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 | 11 ++++++= +++++ > =C2=A03 files changed, 13 insertions(+), 1 deletion(-) >=20 > diff --git a/fs/libfs.c b/fs/libfs.c > index 96a6e9b1535f..8c8789489fdf 100644 > --- a/fs/libfs.c > +++ b/fs/libfs.c > @@ -735,7 +735,7 @@ struct pseudo_fs_context *init_pseudo(struct > fs_context *fc, > =C2=A0 fc->fs_private =3D ctx; > =C2=A0 fc->ops =3D &pseudo_fs_context_ops; > =C2=A0 fc->sb_flags |=3D SB_NOUSER; > - fc->s_iflags |=3D SB_I_NOEXEC | SB_I_NODEV; > + fc_init_pseudo(fc); > =C2=A0 fc->global =3D true; > =C2=A0 } > =C2=A0 return ctx; > diff --git a/include/linux/fs/super_types.h > b/include/linux/fs/super_types.h > index 72f4288e16c5..472200bdf081 100644 > --- a/include/linux/fs/super_types.h > +++ b/include/linux/fs/super_types.h > @@ -359,5 +359,6 @@ struct super_block { > =C2=A0#define SB_I_NOIDMAP 0x00002000 /* No idmapped mounts on > this superblock */ > =C2=A0#define SB_I_ALLOW_HSM 0x00004000 /* Allow HSM events on this > superblock */ > =C2=A0#define SB_I_NO_DATA_INTEGRITY 0x00008000 /* fs cannot guarantee > data persistence on sync */ > +#define SB_I_PSEUDO 0x00010000 /* regular files are kernel > interfaces, not storage */ > =C2=A0 > =C2=A0#endif /* _LINUX_FS_SUPER_TYPES_H */ > diff --git a/include/linux/pseudo_fs.h b/include/linux/pseudo_fs.h > index a651e60d9410..0ca408aa60af 100644 > --- a/include/linux/pseudo_fs.h > +++ b/include/linux/pseudo_fs.h > @@ -2,6 +2,7 @@ > =C2=A0#define __LINUX_PSEUDO_FS__ > =C2=A0 > =C2=A0#include > +#include > =C2=A0 > =C2=A0struct pseudo_fs_context { > =C2=A0 const struct super_operations *ops; > @@ -15,4 +16,14 @@ struct pseudo_fs_context { > =C2=A0struct pseudo_fs_context *init_pseudo(struct fs_context *fc, > =C2=A0 =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 unsigned long magic); > =C2=A0 > +/* > + * Common initialization for all pseudo filesystems, mountable or > not. Their > + * regular files are kernel interfaces. They don't store data and > pseudo > + * filesystems never hold executables or device nodes. > + */ > +static inline void fc_init_pseudo(struct fs_context *fc) > +{ > + fc->s_iflags |=3D SB_I_PSEUDO | SB_I_NOEXEC | SB_I_NODEV; > +} > + > =C2=A0#endif