Re: [PATCH RFC 04/41] fs: add SB_I_PSEUDO

[Alexander Mikhalitsyn <alex@amutable.com>]

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.
> 
> 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.
> 
> Add SB_I_PSEUDO so a filesystem can state that its regular files are
> kernel interfaces and must not be used as storage.
> 
> 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.
> 
> Nothing checks SB_I_PSEUDO yet so no functional changes.
> 
> Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

Reviewed-by: Alexander Mikhalitsyn <alex@amutable.com>

> ---
>  fs/libfs.c                     |  2 +-
>  include/linux/fs/super_types.h |  1 +
>  include/linux/pseudo_fs.h      | 11 +++++++++++
>  3 files changed, 13 insertions(+), 1 deletion(-)
> 
> 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,
>  		fc->fs_private = ctx;
>  		fc->ops = &pseudo_fs_context_ops;
>  		fc->sb_flags |= SB_NOUSER;
> -		fc->s_iflags |= SB_I_NOEXEC | SB_I_NODEV;
> +		fc_init_pseudo(fc);
>  		fc->global = true;
>  	}
>  	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 {
>  #define SB_I_NOIDMAP	0x00002000	/* No idmapped mounts on
> this superblock */
>  #define SB_I_ALLOW_HSM	0x00004000	/* Allow HSM events on this
> superblock */
>  #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 */
>  
>  #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 @@
>  #define __LINUX_PSEUDO_FS__
>  
>  #include <linux/fs_context.h>
> +#include <linux/fs/super_types.h>
>  
>  struct pseudo_fs_context {
>  	const struct super_operations *ops;
> @@ -15,4 +16,14 @@ struct pseudo_fs_context {
>  struct pseudo_fs_context *init_pseudo(struct fs_context *fc,
>  				      unsigned long magic);
>  
> +/*
> + * 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 |= SB_I_PSEUDO | SB_I_NOEXEC | SB_I_NODEV;
> +}
> +
>  #endif