Re: [PATCH v5] erofs: accept source file descriptor via fsconfig

On Wed 29-07-26 09:40:50, Gao Xiang wrote:
> On Tue, Jul 28, 2026 at 06:05:32PM +0200, Giuseppe Scrivano wrote:
> > Allow userspace to pass an already-opened file descriptor as the mount
> > source instead of a path string.  This is useful for tools that already
> > hold an fd to the image, such as composefs reusing an existing erofs
> > backing file.
> > 
> > Signed-off-by: Giuseppe Scrivano <[email protected]>
> > ---
> > v4: https://lore.kernel.org/linux-fsdevel/[email protected]/
> > v3: https://lore.kernel.org/linux-fsdevel/[email protected]/
> > v2: https://lore.kernel.org/linux-fsdevel/[email protected]/
> > v1: https://lore.kernel.org/linux-fsdevel/ak5GfvVfWLJU1EwK@debian/
> > 
> >  Documentation/filesystems/erofs.rst | 15 ++++++
> >  fs/erofs/super.c                    | 73 ++++++++++++++++++++++++-----
> >  2 files changed, 77 insertions(+), 11 deletions(-)
> > 
> > diff --git a/Documentation/filesystems/erofs.rst b/Documentation/filesystems/erofs.rst
> > index 4230884fb359..774e8b236d09 100644
> > --- a/Documentation/filesystems/erofs.rst
> > +++ b/Documentation/filesystems/erofs.rst
> > @@ -139,6 +139,21 @@ inode_share            Enable inode page sharing for this filesystem.  Inodes wi
> >                         page cache.
> >  ===================    =========================================================
> >  
> > +File-backed mounts
> > +==================
> > +
> > +When CONFIG_EROFS_FS_BACKED_BY_FILE is enabled, EROFS file-backed images
> > +can be mounted directly without a loopback block device.  The backing file
> > +can be given either as a path, or as an already-opened file descriptor.
> > +
> > +When a file descriptor is used, the kernel resolves its path and records it
> > +so that /proc/mounts and similar interfaces can still report the mount
> > +source.
> > +
> > +Only regular files are accepted as backing files; to mount an image that
> > +resides on a block device, use the traditional block device mount path
> > +instead.
> 
> Reviewed-by: Gao Xiang <[email protected]>
> 
> Hi Christian, Jan, could you ack this (`fc->source` filling when
> `source_fd` is used) if possible?

Whenever I see a use of d_path() (or file_path() in this case) I'm a bit
watchful because its result is inherently racy and unsafe. But in this case
the result is only used for alloc_vfsmnt() which uses it for mnt_devname so
in the worst case we'll get an unreliable path in /proc/mounts or similar.
I guess that's acceptable. So feel free to add:

Acked-by: Jan Kara <[email protected]>

								Honza
-- 
Jan Kara <[email protected]>
SUSE Labs, CR