[PATCH] erofs: support idmapped mounts

[Gao Xiang]

On Tue, May 17, 2022 at 11:22:03AM +0200, Christian Brauner wrote:
> On Tue, May 17, 2022 at 05:15:02PM +0800, Gao Xiang wrote:
> > Hi Christian,
> > 
> > On Tue, May 17, 2022 at 11:06:22AM +0200, Christian Brauner wrote:
> > > On Tue, May 17, 2022 at 03:32:10PM +0800, Chao Yu wrote:
> > > > This patch enables idmapped mounts for erofs, since all dedicated helpers
> > > > for this functionality existsm, so, in this patch we just pass down the
> > > > user_namespace argument from the VFS methods to the relevant helpers.
> > > > 
> > > > Simple idmap example on erofs image:
> > > > 
> > > > 1. mkdir dir
> > > > 2. touch dir/file
> > > > 3. mkfs.erofs erofs.img dir
> > > > 4. mount -t erofs -o loop erofs.img  /mnt/erofs/
> > > > 
> > > > 5. ls -ln /mnt/erofs/
> > > > total 0
> > > > -rw-rw-r-- 1 1000 1000 0 May 17 15:26 file
> > > > 
> > > > 6. mount-idmapped --map-mount b:0:1001:1 /mnt/erofs/ /mnt/scratch_erofs/
> > > > 
> > > > 7. ls -ln /mnt/scratch_erofs/
> > > > total 0
> > > > -rw-rw-r-- 1 65534 65534 0 May 17 15:26 file
> > > 
> > > Your current example maps id 0 in the filesystem to id 1001 in the
> > > mount. But since no files with id 0 exist in the filesystem you're
> > > illustrating that unmapped ids are correctly reported as overflow{g,u}id.
> > > 
> > > I think what you'd rather want to show is something like this:
> > > 
> > > 5. ls -ln /mnt/erofs/
> > > total 0
> > > -rw-rw-r-- 1 1000 1000 0 May 17 15:26 file
> > > 
> > > 6. mount-idmapped --map-mount b:1000:1001:1 /mnt/erofs/ /mnt/scratch_erofs/
> > > 
> > > 7. ls -ln /mnt/scratch_erofs/
> > > total 0
> > > -rw-rw-r-- 1 1001 1001 0 May 17 15:26 file
> > > 
> > > where id 1000 in the filesystem maps to id 1001 in the mount.

Yeah, I just manually tested, although some steps assume user 1000
and some steps assume the root user. But it works.

I will rephrase such commit messages when applying later...

Thanks,
Gao Xiang