[PATCH] erofs: support idmapped mounts

[Christian Brauner]

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.
> > 
> > > 
> > > Signed-off-by: Chao Yu <chao.yu at oppo.com>
> > > ---
> > 
> > Overall this is currently the smallest patch to support idmapped mounts.
> > 
> > Is erofs integrated with xfstests in any way?
> > For read-only filesystems we probably only need to verify that {g,u}id
> > are correctly reported. All the writable aspects are irrelevant.
> 
> Currently most generic xfstests test cases are unsuitable for erofs.
> 
> Instead we have regression testcases for EROFS specific since it needs
> to generate images with care,
>  https://git.kernel.org/pub/scm/linux/kernel/git/xiang/erofs-utils.git/log/?h=experimental-tests
> 
> Also we have an erofsstress to do long time random stress workloads,
> https://github.com/erofs/erofsstress
> 
> But yeah, it's some awkward that fstests idmapped mount testcases may
> be unsuitable for EROFS for now. I will add some new testcases to build
> images and test for this behavior.
> 
> > 
> > Looks good,
> > Reviewed-by: Christian Brauner (Microsoft) <brauner at kernel.org>
> 
> Thanks for your review!

Thanks for supporting this in erofs!
Christian