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Date:   Thu, 28 Apr 2022 03:46:59 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     "xuyang2018.jy@fujitsu.com" <xuyang2018.jy@fujitsu.com>
Cc:     "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
        "ceph-devel@vger.kernel.org" <ceph-devel@vger.kernel.org>,
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        "jlayton@kernel.org" <jlayton@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Jann Horn <jannh@google.com>
Subject: Re: [PATCH v8 1/4] fs: add mode_strip_sgid() helper
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On Thu, Apr 28, 2022 at 03:12:30AM +0000, Al Viro wrote:

> > Note, BTW, that while XFS has inode_fsuid_set() on the non-inode_init_owner()
> > path, it doesn't have inode_fsgid_set() there.  Same goes for ext4, while
> > ext2 doesn't bother with either in such case...
> > 
> > Let's try to separate the issues here.  Jann, could you explain what makes
> > empty sgid files dangerous?
> 
> Found the original thread in old mailbox, and the method of avoiding the
> SGID removal on modification is usable.  Which answers the question above...

OK, what do we want for grpid mounts?  Aside of "don't forget inode_fsuid_set(),
please", that is.  We don't want inode_fsgid_set() there (whatever went for
the parent directory should be the right value for the child).  Same "strip
SGID from non-directory child, unless we are in the resulting group"?