Re: [Patch v3] vfs: allow file truncations when both suid and write permissions set

[Eric Paris <eparis@redhat.com>]

On Fri, 2009-08-07 at 06:05 -0400, Amerigo Wang wrote:
> V2 -> V3:
> Call notify_change() before clearing suid/sgid.
> Thanks to OGAWA Hirofumi.
> 
> V1 -> V2:
> Introduce dentry_remove_suid(), and use it in do_truncate().
> Thanks to Eric Paris.
> 
> 
> When suid is set and the non-owner user has write permission,
> any writing into this file should be allowed and suid should be
> removed after that.
> 
> However, current kernel only allows writing without truncations,
> when we do truncations on that file, we get EPERM. This is a bug.
> 
> Steps to reproduce this bug:
> 
> % ls -l rootdir/file1
> -rwsrwsrwx 1 root root 3 Jun 25 15:42 rootdir/file1
> % echo h > rootdir/file1
> zsh: operation not permitted: rootdir/file1
> % ls -l rootdir/file1
> -rwsrwsrwx 1 root root 3 Jun 25 15:42 rootdir/file1
> % echo h >> rootdir/file1
> % ls -l rootdir/file1
> -rwxrwxrwx 1 root root 5 Jun 25 16:34 rootdir/file1
> 
> This patch fixes it.
> 
> Signed-off-by: WANG Cong <amwang@redhat.com>
> Cc: Eric Sandeen <esandeen@redhat.com>
> Cc: Eric Paris <eparis@redhat.com>
> Cc: Eugene Teo <eteo@redhat.com>
> Cc: Al Viro <viro@zeniv.linux.org.uk>
> Cc: hirofumi@mail.parknet.co.jp

I was thinking about this and kept telling myself I was going to test v2
before I ack/nak.  Clearly we shouldn't for the dropping of SUID if the
process didn't have permission to change the ATTR_SIZE.

Acked-by: Eric Paris <eparis@redhat.com>