Re: Filesystem setattr/truncate notes and problems

[Nick Piggin <npiggin@suse.de>]

On Thu, Jun 03, 2010 at 09:28:12AM +0200, Christoph Hellwig wrote:
> On Thu, Jun 03, 2010 at 06:08:55AM +1000, Nick Piggin wrote:
> > xfs in the 0 length file shortcut, isn't this missing suid kill case?
> > (ftruncate mandatory mtime update seems like it wasn't working right
> > there either before this patch).
> 
> The wording from Posix for truncate is:
> 
> "Upon successful completion, if the file size is changed, this function
>  shall mark for update the st_ctime and st_mtime fields of the file, and
>  the S_ISUID and S_ISGID bits of the file mode may be cleared."
> 
> and for truncate:
> 
> "Upon successful completion, if fildes refers to a regular file, the
>  ftruncate() function shall mark for update the st_ctime and st_mtime
>  fields of the file and the S_ISUID and S_ISGID bits of the file mode
>  may be cleared."
> 
> Which translates to me that the suid/sgid clearing behaviour is
> exactly the same as the c/mtime update and it can be skipped for
> truncate only if there is no size change.  Except that the wording
> is rather nasty as the suid/sgid bits are only accepted into Posix
> as an ugly headed stepchild and may not actually be present on
> some systems.

Well at least we should do suid bit clearing consistently with ctime/mtime
updates if we do them at all, that part of the wording seems clear.