Re: Filesystem setattr/truncate notes and problems

[Christoph Hellwig <hch@lst.de>]

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.

Which means XFS behaviour for truncate while not strictly against
Posix is at least unexpected.  I'll fix it up.