Sticky/sgid/suid bits safe on regular files?

[Nikita Danilov]

Dan Strick writes:
 > Daniel O'Connor wrote:
 > >>
 > > I think he wanted to use sticky/sgid/suid because they get removed
 > > when the file is changed.
 > >>
 > 
 > and Dag-Erling S?rgrav responded:
 > >>
 > > no they don't.
 > >>
 > 
 > Actually, they do.  (just the sgid/suid bits)
 > 
 > I believe this was an ancient BSD security enhancement in case
 > someone accidentally left a world writeable suid file around.

That is Single Unix Specification requirement:

    Upon successful completion, where nbyte is greater than 0, write()
    shall mark for update the st_ctime and st_mtime fields of the
    file, and if the file is a regular file, the S_ISUID and S_ISGID
    bits of the file mode may be cleared.

However in chmod description it says:

    Implementations that use the S_ISUID bit to indicate some other
    function (for example, mandatory record locking) on non-executable
    files need not clear this bit on writing. They should clear the
    bit for executable files and any other cases where the bit grants
    special powers to processes that change the file contents. Similar
    comments apply to the S_ISGID bit.

 > 
 > Dan Strick

Nikita.