Re: [patch 0/4] Cleanup file_count usage

[Paul Mackerras <paulus@samba.org>]

Ravikiran G Thirumalai writes:

> 2. Read to f_count at drivers/net/ppp_generic.c:
>    The PPPIOCDETACH ioctl is failed if the device fd is duped and
>    being polled by another process -- which is determined by a read
>    to f_count.  The comments in the code indicate that the PPPIOCDETACH 
>    ioctl should be junked and userland can use a workaround
>    by closing the fd and reopening /dev/ppp.  I can make a patch to junk
>    PPPIOCDETACH, but is it okay to break binary compatibility? Paul?

I would very much rather not break binary compatibility.  People using
ppp-2.4.2 or ppp-2.4.3 won't be affected by this change, but ppp-2.4.2
was released in January this year and I wouldn't be surprised if
some people are still using ppp-2.4.1 (or even ppp-2.4.0), which would
be affected.

I suggest that as a temporary compromise, we keep PPPIOCDETACH but
make it zero file->private_data without freeing the ppp structure that
it points to.  That will cause a memory leak but will avoid the
possibility of using that memory after it is freed (in the case where
the descriptor has been dup'd and another process is doing a poll on
it).

We should make the kernel whinge loudly when anyone uses PPPIOCDETACH,
too.  If/when 2.7.0 comes along we can remove it.

Paul.