re: a small C program to test xdm's /dev/mem reading on your architecture

2002-08-26 Thread matthew green 
   
   Be warned: on at least some architectures (notably IA-64), this sort of
   read has been known to cause untrapped machine checks (a.k.a., lockups
   or spontaneous reboots).  Arguably the kernel should trap this sort of
   nonsense, so you may be in the mood to file a bug against kernel after
   running this program.

wow, this is such a bad idea.  the kernel *can't* trap that sort of
thing in a lot of cases.  simply the hardware goes catatonic.  this
is true of ultrasparc machines as well. you may also access some random
device memory causing it do do something unexpected.  read can be
very destructive of device state...


why don't you use /dev/urandom if it exists, as it does on pretty
much all modern UNIX platforms?


*shudder*


.mrg.

re: a small C program to test xdm's /dev/mem reading on your architecture

2002-08-26 Thread matthew green 
   
   On Mon, Aug 26, 2002 at 04:28:38PM +1000, matthew green wrote:
wow, this is such a bad idea.
   
   It originated upstream.

mmm, xdm.
   
   In fact, judging by CVS logs it has been in xdm's source for many, many
   years.

bad ideas often hang around for a long time.  the only surprising
thing to me is how long this one has taken to surface...

   
why don't you use /dev/urandom if it exists, as it does on pretty
much all modern UNIX platforms?

*shudder*
   
   I see you haven't read the thread.


actually, i hadn't, but there wasn't very much there besides the
fact that people found it was xdm reading /dev/mem and a small
patch for debian to enable /dev/random (i'd suggest /dev/urandom).

my point is that on modern systems we simply should not read
from /dev/mem for these purposes _ever_.  so far it has shown to
be unsafe on at least:

- ia64
- arm
- mips
- ultrasparc

i'm sure there are more...

i don't see the purpose in running the program you posted - we
shouldn't care whether it works, just don't do it.  i'm going to
patch NetBSD xsrc shortly to fix this if it isn't already...


.mrg.