[ubuntu-hardened] Re: Collecting NX information

John Richard Moser nigelenki at comcast.net
Tue Mar 29 14:41:15 CST 2005

Hash: SHA1

Arjan van de Ven wrote:
> On Tue, 2005-03-29 at 14:07 -0500, John Richard Moser wrote:


>>/me shrugs.  It's a security blanket for him mostly; he fears automagic
>>security maintainence.
> who is "him" ?

me in third person?  :)

>>>>Remember also I'm very much against "let the compiler guess if you need
>>>>an executable stack"
>>>it's not guessing. the *compiler* emits the stack trampoline. So the
>>>*compiler* knows that it needs that stack.
>>With a trampoline, things like Grub and a few libs need PT_GNU_STACK.
> sure they do. There's about a handful in an entire distro.

Right, so it's a toss-up:  Do you want to fix these few things, or do
you want to let trampolines exist?  Are trampolines that useful?

>>Of course you can't just suddenly say "OH!  Well PT_GNU_STACK should do
>>this instead!" because you'll break everything.
> I'm not a fan of any kind of emutrampoline. At all. But I am open to
> others making a different tradeoff; for me the option to have a
> trampoline at all is just a bypass of the non-exec stack... legit bypass
> one hopes but a bypass regardless. Some time ago we did an eval of how
> much stuff would need the emutramp (well or something equivalent) and
> the list was so small that I decided that the added risk and complexity
> were not worth it and that I rather had those 5 or so apps run with exec
> stack.


>>>again what does tristate mean.. "I don't know" ? But gcc does know, with
>>>very very very few exceptions. Eg old mono is the exception because it
>>>didn't do a proper mprotect. Saying "I don't know" doesn't solve you
>>>anything, since in the end there needs to be a policy enforced anyway,
>>>it's just postponing the inevitable to a point with less knowledge.
>>Remember I'm also thinking of restricted mprotect() situations as well,
>>because I'm trying to get it to the point where one set of markings has
>>a predictable effect on any kernel, be it vanilla, pax, or ES.
> well that is an entirely independent thing again. Really.
> I think mixing all these into one big flag is a mistake. 
> (And thats a lesson I learned the hard way, but Linus was right; don't
> mix independent things into one flag artificially. Extra flags are
> cheap. Don't complicate the world for a dozen bytes.)

two or four dozen bytes, eight dozen bytes in 10 years when 128 bit
systems come along, and 16i dozen planck qubytes when we get quantum
computers :)

Actually when I proposed adding PT_PAX_FLAGS to Ubuntu, the very first
argument I got was "Oh, yeah right, add just a few bytes here for this.
 Then later it'll be a few more bytes for something else.  Then a few
more for another thing.  It all adds up, especially when you have
thousands of binaries."

And if bitfield logic is "complicated," you should stop pretending to be
a programmer.

#define BIG   (1)
#define LONG  (1 << 1)
#define FAT   (1 << 2)
#define TALL  (1 << 3)
#define GREEN (1 << 4)

struct foo {
  char *myname;
  unsigned long flags;

struct foo *newfoo() {
  struct foo *out = malloc(sizeof(struct foo));
  out->myname = malloc(4);
  strcpy(out->myname, "bob");
  out->flags = BIG | TALL | GREEN;
  return out;

Easy enough?

struct foo {
  char myname[10];
  int big;
  int long;
  int fat;
  int tall;
  int green;

struct foo *newfoo() {
  struct foo *out = malloc(sizeof(struct foo));
  strcpy(out->myname, "bob");
  out->big = 1;
  out->tall = 1;
  out->green = 1;
  return out;

I don't find the above to be quite as elegant.  In fact, it looks ugly
and wasteful; even checking ...

if (myfoo->flags & BIG)


if (myfoo->big)


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