Allow ARG_MAX execve string space even with a small stack limit

The new code that removed the limitation on the execve string size
(which was historically 32 pages) replaced it with a much softer limit
based on RLIMIT_STACK which is usually much larger than the traditional
limit.  See commit b6a2fea39318e43fee84fa7b0b90d68bed92d2ba ("mm:
variable length argument support") for details.

However, if you have a small stack limit (perhaps because you need lots
of stacks in a threaded environment), the new heuristic of allowing up
to 1/4th of RLIMIT_STACK to be used for argument and environment strings
could actually be smaller than the old limit.

So just say that it's ok to have up to ARG_MAX strings regardless of the
value of RLIMIT_STACK, and check the rlimit only when going over that
traditional limit.

(Of course, if you actually have a *really* small stack limit, the whole
stack itself will be limited before you hit ARG_MAX, but that has always
been true and is clearly the right behaviour anyway).

Acked-by: Carlos O'Donell <carlos@codesourcery.com>
Cc: Michael Kerrisk <michael.kerrisk@googlemail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Ollie Wild <aaw@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/exec.c

@@ -173,8 +173,15 @@ static struct page *get_arg_page(struct linux_binprm *bprm, unsigned long pos,
 		return NULL;
 
 	if (write) {
-		struct rlimit *rlim = current->signal->rlim;
 		unsigned long size = bprm->vma->vm_end - bprm->vma->vm_start;
+		struct rlimit *rlim;
+
+		/*
+		 * We've historically supported up to 32 pages (ARG_MAX)
+		 * of argument strings even with small stacks
+		 */
+		if (size <= ARG_MAX)
+			return page;
 
 		/*
 		 * Limit to 1/4-th the stack size for the argv+env strings.
@@ -183,6 +190,7 @@ static struct page *get_arg_page(struct linux_binprm *bprm, unsigned long pos,
 		 *  - the program will have a reasonable amount of stack left
 		 *    to work from.
 		 */
+		rlim = current->signal->rlim;
 		if (size > rlim[RLIMIT_STACK].rlim_cur / 4) {
 			put_page(page);
 			return NULL;