exec: simplify initial stack size expansion

I had a hard time trying to understand completely why it is using vm_end in
one side of the expression and vm_start in the other one, and using
something in the "if" clause that is not an exact copy of what is used
below. The whole point is that the stack_size variable that was used in the
"if" clause is the difference between vm_start and vm_end, which is not far
away but makes this thing harder to read than it must be.

Signed-off-by: Rolf Eike Beer <eb@emlix.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/2017429.gqNitNVd0C@mobilepool36.emlix.com

diff --git a/fs/exec.c b/fs/exec.c
index 1644bf1..9585bc1 100644 (file)
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -843,16 +843,13 @@ int setup_arg_pages(struct linux_binprm *bprm,
         * will align it up.
         */
        rlim_stack = bprm->rlim_stack.rlim_cur & PAGE_MASK;
+
+       stack_expand = min(rlim_stack, stack_size + stack_expand);
+
 #ifdef CONFIG_STACK_GROWSUP
-       if (stack_size + stack_expand > rlim_stack)
-               stack_base = vma->vm_start + rlim_stack;
-       else
-               stack_base = vma->vm_end + stack_expand;
+       stack_base = vma->vm_start + stack_expand;
 #else
-       if (stack_size + stack_expand > rlim_stack)
-               stack_base = vma->vm_end - rlim_stack;
-       else
-               stack_base = vma->vm_start - stack_expand;
+       stack_base = vma->vm_end - stack_expand;
 #endif
        current->mm->start_stack = bprm->p;
        ret = expand_stack(vma, stack_base);