Re: [PATCH v2 1/4] lib: introduce copy_struct_from_user() helper

On 2019-09-26, Christian Brauner [off-list ref] wrote:

On Thu, Sep 26, 2019 at 01:03:29AM +0200, Aleksa Sarai wrote:

quoted

+int is_zeroed_user(const void __user *from, size_t size)
+{
+	unsigned long val;
+	uintptr_t align = (uintptr_t) from % sizeof(unsigned long);
+
+	if (unlikely(!size))
+		return true;

You're returning "true" and another implicit boolean with (val == 0)
down below but -EFAULT in other places. But that function is int
is_zeroed_user() Would probably be good if you either switch to bool
is_zeroed_user() as the name suggests or rename the function and have
it return an int everywhere.

I just checked, and in C11 (and presumably in older specs) it is
guaranteed that "true" and "false" from <stdbool.h> have the values 1
and 0 (respectively) [§7.18]. So this is perfectly well-defined.

Personally, I think it's more readable to have:

  if (unlikely(size == 0))
    return true;
  /* ... */
  return (val == 0);

compared to:

  if (unlikely(size == 0))
    return 1;
  /* ... */
  return val ? 0 : 1;

But I will change the function name (to check_zeroed_user) to make it
clearer that it isn't returning a boolean and that you need to check for
negative returns.

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>