On Mon, Aug 07, 2023 at 11:20:58AM +0100, Szabolcs Nagy wrote:

The 07/31/2023 14:43, Mark Brown wrote:

quoted

+SYSCALL_DEFINE3(map_shadow_stack, unsigned long, addr, unsigned long, size, unsigned int, flags)
+{
+	unsigned long alloc_size;
+	unsigned long __user *cap_ptr;
+	unsigned long cap_val;
+	int ret;
+
+	if (!system_supports_gcs())
+		return -EOPNOTSUPP;
+
+	if (flags)
+		return -EINVAL;
+
+	if (addr % 16)
+		return -EINVAL;

mmap addr must be page aligned (and there is no align req on size).

i'd expect similar api here.

That's not what the manual page or a quick check of the code suggest
that mmap() does, they say that the kernel just takes it as a hint and
chooses a nearby page boundary, though I didn't test.  I'm not sure why
I have that alignment check at all TBH, and to the extent it's needed I
could just be 8 - this level of code doesn't really care.

quoted

+	if (size == 16 || size % 16)
+		return -EINVAL;

why %16 and not %8 ?

I don't think that's needed any more - there was some stuff in an
earlier version of the code but no longer.