Re: [PATCH v19 02/15] arm64: Introduce prctl() options to control the tagged user addresses ABI

On 7/23/19 10:58 AM, Andrey Konovalov wrote:

+long set_tagged_addr_ctrl(unsigned long arg) +{

• if (!tagged_addr_prctl_allowed)

• return -EINVAL;
  

• if (is_compat_task())

• return -EINVAL;
  

• if (arg & ~PR_TAGGED_ADDR_ENABLE)

• return -EINVAL;
  

• update_thread_flag(TIF_TAGGED_ADDR, arg & PR_TAGGED_ADDR_ENABLE);
• return 0;

+}

Instead of a plain enable/disable, a more flexible ABI would be to have the tag mask be passed in. That way, an implementation that has a flexible tag size can select it. It also ensures that userspace actually knows what the tag size is and isn't surprised if a hardware implementation changes the tag size or position.

Also, this whole set deals with tagging/untagging, but there's an effective loss of address space when you do this. Is that dealt with anywhere? How do we ensure that allocations don't get placed at a tagged address before this gets turned on? Where's that checking?