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

[Kevin Brodsky <kevin.brodsky@arm.com>]

On 31/07/2019 18:05, Dave Hansen wrote:
> 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?

This patch series only changes what is allowed or not at the syscall interface. It 
does not change the address space size. On arm64, TBI (Top Byte Ignore) has always 
been enabled for userspace, so it has never been possible to use the upper 8 bits of 
user pointers for addressing.

If other architectures were to support a similar functionality, then I agree that a 
common and more generic interface (if needed) would be helpful, but as it stands this 
is an arm64-specific prctl, and on arm64 the address tag is defined by the 
architecture as bits [63:56].

Kevin