Re: untagged_addr_remote() in do_madvise()

[Dave Hansen]

On 1/14/25 12:41, Lorenzo Stoakes wrote:
...
> However, MADV_HWPOISON, MADV_SOFT_OFFLINE seems fundamentally broken for tagged
> addresses:
> 
> #ifdef CONFIG_MEMORY_FAILURE
> 	if (behavior == MADV_HWPOISON || behavior == MADV_SOFT_OFFLINE)
> 		return madvise_inject_error(behavior, start, start + len_in);
> #endif
> 
> ^ this is invoked before untagged_addr_remote() is called (as no mmap lock is
> acquired) and so no attempt at untagging happens at all...!

Except this call path:

	madvise_inject_error() ->
	get_user_pages_fast() ->
	gup_fast_fallback()

does its own untagging:

        start = untagged_addr(start) & PAGE_MASK;

It might also have some funky behavior if start+len_in overflows. But,
just as in the other case, it's invalid to begin with so I think
userspace kinda gets to keep the pieces.

But I do 100% agree that this is non-obvious. In a perfect world, tagged
addresses would get untagged at the user/kernel boundary in _one_ choke
point. But the world is hard and that would make things too easy and
then we wouldn't get paid the big bucks. ;)

To clarify things, I don't think it'd be the worst thing to just move
the madvise_inject_error() down and have that case acquire
mmap_read_lock(). Sure, it's not required, but it's basically debugging
code and I can't imagine it's avoiding the lock for performance reasons.