Re: [PATCH v9 04/17] iov_iter: Turn iov_iter_fault_in_readable into fault_in_iov_iter_readable

On Tue, Nov 02, 2021 at 01:29:32PM +0100, Andreas Gruenbacher wrote:
> Turn iov_iter_fault_in_readable into a function that returns the number
> of bytes not faulted in, similar to copy_to_user, instead of returning a
> non-zero value when any of the requested pages couldn't be faulted in.
> This supports the existing users that require all pages to be faulted in
> as well as new users that are happy if any pages can be faulted in.
> 
> Rename iov_iter_fault_in_readable to fault_in_iov_iter_readable to make
> sure this change doesn't silently break things.
> 
> Signed-off-by: Andreas Gruenbacher <[email protected]>
[...]
> diff --git a/mm/filemap.c b/mm/filemap.c
> index ff34f4087f87..4dd5edcd39fd 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -3757,7 +3757,7 @@ ssize_t generic_perform_write(struct file *file,
>  		 * same page as we're writing to, without it being marked
>  		 * up-to-date.
>  		 */
> -		if (unlikely(iov_iter_fault_in_readable(i, bytes))) {
> +		if (unlikely(fault_in_iov_iter_readable(i, bytes))) {
>  			status = -EFAULT;
>  			break;
>  		}

Now that fault_in_iov_iter_readable() returns the number of bytes, we
could change the above test to:

		if (unlikely(fault_in_iov_iter_readable(i, bytes) = bytes)) {

Assuming we have a pointer 'a', accessible, and 'a + PAGE_SIZE' unmapped:

	write(fd, a + PAGE_SIZE - 1, 2);

can still copy one byte but it returns -EFAULT instead since the second
page is not accessible.

While writing some test-cases for MTE (sub-page faults, 16-byte
granularity), we noticed that reading 2 bytes from 'a + 15' with
'a + 16' tagged for faulting:

	write(fd, a + 15, 2);

succeeds as long as 'a + 16' is not at a page boundary. Checking against
'bytes' above makes this consistent.

The downside is that it's an ABI change though not sure anyone is
relying on it.

-- 
Catalin