On Sat, 2020-07-11 at 11:31 +0200, Dmitry Vyukov wrote:
> rings_size() sets sq_offset to the total size of the rings
> (the returned value which is used for memory allocation).
> This is wrong: sq array should be located within the rings,
> not after them. Set sq_offset to where it should be.
> 
> Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
> Cc: io-uring@vger.kernel.org
> Cc: Hristo Venev <hristo@venev.name>
> Fixes: 75b28affdd6a ("io_uring: allocate the two rings together")

Oops.

Acked-by: Hristo Venev <hristo@venev.name>

> 
> ---
> This looks so wrong and yet io_uring works.
> So I am either missing something very obvious here,
> or io_uring worked only due to lucky side-effects
> of rounding size to power-of-2 number of pages
> (which gave it enough slack at the end),
> maybe reading/writing some unrelated memory
> with some sizes.
> If I am wrong, please poke my nose into what I am not seeing.
> Otherwise, we probably need to CC stable as well.
> ---
>  fs/io_uring.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index ca8abde48b6c7..c4c3731ed41e9 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -7063,6 +7063,9 @@ static unsigned long rings_size(unsigned
> sq_entries, unsigned cq_entries,
>  		return SIZE_MAX;
>  #endif
>  
> +	if (sq_offset)
> +		*sq_offset = off;
> +
>  	sq_array_size = array_size(sizeof(u32), sq_entries);
>  	if (sq_array_size == SIZE_MAX)
>  		return SIZE_MAX;
> @@ -7070,9 +7073,6 @@ static unsigned long rings_size(unsigned
> sq_entries, unsigned cq_entries,
>  	if (check_add_overflow(off, sq_array_size, &off))
>  		return SIZE_MAX;
>  
> -	if (sq_offset)
> -		*sq_offset = off;
> -
>  	return off;
>  }
>