Subject: Re: [PATCH net] tls: Skip tls_append_frag on zero copy size

From: Jakub Kicinski <[email-masked]>
Date: Thu, 14 Apr 2022 12:28:08 +0200

On Wed, 13 Apr 2022 16:49:56 +0300 Maxim Mikityanskiy wrote:
> Calling tls_append_frag when max_open_record_len == record->len might
> add an empty fragment to the TLS record if the call happens to be on the
> page boundary. Normally tls_append_frag coalesces the zero-sized
> fragment to the previous one, but not if it's on page boundary.
> 
> If a resync happens then, the mlx5 driver posts dump WQEs in
> tx_post_resync_dump, and the empty fragment may become a data segment
> with byte_count == 0, which will confuse the NIC and lead to a CQE
> error.
> 
> This commit fixes the described issue by skipping tls_append_frag on
> zero size to avoid adding empty fragments. The fix is not in the driver,
> because an empty fragment is hardly the desired behavior.
> 
> Fixes: e8f69799810c ("net/tls: Add generic NIC offload infrastructure")
> Signed-off-by: Maxim Mikityanskiy <[email-masked]>
> Reviewed-by: Tariq Toukan <[email-masked]>
> ---
>  net/tls/tls_device.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/net/tls/tls_device.c b/net/tls/tls_device.c
> index 12f7b56771d9..af875ad4a822 100644
> --- a/net/tls/tls_device.c
> +++ b/net/tls/tls_device.c
> @@ -483,11 +483,13 @@ static int tls_push_data(struct sock *sk,
>  		copy = min_t(size_t, size, (pfrag->size - pfrag->offset));
>  		copy = min_t(size_t, copy, (max_open_record_len - record->len));
>  
> -		rc = tls_device_copy_data(page_address(pfrag->page) +
> -					  pfrag->offset, copy, msg_iter);
> -		if (rc)
> -			goto handle_error;
> -		tls_append_frag(record, pfrag, copy);
> +		if (copy) {
> +			rc = tls_device_copy_data(page_address(pfrag->page) +
> +						  pfrag->offset, copy, msg_iter);
> +			if (rc)
> +				goto handle_error;
> +			tls_append_frag(record, pfrag, copy);
> +		}

I appreciate you're likely trying to keep the fix minimal but Greg
always says "fix it right, worry about backports later".

I think we should skip more, we can reorder the mins and if 
min(size, rec space) == 0 then we can skip the allocation as well.
Maybe some application wants to do zero-length sends to flush the
MSG_MORE and would benefit that way?

>  		size -= copy;
>  		if (!size) {