netdev - Re: [PATCH] ionic: fix 16bit math issue when PAGE

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Date: Tue, 12 Sep 2023 15:31:31 -0700
From: David Christensen <drc@...ux.vnet.ibm.com>
To: "Nelson, Shannon" <shannon.nelson@....com>, brett.creeley@....com,
        drivers@...sando.io
Cc: netdev@...r.kernel.org
Subject: Re: [PATCH] ionic: fix 16bit math issue when PAGE_SIZE >= 64KB



On 9/11/23 5:24 PM, Nelson, Shannon wrote:

>> @@ -452,7 +452,7 @@ void ionic_rx_fill(struct ionic_queue *q)
>>
>>                  /* fill main descriptor - buf[0] */
>>                  desc->addr = cpu_to_le64(buf_info->dma_addr + 
>> buf_info->page_offset);
>> -               frag_len = min_t(u16, len, IONIC_PAGE_SIZE - 
>> buf_info->page_offset);
>> +               frag_len = min_t(u32, len, IONIC_PAGE_SIZE - 
>> buf_info->page_offset);
>>                  desc->len = cpu_to_le16(frag_len);
> 
> Hmm... using cpu_to_le16() on a 32-bit value looks suspect - it might 
> get forced to 16-bit, but looks funky, and might not be as successful in 
> a BigEndian environment.
> 
> Since the descriptor and sg_elem length fields are limited to 16-bit, 
> there might need to have something that assures that the resulting 
> lengths are never bigger than 64k - 1.
> 

What do you think about this:

  frag_len = min_t(u16, len, min_t(u32, 0xFFFF, IONIC_PAGE_SIZE - 
buf_info->page_offset));

Can you think of a test case where buf_info->page_offset will be 
non-zero that I can test locally?

Dave