Re: [PATCH RFC net] igc: Avoid dereference of ptr_err in igc_clean_rx_irq()

[Dan Carpenter <dan.carpenter@linaro.org>]

The original code is okay.  Passing zero to ERR_PTR() is intentional.

On Thu, Jun 15, 2023 at 11:45:36AM +0200, Simon Horman wrote:
> In igc_clean_rx_irq() the result of a call to igc_xdp_run_prog() is assigned
> to the skb local variable. This may be an ERR_PTR.
> 
> A little later the following is executed, which seems to be a
> possible dereference of an ERR_PTR.
> 
> 	total_bytes += skb->len;


There is an IS_ERR() check in igc_cleanup_headers() which prevents
this.  Sort of tricky to see.  Do you have the cross function database
set up?  If so then Smatch shouldn't warn about this dereference.

> 
> Avoid this problem by continuing the loop in which all of the
> above occurs once the handling of the NULL case completes.
> 
> This proposed fix is speculative - I do not have deep knowledge of this
> driver.  And I am concerned about the effect of skipping the following
> logic:
> 
>   igc_put_rx_buffer(rx_ring, rx_buffer, rx_buffer_pgcnt);
>   cleaned_count++;
> 
> Flagged by Smatch as:
> 
>   .../igc_main.c:2467 igc_xdp_run_prog() warn: passing zero to 'ERR_PTR'

Linus once complained to me that this check is bogus and passing zero to
ERR_PTR() is fine and an intended use case.  But actually this test
does really find a lot of bugs.  I think for new warnings it is less
than 10% false positives.  But we fix the bugs so warnings which are
over three month old are probably 97% false positives.

regards,
dan carpenter