Re: [PATCH net 3/4] net: ipa: use the right accessor in ipa_endpoint_status_skip()

[Amy Parker]

On Mon, Feb 1, 2021 at 4:15 PM Alex Elder <elder@xxxxxxxxxx> wrote:
>
> On 2/1/21 6:02 PM, Amy Parker wrote:
> > On Mon, Feb 1, 2021 at 3:32 PM Alex Elder <elder@xxxxxxxxxx> wrote:
> >>
> >> When extracting the destination endpoint ID from the status in
> >> ipa_endpoint_status_skip(), u32_get_bits() is used.  This happens to
> >> work, but it's wrong: the structure field is only 8 bits wide
> >> instead of 32.
> >>
> >> Fix this by using u8_get_bits() to get the destination endpoint ID.
> >
> > Isn't
>
> (I saw your second message.)
>
> >> Signed-off-by: Alex Elder <elder@xxxxxxxxxx>
> >> ---
> >>   drivers/net/ipa/ipa_endpoint.c | 4 ++--
> >>   1 file changed, 2 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/drivers/net/ipa/ipa_endpoint.c b/drivers/net/ipa/ipa_endpoint.c
> >> index 448d89da1e456..612afece303f3 100644
> >> --- a/drivers/net/ipa/ipa_endpoint.c
> >> +++ b/drivers/net/ipa/ipa_endpoint.c
> >> @@ -1164,8 +1164,8 @@ static bool ipa_endpoint_status_skip(struct ipa_endpoint *endpoint,
> >>                  return true;
> >
> > A few lines above this, endpoint_id is initialized as u32. If we're
> > going for "correctness", endpoint_id should be a u8. But of course,
> > this would contrast with ipa_endpoint having it as a u32.
>
> You are correct, endpoint_id is *defined* as type u32.
>
> But the issue here is that the field status->endp_dst_idx
> has type u8.  u32_get_bits() assumes the field it is
> passed has type u32; while u8_get_bits() takes a u8.

Ah, missed that bit. Thanks for clarifying.

>
> The return value of u8_get_bits() is u8, as you might
> suspect.  The C standard guarantees that the u8 value
> will be promoted to the u32 target type here.

Yes, it does, and so it wouldn't be a theoretical issue - just an
issue of developer confusion at first when working with it. But the
above outlined point of the types taken is more important.

>
> >>          if (!status->pkt_len)
> >>                  return true;
> >> -       endpoint_id = u32_get_bits(status->endp_dst_idx,
> >> -                                  IPA_STATUS_DST_IDX_FMASK);
> >> +       endpoint_id = u8_get_bits(status->endp_dst_idx,
> >> +                                 IPA_STATUS_DST_IDX_FMASK);
> >>          if (endpoint_id != endpoint->endpoint_id)
> >>                  return true;
> >>
> >> --
> >> 2.27.0
> >>
> >
> > As far as I see it, using u32_get_bits instead of u8_get_bits simply
> > eliminates confusion about the type of endpoint_id. Perhaps instead of
> > this patch, send a patch with a comment that while u32_get_bits is
> > used, the field is only 8 bits?
>
> No.  We really want to extract a sub-field from the u8
> value passed to u8_get_bits() (not u32_get_bits()).
>
> Does that make sense?
>
>                                         -Alex

Yes, it does. Thank you.

Best regards,
Amy Parker
(she/her/hers)