Re: [dpdk-dev] [PATCH 2/3] mbuf: avoid cast-align warning in pktmbuf mtod offset macro

[Eli Britstein <elibr@nvidia.com>]

On 7/30/2021 2:10 PM, Olivier Matz wrote:
> External email: Use caution opening links or attachments
>
>
> Hi Eli,
>
> On Thu, Jul 29, 2021 at 10:13:45AM +0300, Eli Britstein wrote:
>> On 7/28/2021 6:28 PM, Olivier Matz wrote:
>>> External email: Use caution opening links or attachments
>>>
>>>
>>> On Tue, Jul 13, 2021 at 09:49:09AM +0300, Eli Britstein wrote:
>>>> In rte_pktmbuf_mtod_offset macro, there is a casting from char * to type
>>>> 't', which may cause cast-align warning when using gcc flags
>>>> '-Werror -Wcast-align':
>>>>
>>>> .../include/rte_mbuf_core.h:723:3: error: cast increases required alignment
>>>>       of target type [-Werror=cast-align]
>>>>     723 |  ((t)((char *)(m)->buf_addr + (m)->data_off + (o)))
>>>>         |   ^
>>>>
>>>> As the code assumes correct alignment, add first a (void *) casting, to
>>>> avoid the warning.
>>>>
>>>> Fixes: af75078fece3 ("first public release")
>>>> Cc: stable@dpdk.org
>>>>
>>>> Signed-off-by: Eli Britstein <elibr@nvidia.com>
>>> My initial thinking was that it's the problem of the application: if
>>> -Werror=cast-align is used, it is up to the application to cast the
>>> return value of rte_pktmbuf_mtod_offset() to (void *) before casting it
>>> to the network type.
>>>
>>> But, if I understand correctly, the problem is not about the application
>>> code itself, but about inlined code in the header files of dpdk
>>> (i.e. compiling an empty C file that just includes the dpdk headers with
>>> -Werror=cast-align). Is it correct? If yes I think it should be
>>> highlighted in the commit log.
>> I think yes, though in this specific patch it is not even an inline
>> function, but a macro.
>>
>> However, I don't have a synthetic application example to show those
>> warnings, thus didn't put such in the commit msg.
> For this patch, I think it would be useful to have a way to reproduce
> the issue first, so we can check whether it is the proper place to fix
> the problem.
--- a/examples/l2fwd/Makefile
+++ b/examples/l2fwd/Makefile
@@ -22,6 +22,7 @@ static: build/$(APP)-static
         ln -sf $(APP)-static build/$(APP)

  PC_FILE := $(shell $(PKGCONF) --path libdpdk 2>/dev/null)
+CFLAGS += -Wcast-align=strict
  CFLAGS += -O3 $(shell $(PKGCONF) --cflags libdpdk)

gcc (Ubuntu 9.3.0-10ubuntu2) 9.3.0
Copyright (C) 2019 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

make -C examples/l2fwd clean static

>
> To me, it is assumed in the DPDK project that we can mmap a network
> structure on mbuf data (maybe I'm wrong?). If an external application
> like OVS wants to use -Werror=cast-align, it has to cast the result of
> calls to rte_pktmbuf_mtod() family.
>
> The only corner cases are DPDK header files which have static inline
> functions or macro that forces the use of rte_pktmbuf_mtod() family
> without a cast (like for your patch 1/3), because it cannot be fixed in
> the external project.
>
> I think we have to make our header files compliant to projects that want
> to use -Werror=cast-align, like we do to make our header files compliant
> to C++.
>
> What you suggest in this patch forces the cast to (void *) for all users
> of rte_pktmbuf_mtod() family. This could be a problem for projects that
> want to see these warnings.
>
> Would it be possible instead to add a cast in DPDK headers, in inline
> functions that make use of these mtod functions?
>
> Regards,
> Olivier
>
>
>
>>> Out of curiosity, how did you find the errors? I mean, is it possible
>>> that some casts are missing some other headers, or is this patchset
>>> exhaustive?
>> Currently OVS-DPDK is compiled only with -Wno-cast-align.
>>
>> Following complaint that a recent commit introduced a degradation in OVS
>> [1], I compiled OVS without this warning deprecation.
>> The fixes in OVS are [2] and [3] (already merged). The fixes in DPDK are in
>> this patch-set.
>>
>> [1] https://mail.openvswitch.org/pipermail/ovs-dev/2021-July/385084.html
>> [2] https://mail.openvswitch.org/pipermail/ovs-dev/2021-July/386278.html
>>      e8cccd3a3589 ("netdev-offload-dpdk: Fix IPv6 rewrite cast-align
>> warning.")
>> [3] https://mail.openvswitch.org/pipermail/ovs-dev/2021-July/386279.html
>>      1f7f557603a5 ("netdev-offload-dpdk: Fix vxlan vni cast-align warnings.")
>>> Thanks,
>>> Olivier
>>>
>>>
>>>> ---
>>>>    lib/mbuf/rte_mbuf_core.h | 2 +-
>>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/lib/mbuf/rte_mbuf_core.h b/lib/mbuf/rte_mbuf_core.h
>>>> index bb38d7f581..dabdeee604 100644
>>>> --- a/lib/mbuf/rte_mbuf_core.h
>>>> +++ b/lib/mbuf/rte_mbuf_core.h
>>>> @@ -720,7 +720,7 @@ struct rte_mbuf_ext_shared_info {
>>>>     *   The type to cast the result into.
>>>>     */
>>>>    #define rte_pktmbuf_mtod_offset(m, t, o)     \
>>>> -     ((t)((char *)(m)->buf_addr + (m)->data_off + (o)))
>>>> +     ((t)(void *)((char *)(m)->buf_addr + (m)->data_off + (o)))
>>>>
>>>>    /**
>>>>     * A macro that points to the start of the data in the mbuf.
>>>> --
>>>> 2.28.0.2311.g225365fb51
>>>>