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

On 7/30/2021 2:10 PM, Olivier Matz wrote:

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Hi Eli,

On Thu, Jul 29, 2021 at 10:13:45AM +0300, Eli Britstein wrote:

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On 7/28/2021 6:28 PM, Olivier Matz wrote:

quoted

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On Tue, Jul 13, 2021 at 09:49:09AM +0300, Eli Britstein wrote:

quoted

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 <redacted>

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

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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.")

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Thanks,
Olivier

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---
   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