Hi Eli,

On Tue, Oct 19, 2021 at 09:41:56AM +0300, Eli Britstein wrote:

Hi Olivier,

On 8/1/2021 11:06 AM, Eli Britstein wrote:

quoted

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

quoted

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Hi Eli,

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

quoted

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 reproduce locally with DPDK only, no need to change any file. Only run:

CFLAGS="-Wcast-align=strict" make V=1 -C examples/l2fwd clean static

How would you like to proceed?

Sorry, I missed your previous message. I reproduced the issue, with
a slightly modified command:

  # no error, my gcc is 8.3.0-6 (debian)
  CFLAGS="-Wcast-align=strict" make V=1 -C examples/l2fwd clean static


  # bad option name with clang
  CC=clang CFLAGS="-Wcast-align=strict" make V=1 -C examples/l2fwd clean static
  ...
  warning: unknown warning option '-Wcast-align=strict'; did you mean '-Wcast-align'? [-Wunknown-warning-option]


  # problem reproduced with clang
  CC=clang CFLAGS="-Wcast-align" make V=1 -C examples/l2fwd clean static
  main.c:170:8: warning: cast from 'char *' to 'struct rte_ether_hdr *' increases required alignment from 1 to 2 [-Wcast-align]
          eth = rte_pktmbuf_mtod(m, struct rte_ether_hdr *);
                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  /usr/local/include/rte_mbuf_core.h:830:32: note: expanded from macro 'rte_pktmbuf_mtod'
  #define rte_pktmbuf_mtod(m, t) rte_pktmbuf_mtod_offset(m, t, 0)
                                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  /usr/local/include/rte_mbuf_core.h:816:3: note: expanded from macro 'rte_pktmbuf_mtod_offset'
          ((t)((char *)(m)->buf_addr + (m)->data_off + (o)))


I confirm the patch fixes the issue.

Acked-by: Olivier Matz <redacted>

Thanks