Re: [dpdk-dev] [PATCH v2] mk: report address of packed member as warning

[David Marchand <david.marchand@redhat.com>]

On Thu, May 2, 2019 at 4:42 PM Bruce Richardson <bruce.richardson@intel.com>
wrote:

> On Thu, May 02, 2019 at 04:24:37PM +0200, David Marchand wrote:
> >    On Thu, May 2, 2019 at 4:13 PM Reshma Pattan
> >    <[1]reshma.pattan@intel.com> wrote:
> >
> >      gcc 9 on Fedora 30 gives an error
> >      "taking address of packed member may result in an
> >      unaligned pointer value" for -Waddress-of-packed-member.
> >      Report it as warning instead of error to fix the build.
> >      Snippet of build before fix
> >      ...lib/librte_eal/linux/eal/eal_memalloc.c: In function
> >      ‘alloc_seg_walk’:
> >      ...lib/librte_eal/linux/eal/eal_memalloc.c:768:12: error: taking
> >      address
> >      of packed member of ‘struct rte_mem_config’ may result in an
> >      unaligned
> >      pointer value [-Werror=address-of-packed-member]
> >        768 |  cur_msl = &mcfg->memsegs[msl_idx];
> >            |            ^~~~~~~~~~~~~~~~~~~~~~~
> >      Snippet of build after fix
> >      ..lib/librte_eal/linux/eal/eal_memory.c: In function
> >      ‘remap_segment’:
> >      ..lib/librte_eal/linux/eal/eal_memory.c:685:9: warning: taking
> >      address
> >      of packed member of ‘struct rte_mem_config’ may result in an
> >      unaligned
> >      pointer value [-Waddress-of-packed-member]
> >        685 |   msl = &mcfg->memsegs[msl_idx];
> >      Signed-off-by: Reshma Pattan <[2]reshma.pattan@intel.com>
> >      ---
> >       mk/toolchain/gcc/[3]rte.vars.mk | 4 ++++
> >       1 file changed, 4 insertions(+)
> >      diff --git a/mk/toolchain/gcc/[4]rte.vars.mk
> >      b/mk/toolchain/gcc/[5]rte.vars.mk
> >      index d8b99faf6..61032bbbc 100644
> >      --- a/mk/toolchain/gcc/[6]rte.vars.mk
> >      +++ b/mk/toolchain/gcc/[7]rte.vars.mk
> >      @@ -87,5 +87,9 @@ WERROR_FLAGS += -Wimplicit-fallthrough=2
> >       WERROR_FLAGS += -Wno-format-truncation
> >       endif
> >      +#Report "taking address of packed member may result
> >      +#in an unaligned pointer value" issues as warnings.
> >      +WERROR_FLAGS += -Wno-error=address-of-packed-member
> >      +
> >       export CC AS AR LD OBJCOPY OBJDUMP STRIP READELF
> >       export TOOLCHAIN_CFLAGS TOOLCHAIN_LDFLAGS TOOLCHAIN_ASFLAGS
> >      --
> >      2.21.0
> >
> >    No, you can't do this unconditionally.
> >    Looked at clang (in make build system) and meson (globally) and those
> >    completely disable the warning.
> >    $ git grep address-of-packed-member origin/master --
> >    origin/master:config/meson.build:
>  '-Wno-address-of-packed-member'
> >    origin/master:mk/toolchain/clang/rte.vars.mk:WERROR_FLAGS +=
> >    -Wno-address-of-packed-member
> >    It should be something like (untested):
> >    @@ -87,5 +87,10 @@ WERROR_FLAGS += -Wimplicit-fallthrough=2
> >     WERROR_FLAGS += -Wno-format-truncation
> >     endif
> >
> >    +# disable packed member unalign warnings
> >    +ifeq ($(shell test $(GCC_VERSION) -ge 90 && echo 1), 1)
> >    +WERROR_FLAGS += -Wno-address-of-packed-member
> >    +endif
> >    +
> Actually, you should not need to check the GCC version for this.
> GCC always ignores silently any warning disable flags it doesn't recognise,
> only printing information about them if there is another error to report.
> [This is why meson, when you ask it if the compiler supports "-Wno-foo",
> will
> check the flag and also the -Wfoo flag, as gcc won't report an error with
> just the "no" form]
>

WERROR_FLAGS += -Wno-error=address-of-packed-member triggers an error.

[dmarchan@dmarchan dpdk]$ make EXTRA_CFLAGS="-g -O0" O=master
cc1: error: -Werror=address-of-packed-member: no option
-Waddress-of-packed-member
== Build lib
cc1: error: -Werror=address-of-packed-member: no option
-Waddress-of-packed-member
== Build lib/librte_kvargs
cc1: error: -Werror=address-of-packed-member: no option
-Waddress-of-packed-member
  CC rte_kvargs.o
cc1: error: -Werror=address-of-packed-member: no option
-Waddress-of-packed-member
make[3]: *** [rte_kvargs.o] Error 1
make[2]: *** [librte_kvargs] Error 2
make[1]: *** [lib] Error 2
make: *** [all] Error 2


WERROR_FLAGS += -Wno-address-of-packed-member is ignored.

Marvellous :-)
Sorry for the noise, let's go with your initial patch.

-- 
David Marchand