[Bug general/23914] Add --disable-werror to ./configure support (example trigger: CFLAGS=-Og

["slyfox at inbox dot ru" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=23914

--- Comment #2 from Sergei Trofimovich <slyfox at inbox dot ru> ---
(In reply to Mark Wielaard from comment #1)
> I rather have that people report such issues and we fix them.

Totally understandable. My intent is to only still be able to build old
releases with new toolchains or against exotic yet safe CFLAGS  without resort
to manual patching of build system.

I would suggest having --enable-werror enabled by default. That way we would be
able to disable it downstream for older packages on first offense.

> In this case it really was just that one gcc warning (plus an almost
> identical one in dwarf_getattrs):
> https://sourceware.org/ml/elfutils-devel/2018-q4/msg00174.html
> 
> And that build flag actually showed a very interesting nasty bug that we
> might not have found otherwise:
> https://sourceware.org/ml/elfutils-devel/2018-q4/msg00175.html
> 
> Does Gentoo show any other compile errors?

Yes. I can trigger a lot of them passing various warning flags (I'll post those
below).

Gentoo allows users to control CC and CFLAGS and thus the space for getting a
warning is wide. People frequently use things like -Wcast-qual or other high
signal-to-noise flags for their purposes.

My favourite example is
    ./configure CFLAGS="-g -Wall" # works today without failures
or even ./configure CC=clang CFLAGS="-g -Weverything" but elfutils does not
seem to support clang.

Real-world examples used by people:

1. CFLAGS="-g -Wall -Wcast-qual"

  In file included from gelf_xlate.c:166:
  version_xlate.h: In function 'elf_cvt_Verdef':
  version_xlate.h:74:31: error: cast discards 'const' qualifier from pointer
target type [-Werror=cast-qual]
         dsrc = (GElf_Verdef *) ((char *) src + def_offset);
                                 ^

2. CFLAGS="-g -O2 -Wstack-protector"

    CC       readelf.o
  readelf.c: In function 'open_input_section':
  readelf.c:581:1: error: stack protector not protecting local variables:
variable length buffer [-Werror=stack-protector]
   open_input_section (int fd)
   ^~~~~~~~~~~~~~~~~~

> BTW for make check to work correctly you do of course need debuginfo, so you
> really need CLAGS="-g -Og".

Yes, sorry. Forgot about the unconditional -g passing (we do it all the time).

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