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

["mark at klomp dot org" <sourceware-bugzilla at sourceware dot org>]

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

--- Comment #3 from Mark Wielaard <mark at klomp dot org> ---
(In reply to Sergei Trofimovich from comment #2)
> 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.

If they do and don't care about the warnings, then why don't they simply add
-Wno-error too?

> 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.

Yes, -Wall is one of the warnings we explicitly enable. See config/eu.am for
the full list. Some have configure checks to make sure the compiler actually
supports it. And some sadly have to be disabled for some specific source files.
If you have concrete warning flags you would like to see enabled by default
please do submit them, but we do like to enable them only once the code base is
clean. Then we enable them by default and will always catch whenever new code
produces a warning (because of -Werror).

An -Weverything flag seems silly, some warnings don't really mix, some are for
style issues. IMHO warnings are only interesting if you can take some action to
correct the code.

clang support would be nice, but clang doesn't support various GNU C
extensions, there is a configure check for it though, so as soon as clang
actually support -std=gnu99 it should work.

> 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);
>                                  ^

There are a lot of cast-qual warnings. It might make sense to clean them up.
But it might be hard since in some cases we support overlapping src/dest
buffers which might be marked "wrongly" in the external API.

> 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)
>    ^~~~~~~~~~~~~~~~~~

That in itself wouldn't warn. I assume you are using
-fstack-protector[-all|strong] too.

The warning is correct. We do already support -Wstack-usage. But it is disabled
for a couple of files. readelf.c is one of them (see src/Makefile.am).

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