cmrtlib/linux/CMakelists.txt: respect MEDIA_BUILD_FATAL_WARNINGS

[ffontaine]

Respect MEDIA_BUILD_FATAL_WARNINGS to avoid the following build failure when the user provides _FORTIFY_SOURCE:

In file included from /home/buildroot/autobuild/instance-0/output-1/host/x86_64-buildroot-linux-gnu/sysroot/usr/include/dlfcn.h:22,
                 from /home/buildroot/autobuild/instance-0/output-1/build/intel-mediadriver-19.4.0r/cmrtlib/linux/../linux/share/cm_include.h:30,
                 from /home/buildroot/autobuild/instance-0/output-1/build/intel-mediadriver-19.4.0r/cmrtlib/agnostic/share/cm_printf_host.cpp:23:
/home/buildroot/autobuild/instance-0/output-1/host/x86_64-buildroot-linux-gnu/sysroot/usr/include/features.h:397:4: error: #warning _FORTIFY_SOURCE requires compiling with optimization (-O) [-Werror=cpp]
  397 | #  warning _FORTIFY_SOURCE requires compiling with optimization (-O)
      |    ^~~~~~~

Fixes:

• http://autobuild.buildroot.org/results/52638d95312e464626d1c4047b3b26d4f57a1cd2

Signed-off-by: Fabrice Fontaine fontaine.fabrice@gmail.com

[XinfengZhang]

from my understanding, we should resolve warning instead of remove the Werror option. could you help to explain why we could not fix the warning directly?
@HeFan2017 could you help to review it.

[FurongZhang]

Why need to remove -Werror instead of fixing in the code.

[ffontaine]

Hardcoding -Werror is generally seen as a bad practice from a downstream user perspective. Indeed, this will break your build with old or new gcc. This will also break the build if the user provides some kind of options that were not foreseen (e.g. if the user passes -D_FORTIFY_SOURCE=1 in CFLAGS, you'll get a warning because some parts of intel-mediadriver hardcodes -D_FORTIFY_SOURCE=2. Due to -Werror, this warning will become an error). You can read more info about it here: https://embeddedartistry.com/blog/2017/05/22/werror-is-not-your-friend.

[dvrogozh]

Warning should be fixed, no doubt.

However, I fully agree that having -Werror by default is a bad practice. Instead, in our CI we should add -Werror passing custom build flags, something like:

cmake -DCMAKE_C_FLAGS="-Werror" ...

This advise goes to majority of the build flags by the way. Imho, cmake setup should be minimal and if we have any additional requirements these should be handled in CI setup, not in the cmake setup.

As of now media-drviver does not follow that, but it has MEDIA_BUILD_FATAL_WARNINGS cmake variable to allow disabling of -Werror. By default, we force warnings as errors. That's good compromise. So, I believe this PR is a good one since it actually won't affect our CI builds, but allow community make experiments if needed (on their own risk). So, I approve the request.

[ffontaine]

Thanks for your feedback, it should also be noted that we're carrying a patch in buildroot to drop hardening flags because some embedded toolchains don't support it: https://git.buildroot.net/buildroot/tree/package/intel-mediadriver/0001-Drop-hardening-related-flags.patch. It would be great if those kind of flags could also be disabled through a dedicated option even if stack-protection should be enabled by default for obvious security reasons.