Re: [RFC PATCH v2] configure: Use clang for sanitizer builds or disable Werror

["Daniel P. Berrangé" <berrange@redhat.com>]

On Mon, Mar 02, 2026 at 07:20:54PM -0600, Yodel Eldar wrote:
> From: Yodel Eldar <yodel.eldar@yodel.dev>
> 
> Builds with --enable-{asan,tsan,safe-stack} fail under GCC, so use
> clang if available, otherwise disable the treatment of warnings as
> errors.
> 
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3006
> Suggested-by: Peter Maydell <peter.maydell@linaro.org>
> Signed-off-by: Yodel Eldar <yodel.eldar@yodel.dev>
> ---
> Hi,
> 
> The previous version only disabled Werror whenever `--skip-meson` wasn't
> used and the build occurred in a git repo, but this change should
> probably apply to all types of builds. So, let's use meson_option_add
> to globally disable Werror instead; IIUC (and according to my testing),
> this will override the value in config-meson.cross.new.
> 
> I'm still not sure if we should be disabling Werror for ubsan, even
> though it's not currently breaking builds with GCC; please let me know
> what you think.
> 
> Special thanks to Peter for looking into the cause of the reports around
> this, for sharing the findings, and suggesting approaches to resolve it.
> I couldn't pick one over the other, so I went with using clang when
> available with Werror disable as a fallback; please let me know if you
> think this is an XOR kind of policy decision.

In the bug 

> 
> Link to RFCv1:
> https://lore.kernel.org/qemu-devel/20260302210039.261325-1-yodel.eldar@yodel.dev/
> 
> Link to mentioned discussion:
> https://lore.kernel.org/qemu-devel/CAFEAcA88hc4UsgpuPXBWpbeN0tW26159kPn7jx2J9erBA5DLBw@mail.gmail.com/
> 
> v2:
> - Fix misnomer in commit message
> - Simplify condition by using the same variable for all sanitizers
> - Use meson_option_add to disable Werror
> 
> Thanks,
> Yodel
> ---
>  configure | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/configure b/configure
> index 5e114acea2..e457e8a17d 100755
> --- a/configure
> +++ b/configure
> @@ -762,6 +762,12 @@ for opt do
>    ;;
>    --wasm64-32bit-address-limit)
>    ;;
> +  --enable-asan) use_sanitizer="yes"
> +  ;;
> +  --enable-tsan) use_sanitizer="yes"
> +  ;;
> +  --enable-safe-stack) use_sanitizer="yes"
> +  ;;
>    # everything else has the same name in configure and meson
>    --*) meson_option_parse "$opt" "$optarg"
>    ;;
> @@ -771,6 +777,18 @@ for opt do
>    esac
>  done
>  
> +if test "$use_sanitizer" = "yes"; then
> +    if has clang; then
> +        echo "Sanitizer requested: setting compiler suite to clang"
> +        cc=clang
> +        cxx=clang++
> +        host_cc=clang
> +    else
> +        echo "Sanitizer requested: disabling Werror for non-clang compilers"
> +        meson_option_add -Dwerror=false
> +    fi
> +fi

GCC documents the possibility of false positives, but I would consider
that caveat to apply to CLang too. It may simply have different false
positives or at a lower rate.

I don't think it is acceptable to override the user's choice of compiler,
and I'm not really a fan of auto-changing use of -Werror either.  

If using sanitizers, it is expected behaviour that there can be false
positives, and if the user doesn't want the build to break as a result
of that, then *the user* can choose to disable -Werror with the suitable
configure warnings.

So IMHO the issue above is not a bug - it is sanitizers doing what they
are expected to do.

If we want to inform the user of the possible fallout, then it would
suffice to print a warning message at the end of meson:

  if sanitizers and -Werror
  
     warning("Note: sanitizers may produce false positives."
             "It is recommended to disable -Werror if requiring"
	     "a complete build of QEMU without errors")

  endif

With regards,
Daniel
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