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

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

On Tue, Mar 03, 2026 at 09:36:45AM +0000, Peter Maydell wrote:
> On Tue, 3 Mar 2026 at 08:41, Daniel P. Berrangé <berrange@redhat.com> wrote:
> >
> > 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'm told that clang/LLVM doesn't have any "middle-end warnings", so
> it won't have the same problem that the sanitizers might change IR code
> in a way that results in a warning firing that didn't before. So I
> think this "sanitizers change warning behaviour" is specific to GCC.
> 
> > 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.
> 
> It is expected *for GCC*. I don't think it's expected for LLVM/clang.

Oh, so you mean CLang errors from sanitizers are not influenced by
the use of -Werror ?

With regards,
Daniel
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