Re: [Qemu-devel] [PATCH] configure: workaround for Clang 3.5.0

[John Snow <jsnow@redhat.com>]

On 10/29/2015 04:22 PM, John Snow wrote:
> Clang++ 3.5 on Fedora 22 appears to have difficulty tolerating
> D_FORTIFY_SOURCE for certain glibc headers, such as stdio.
> 
> This interferes, currently, with any arm target build.
> 
> Work around this by disabling FORTIFY_SOURCE for clang builds
> if a problem is observed.
> 
> Newer versions of clang such as 3.5.2 (As seen in debian-testing)
> or 3.7.0 (As seen in Fedora 23 Beta) are unaffected and will not
> trigger this workaround.
> 
> Signed-off-by: John Snow <jsnow@redhat.com>
> ---

As a meta-cover-letter, this fix is a little weird in that it will
disable FORTIFY_SOURCE (silently!) for non-debug builds. Not great.

(Maybe I could have it fail and print a warning encouraging users to
either use --enable-debug or --disable-fortify-source?)

Still, It'd be nice to have Clang builds working out of the box for ARM
builds. It just so happens that ARM is the only target that happens to
trip this specific unfortunate chain of events.

Is it sane to check for clang-and-arm-targets only? Maybe it's a moot
point -- newer (and older, I believe) versions of Clang won't trigger
this at all.

Anyway, see these links (Dredged up by Laszlo Ersek, thanks!)
https://llvm.org/bugs/show_bug.cgi?id=7219
https://llvm.org/bugs/show_bug.cgi?id=16821
https://llvm.org/bugs/show_bug.cgi?id=23277#c2

Here's a good takeaway quote:

'Also, _FORTIFY_SOURCE + glibc + clang is not supported and does not
work (for instance, it relies on __builtin_va_pack_len and friends,
which we have no intention of supporting), so glibc compatibility is
unlikely to be a strong motivator for a change here.'

So long story short, we have a weird hacky workaround where we disable
FORTIFY_SOURCE for compilers that don't appear to be able to support it.

--js

>  configure | 25 ++++++++++++++++++++++++-
>  1 file changed, 24 insertions(+), 1 deletion(-)
> 
> diff --git a/configure b/configure
> index 7a1d08d..7abfcc3 100755
> --- a/configure
> +++ b/configure
> @@ -107,6 +107,11 @@ compile_object() {
>    do_cc $QEMU_CFLAGS $local_cflags -c -o $TMPO $TMPC
>  }
>  
> +compile_cxx_object() {
> +  local_cflags="$1"
> +  do_cxx $QEMU_CXXFLAGS $local_cflags -c -o $TMPO $TMPC
> +}
> +
>  compile_prog() {
>    local_cflags="$1"
>    local_ldflags="$2"
> @@ -4436,13 +4441,31 @@ if ! compile_object "-Werror"; then
>  fi
>  
>  ##########################################
> +# Test that we can use FORTIFY_SOURCE,
> +# which might break Clang.
> +
> +if test "$debug" = "no"; then
> +  cat > $TMPC << EOF
> +#include <cstdio>
> +int main(int argc, char*argv[]) {
> +  fprintf(stdout, "Hello World\n");
> +  return 0;
> +}
> +EOF
> +
> +  if ! compile_cxx_object "-O2 -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2"; then
> +      fortify_source="no";
> +  fi
> +fi
> +
> +##########################################
>  # End of CC checks
>  # After here, no more $cc or $ld runs
>  
>  if test "$gcov" = "yes" ; then
>    CFLAGS="-fprofile-arcs -ftest-coverage -g $CFLAGS"
>    LDFLAGS="-fprofile-arcs -ftest-coverage $LDFLAGS"
> -elif test "$debug" = "no" ; then
> +elif test "$debug" = "no" && test "$fortify_source" != "no" ; then
>    CFLAGS="-O2 -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2 $CFLAGS"
>  fi
>  
>