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

[Laszlo Ersek <lersek@redhat.com>]

Aargh, clicked "Reply" instead of "Reply All". Resending.

On 10/29/15 21:29, John Snow wrote:
> 
> 
> 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!)

Thanks for the credit.

I didn't notice this patch on the list (and I've been CC'd just now), so
I'm unsure if I'm supposed to (try to) review it. If so, then:

> 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.'

this quote would be compelling enough for me to disable _FORTIFY_SOURCE
when clang is seen, no questions asked. The above is a public no-support
statement from an apparently core clang developer, so "it happens to
build without errors with version X.Y.Z." just don't cut it. A positive
claim (bugzilla comment, release note, etc) would be necessary.

... As far as I'm concerned, of course. :)

Cheers
Laszlo


> 
> 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
>>  
>>