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

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

[John Snow]

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>
---
 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
 
-- 
2.4.3

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

[John Snow]

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
>  
> 

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

[Laszlo Ersek]

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
>>  
>>

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

[Peter Maydell]

On 29 October 2015 at 21:20, Laszlo Ersek <lersek@redhat.com> wrote:
> On 10/29/15 21:29, John Snow wrote:
>> 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. :)

I think I would agree with that.

thanks
-- PMM

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

[John Snow]

On 10/29/2015 05:25 PM, Peter Maydell wrote:
> On 29 October 2015 at 21:20, Laszlo Ersek <lersek@redhat.com> wrote:
>> On 10/29/15 21:29, John Snow wrote:
>>> 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. :)
> 
> I think I would agree with that.
> 
> thanks
> -- PMM
> 

OK. I'll spin a new one that disables FORTIFY SOURCE without test if it
detects you are using Clang.

(I am curious as to why it now magically works in later versions of
Clang though, despite such a strong "no support" statement from the
developer above. I might do a little digging.)

--js