Re: [PATCH] configure: Improve alias attribute check

[Gavin Shan]

Hi Thomas,

On 3/20/21 3:48 PM, Thomas Huth wrote:
> On 20/03/2021 05.27, Gavin Shan wrote:
> > It's still possible that the wrong value is returned from the alias
> > of variable even if the program can be compiled without issue. This
> > improves the check by executing the binary to check the result.
> >
> > If alias attribute can't be working properly, the @target_page in
> > exec-vary.c will always return zeroes when we have the following gcc
> > version.
> >
> >    # gcc --version
> >    gcc (GCC) 11.0.0 20210210 (Red Hat 11.0.0-0)
> >
> > This abstracts the code from exec-vary.c and use it as indicator to
> > enable gcc alias attribute or not.
> >
> > Signed-off-by: Gavin Shan <gshan@redhat.com>
> > ---
> >   configure | 34 ++++++++++++++++++++++++++++++----
> >   1 file changed, 30 insertions(+), 4 deletions(-)
> >
> > diff --git a/configure b/configure
> > index f7d022a5db..8321f380d5 100755
> > --- a/configure
> > +++ b/configure
> > @@ -75,6 +75,7 @@ fi
> >   TMPB="qemu-conf"
> >   TMPC="${TMPDIR1}/${TMPB}.c"
> > +TMPC_B="${TMPDIR1}/${TMPB}_b.c"
> >   TMPO="${TMPDIR1}/${TMPB}.o"
> >   TMPCXX="${TMPDIR1}/${TMPB}.cxx"
> >   TMPE="${TMPDIR1}/${TMPB}.exe"
> > @@ -4878,13 +4879,38 @@ fi
> >   attralias=no
> >   cat > $TMPC << EOF
> > -int x = 1;
> > +static int x;
> >   extern const int y __attribute__((alias("x")));
> > -int main(void) { return 0; }
> > +extern int read_y(void);
> > +void write_x(int val);
> > +
> > +void write_x(int val)
> > +{
> > +    x = val;
> > +}
> > +
> > +int main(void)
> > +{
> > +    return read_y();
> > +}
> >   EOF
> > -if compile_prog "" "" ; then
> > -    attralias=yes
> > +cat > $TMPC_B << EOF
> > +extern const int y;
> > +extern void write_x(int val);
> > +int read_y(void);
> > +
> > +int read_y(void)
> > +{
> > +     write_x(1);
> > +     return y;
> > +}
> > +EOF
> > +
> > +TMPC+=" ${TMPC_B}"
> > +if compile_prog "" "" && ! $TMPE; then
>
> What about cross-compiling? Running an executable won't work if QEMU gets 
> cross-compiled...

Executing the cross-compiled binary returns 126, which means we will
disable gcc alias attribute for cross-compiling case with the following
changes included into v2:

int main(void) { return (read_y() == 1) ? 0 : 1; }

if compile_prog "" "" && $TMPE >/dev/null 2>/dev/null; then
   attralias=yes
fi

Thanks,
Gavin