Re: [PATCH] configure: Improve alias attribute check

[Gavin Shan <gshan@redhat.com>]

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