Re: [PATCH] block-qdict: Fix -Werror=maybe-uninitialized build failure

["Murilo Opsfelder Araújo" <muriloo@linux.ibm.com>]

Hi, Markus.

On 3/11/22 06:33, Markus Armbruster wrote:
> Murilo Opsfelder Araujo <muriloo@linux.ibm.com> writes:
> 
>> Building QEMU on Fedora 37 (Rawhide Prerelease) ppc64le failed with the
>> following error:
>>
>>      $ ../configure --prefix=/usr/local/qemu-disabletcg --target-list=ppc-softmmu,ppc64-softmmu --disable-tcg --disable-linux-user
>>      ...
>>      $ make -j$(nproc)
>>      ...
>>      FAILED: libqemuutil.a.p/qobject_block-qdict.c.o
>>      cc -m64 -mlittle-endian -Ilibqemuutil.a.p -I. -I.. -Isubprojects/libvhost-user -I../subprojects/libvhost-user -Iqapi -Itrace -Iui -Iui/shader -I/usr/include/glib-2.0 -I/usr/lib64/glib-2.0/include -I/usr/include/sysprof-4 -I/usr/include/lib
>>      mount -I/usr/include/blkid -I/usr/include/gio-unix-2.0 -I/usr/include/p11-kit-1 -I/usr/include/pixman-1 -fdiagnostics-color=auto -Wall -Winvalid-pch -Werror -std=gnu11 -O2 -g -isystem /root/qemu/linux-headers -isystem linux-headers -iquote
>>       . -iquote /root/qemu -iquote /root/qemu/include -iquote /root/qemu/disas/libvixl -pthread -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2 -D_GNU_SOURCE -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -Wstrict-prototypes -Wredundant-decls -Wundef -Wwrite
>>      -strings -Wmissing-prototypes -fno-strict-aliasing -fno-common -fwrapv -Wold-style-declaration -Wold-style-definition -Wtype-limits -Wformat-security -Wformat-y2k -Winit-self -Wignored-qualifiers -Wempty-body -Wnested-externs -Wendif-label
>>      s -Wexpansion-to-defined -Wimplicit-fallthrough=2 -Wno-missing-include-dirs -Wno-shift-negative-value -Wno-psabi -fstack-protector-strong -fPIE -MD -MQ libqemuutil.a.p/qobject_block-qdict.c.o -MF libqemuutil.a.p/qobject_block-qdict.c.o.d -
>>      o libqemuutil.a.p/qobject_block-qdict.c.o -c ../qobject/block-qdict.c
>>      In file included from /root/qemu/include/qapi/qmp/qdict.h:16,
>>                       from /root/qemu/include/block/qdict.h:13,
>>                       from ../qobject/block-qdict.c:11:
>>      /root/qemu/include/qapi/qmp/qobject.h: In function ‘qdict_array_split’:
>>      /root/qemu/include/qapi/qmp/qobject.h:49:17: error: ‘subqdict’ may be used uninitialized [-Werror=maybe-uninitialized]
>>         49 |     typeof(obj) _obj = (obj);                                   \
>>            |                 ^~~~
>>      ../qobject/block-qdict.c:227:16: note: ‘subqdict’ declared here
>>        227 |         QDict *subqdict;
>>            |                ^~~~~~~~
>>      cc1: all warnings being treated as errors
>>
>> Fix build failure by initializing the QDict variable with NULL.
>>
>> Signed-off-by: Murilo Opsfelder Araujo <muriloo@linux.ibm.com>
>> Cc: Kevin Wolf <kwolf@redhat.com>
>> Cc: Hanna Reitz <hreitz@redhat.com>
>> Cc: Markus Armbruster <armbru@redhat.com>
>> ---
>>   qobject/block-qdict.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/qobject/block-qdict.c b/qobject/block-qdict.c
>> index 1487cc5dd8..b26524429c 100644
>> --- a/qobject/block-qdict.c
>> +++ b/qobject/block-qdict.c
>> @@ -224,7 +224,7 @@ void qdict_array_split(QDict *src, QList **dst)
>>       for (i = 0; i < UINT_MAX; i++) {
>>           QObject *subqobj;
>>           bool is_subqdict;
>> -        QDict *subqdict;
>> +        QDict *subqdict = NULL;
>>           char indexstr[32], prefix[32];
>>           size_t snprintf_ret;
> 
> The compiler's warning is actually spurious.  Your patch is the
> minimally invasive way to shut it up.  But I wonder whether we can
> make the code clearer instead.  Let's have a look:
> 
>             /*
>              * There may be either a single subordinate object (named
>              * "%u") or multiple objects (each with a key prefixed "%u."),
>              * but not both.
>              */
>             if (!subqobj == !is_subqdict) {
>                 break;
> 
> Because of this, ...
> 
>             }
> 
>             if (is_subqdict) {
> 
> ... subqobj is non-null here, and ...
> 
>                 qdict_extract_subqdict(src, &subqdict, prefix);
>                 assert(qdict_size(subqdict) > 0);
>             } else {
> 
> ... null here.
> 
>                 qobject_ref(subqobj);
>                 qdict_del(src, indexstr);
>             }
> 
>             qlist_append_obj(*dst, subqobj ?: QOBJECT(subqdict));
> 
> What about this:
> 
>             if (is_subqdict) {
>                 qdict_extract_subqdict(src, &subqdict, prefix);
>                 assert(qdict_size(subqdict) > 0);
>                 qlist_append_obj(*dst, subqobj);
>             } else {
>                 qobject_ref(subqobj);
>                 qdict_del(src, indexstr);
>                 qlist_append_obj(*dst, QOBJECT(subqdict));
>             }
> 

The logic looks inverted but I think I got what you meant.

I've sent a v2 with changes that made the compiler happy and also passed check-unit tests.

Thank you!

-- 
Murilo