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

[Markus Armbruster]

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));
           }