Re: [PATCH] selftests/mm: Fix compiler -Wmaybe-uninitialized warning

[David Hildenbrand]

On 09.04.25 12:09, Anshuman Khandual wrote:
> On 4/9/25 15:27, David Hildenbrand wrote:
> > On 09.04.25 11:50, Anshuman Khandual wrote:
> > > Following build warning comes up for cow test as 'transferred' variable has
> > > not been initialized. Fix the warning via zero init for the variable.
> > >
> > >     CC       cow
> > > cow.c: In function ‘do_test_vmsplice_in_parent’:
> > > cow.c:365:61: warning: ‘transferred’ may be used uninitialized [-Wmaybe-uninitialized]
> > >     365 |                 cur = read(fds[0], new + total, transferred - total);
> > >         |                                                 ~~~~~~~~~~~~^~~~~~~
> > > cow.c:296:29: note: ‘transferred’ was declared here
> > >     296 |         ssize_t cur, total, transferred;
> > >         |                             ^~~~~~~~~~~
> > >     CC       compaction_test
> > >     CC       gup_longterm
> > >
> > > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > > Cc: Shuah Khan <shuah@xxxxxxxxxx>
> > > Cc: linux-mm@xxxxxxxxx
> > > Cc: linux-kselftest@xxxxxxxxxxxxxxx
> > > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > > Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> > > ---
> > >    tools/testing/selftests/mm/cow.c | 2 +-
> > >    1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/tools/testing/selftests/mm/cow.c b/tools/testing/selftests/mm/cow.c
> > > index f0cb14ea8608..b6cfe0a4b7df 100644
> > > --- a/tools/testing/selftests/mm/cow.c
> > > +++ b/tools/testing/selftests/mm/cow.c
> > > @@ -293,7 +293,7 @@ static void do_test_vmsplice_in_parent(char *mem, size_t size,
> > >            .iov_base = mem,
> > >            .iov_len = size,
> > >        };
> > > -    ssize_t cur, total, transferred;
> > > +    ssize_t cur, total, transferred = 0;
> > >        struct comm_pipes comm_pipes;
> > >        char *old, *new;
> > >        int ret, fds[2];
> >
> >
> > if (before_fork) {
> >      transferred = vmsplice(fds[1], &iov, 1, 0);
> > ...
> >
> > if (!before_fork) {
> >      transferred = vmsplice(fds[1], &iov, 1, 0);
> > ...
> >
> > for (total = 0; total < transferred; total += cur) {
> > ...
> >
> >
> > And I don't see any jump label that could jump to code that would ve using transferred.
> >
> > What am I missing?
>
> Probably because both those conditional statements are not mutually
> exclusive above with an if-else construct. Hence compiler flags it
> rather as a false positive ? Initializing with 0 just works around
> that false positive.

This is something the compiler should clearly be able to verify. 
before_fork is never changed in that function.

We should not work around wrong compilers.

Which compiler are you using such that you run into this issue?

--
Cheers,

David / dhildenb