Re: [PATCH] accel/qaic: initialize ret variable to 0

[Jeffrey Hugo]

On 4/18/2023 2:48 PM, Nick Desaulniers wrote:
> On Tue, Apr 18, 2023 at 1:46 PM Jeffrey Hugo <quic_jhugo@xxxxxxxxxxx> wrote:
> > On 4/18/2023 1:20 PM, Tom Rix wrote:
> > > clang static analysis reports
> > > drivers/accel/qaic/qaic_data.c:610:2: warning: Undefined or garbage
> > >     value returned to caller [core.uninitialized.UndefReturn]
> > >           return ret;
> > >           ^~~~~~~~~~
> > >
> > > The ret variable is only set some of the time but is always returned.
> > > So initialize ret to 0.
> >
> > This does not appear to be entirely accurate to me.
> >
> > Do you know what analysis Clang is doing?  Is it limited to the function
> > itself?
> >
> > remap_pfn_range, which initializes ret, will always run at-least once.
>
> What happens if the loop body is never executed, say if `bo->sgt->sgl` is NULL?

qaic_gem_object_mmap() doesn't get called unless a valid GEM object is 
provided by userspace.  For userspace to get a valid GEM object, it has 
to go through qaic_create_bo_ioctl().  qaic_create_bo_ioctl() will not 
return a valid GEM object unless sgt is allocated and initialized.

The loop body will execute at-least once.  The if body will execute 
at-least once.  remap_pfn_range() will run at-least once.  Therefore, 
ret is always initialized.

This is how the code is intended to operate, and how it appears to be 
implemented from what I see.  Unless I'm missing something.

I can see how Clang might not be able to infer these things, but I don't 
believe the code is broken.  I feel that the commit text is unclear and 
suggests that the code is infact, broken.

Userspace should not call mmap() in a critical path thus I don't see a 
true performance concern here.  So while my understanding of the coding 
style is that redundant initialization of variables are to be avoided, I 
think we can say that this is not redundant because it silences a 
warning (because Clang is limited).

> > Feels more accurate to say that Clang cannot detect that ret will be
> > initialized, and we want to quiet the warning from the false positive.
> >
> > > Fixes: ff13be830333 ("accel/qaic: Add datapath")
> > > Signed-off-by: Tom Rix <trix@xxxxxxxxxx>
> > > ---
> > >    drivers/accel/qaic/qaic_data.c | 2 +-
> > >    1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/accel/qaic/qaic_data.c b/drivers/accel/qaic/qaic_data.c
> > > index c0a574cd1b35..b46a16fb3080 100644
> > > --- a/drivers/accel/qaic/qaic_data.c
> > > +++ b/drivers/accel/qaic/qaic_data.c
> > > @@ -591,7 +591,7 @@ static int qaic_gem_object_mmap(struct drm_gem_object *obj, struct vm_area_struc
> > >        struct qaic_bo *bo = to_qaic_bo(obj);
> > >        unsigned long offset = 0;
> > >        struct scatterlist *sg;
> > > -     int ret;
> > > +     int ret = 0;
> > >
> > >        if (obj->import_attach)
> > >                return -EINVAL;