Re: [PATCH 5/6] hw/pci-bridge/cxl_upstream: Fix missing ERRP_GUARD() in cxl_usp_realize()

[Zhao Liu <zhao1.liu@linux.intel.com>]

On Wed, Feb 21, 2024 at 12:49:46PM +0100, Markus Armbruster wrote:
> Date: Wed, 21 Feb 2024 12:49:46 +0100
> From: Markus Armbruster <armbru@redhat.com>
> Subject: Re: [PATCH 5/6] hw/pci-bridge/cxl_upstream: Fix missing
>  ERRP_GUARD() in cxl_usp_realize()
> 
> Zhao Liu <zhao1.liu@linux.intel.com> writes:
> 
> > From: Zhao Liu <zhao1.liu@intel.com>
> >
> > As the comment in qapi/error, dereferencing @errp requires
> > ERRP_GUARD():
> >
> > * = Why, when and how to use ERRP_GUARD() =
> > *
> > * Without ERRP_GUARD(), use of the @errp parameter is restricted:
> > * - It must not be dereferenced, because it may be null.
> > * - It should not be passed to error_prepend() or
> > *   error_append_hint(), because that doesn't work with &error_fatal.
> > * ERRP_GUARD() lifts these restrictions.
> > *
> > * To use ERRP_GUARD(), add it right at the beginning of the function.
> > * @errp can then be used without worrying about the argument being
> > * NULL or &error_fatal.
> > *
> > * Using it when it's not needed is safe, but please avoid cluttering
> > * the source with useless code.
> >
> > Currently, since cxl_usp_realize() - as a PCIDeviceClass.realize()
> > method - doesn't get the NULL errp parameter, it doesn't trigger the
> > dereference issue.
> >
> > To follow the requirement of errp, add missing ERRP_GUARD() in
> > cxl_usp_realize()().
> >
> > Suggested-by: Markus Armbruster <armbru@redhat.com>
> > Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
> > ---
> > Suggested by credit:
> >  Markus: Referred his explanation about ERRP_GUARD().
> > ---
> >  hw/pci-bridge/cxl_upstream.c | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/hw/pci-bridge/cxl_upstream.c b/hw/pci-bridge/cxl_upstream.c
> > index e87eb4017713..03d123cca0ef 100644
> > --- a/hw/pci-bridge/cxl_upstream.c
> > +++ b/hw/pci-bridge/cxl_upstream.c
> > @@ -289,6 +289,7 @@ static void free_default_cdat_table(CDATSubHeader **cdat_table, int num,
> >  
> >  static void cxl_usp_realize(PCIDevice *d, Error **errp)
> >  {
> > +    ERRP_GUARD();
> >      PCIEPort *p = PCIE_PORT(d);
> >      CXLUpstreamPort *usp = CXL_USP(d);
> >      CXLComponentState *cxl_cstate = &usp->cxl_cstate;
> 
> The dereference is
> 
>        cxl_doe_cdat_init(cxl_cstate, errp);
>        if (*errp) {
>            goto err_cap;
>        }
> 
> As noted in review of PATCH 3, we check *errp, because
> cxl_doe_cdat_init() returns void.  Could be improved to return bool,
> along with its callees ct3_load_cdat() and ct3_build_cdat(), but that's
> a slightly more ambitious cleanup, so
> 
> Reviewed-by: Markus Armbruster <armbru@redhat.com>
>

Thanks! It's a good idea and I can continue to consider such the cleanup
in the follow up.

Will also add the dereference description in commit message to make
review easier.

Regards,
Zhao