Re: struct mctelem_cookie missing definition

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Thu, 13 Feb 2025, Stefano Stabellini wrote:
> > > diff --git a/xen/arch/x86/cpu/mcheck/mctelem.h 
> > > b/xen/arch/x86/cpu/mcheck/mctelem.h
> > > index f4c5ff848d..2ccd490e5d 100644
> > > --- a/xen/arch/x86/cpu/mcheck/mctelem.h
> > > +++ b/xen/arch/x86/cpu/mcheck/mctelem.h
> > > @@ -52,7 +52,7 @@
> > >   * the element from the processing list.
> > >   */
> > >  
> > > -typedef struct mctelem_cookie *mctelem_cookie_t;
> > > +typedef uint64_t *mctelem_cookie_t;
> > 
> > Yet that makes it possible to de-reference the pointer. Which, as Andrew
> > explained, is intended to be impossible. If this could be properly
> > replaced (not exactly what Andrew indicated by "file it in /dev/null"),
> > fine. Truly purging the code (i.e. as Andrew suggests) may still be an
> > option, with appropriate justification. But simply adjusting the type
> > and then moving on is too little, imo. Even if you used void * (to make
> > de-referencing impossible) I'd view it as largely papering over an issue;
> > then converting to other pointers (without explicit cast, and hence
> > without making apparent the badness of doing so) would become possible.
> 
> What about converting to uintptr_t (not a pointer)?
> 
> 
> In general, there are quite a few MISRA rules that we could mark as
> blocking (clean) in our GitLab scan with just a few code changes like
> this one. My goal is to make these rules blocking as soon as possible.
> If I can improve the code in the process, that is even better, but it is
> not mandatory. And I would rather spend one more hour marking a second
> rule as blocking instead. 
> 
> What I mean is that I believe it would be acceptable to make some
> compromises and accept non-perfect changes to the code if it helps us
> enforce more rules as blocking in GitLab CI.

After briefly speaking with Andrew about this, and re-reading Jan's
email above, I think it is best to resolve this as a deviation.

Would this deviation work for you? Please suggest a better wording if
you prefer.

Nicola, in reality I think it would be better to use deviations.rst
because the SAF comment below would need to be replicated at every call
side, if I am not mistaken.


diff --git a/docs/misra/safe.json b/docs/misra/safe.json
index b8a4f878ea..d9fbe959d1 100644
--- a/docs/misra/safe.json
+++ b/docs/misra/safe.json
@@ -92,6 +92,14 @@
         },
         {
             "id": "SAF-11-safe",
+            "analyser": {
+                "eclair": "MC3A2.R11.2"
+            },
+            "name": "Rule 11.2: purposely impossible to dereference",
+            "text": "Certain pointers points to incomplete types purposely so 
that they are impossible to dereference."
+        },
+        {
+            "id": "SAF-12-safe",
             "analyser": {},
             "name": "Sentinel",
             "text": "Next ID to be used"
diff --git a/xen/arch/x86/cpu/mcheck/mctelem.h 
b/xen/arch/x86/cpu/mcheck/mctelem.h
index f4c5ff848d..e845360c7d 100644
--- a/xen/arch/x86/cpu/mcheck/mctelem.h
+++ b/xen/arch/x86/cpu/mcheck/mctelem.h
@@ -52,6 +52,7 @@
  * the element from the processing list.
  */
 
+/* SAF-11-safe: impossible to dereference */
 typedef struct mctelem_cookie *mctelem_cookie_t;
 
 typedef enum mctelem_class {