[PATCH] diagnosing flexible array assignments

[Eli Friedman]

On Tue, Sep 10, 2013 at 2:33 PM, Aaron Ballman <aaron at aaronballman.com>wrote:

> On Tue, Sep 10, 2013 at 5:21 PM, Eli Friedman <eli.friedman at gmail.com>
> wrote:
> > On Tue, Sep 10, 2013 at 2:03 PM, Aaron Ballman <aaron at aaronballman.com>
> > wrote:
> >>
> >> Currently, if you have a value assignment involving structures
> >> containing a flexible array, no diagnostic is emitted.  However, since
> >> the array members won't be copied as part of the assignment, a
> >> diagnostic will help programmers avoid bugs.  This patch emits said
> >> diagnostic.
> >>
> >
> > +def warn_flexible_array_assignment : ExtWarn<
> > +  "assignment of flexible arrays does not copy the array members">,
> > +  InGroup<FlexibleArrayExtensions>;
> >
> > I assume this is supposed to be a Warning, not an ExtWarn?
>
> Yes, thanks for pointing that out.
>
> > Also, I'm not sure putting this into the FlexibleArrayExtensions
> diagnostic
> > group is the best idea.
>
> I wasn't certain either, but it was the closest existing group.  Would
> it make more sense to add a new group for it?
>

I think so.


> > What sort of mistakes do you expect this to catch in practice?
>
> Simple programmer mistakes, mostly.  It also nicely covers a CERT
> secure C coding rule:
>
>
> https://www.securecoding.cert.org/confluence/display/seccode/MEM33-C.++Allocate+and+copy+structures+containing+flexible+array+members+dynamically


Okay.

Do you have any idea what the false positive rate is?


>
>
> > Have you considered warning about the variable declaration rather than
> the
> > assignment?  In your testcase, it's impossible to call foo() without
> > triggering the warning, so it seems better to warn about foo() itself
> rather
> > than the call.
>
> Hmmm, would there ever be a case where it would make sense to declare
> a structure with a flexible array member as a value type?The only
> situation I could think of would be overlaying the value type with
> stack-allocated memory in some sort of bizarre union type punning
> scenario.  So I'm thinking that may be a better approach than checking
> on assignment, unless there are intermediary ways you could get this
> assignment to happen in C.
>
>
Well, you can write something like "*a = *b;", which doesn't involve
declaring a variable...

-Eli
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