[PATCH] diagnosing flexible array assignments
eli.friedman at gmail.com
Tue Sep 10 14:40:37 PDT 2013
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>
> > 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
> > 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:
Do you have any idea what the false positive rate is?
> > Have you considered warning about the variable declaration rather than
> > assignment? In your testcase, it's impossible to call foo() without
> > triggering the warning, so it seems better to warn about foo() itself
> > 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...
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