On Tue, Sep 10, 2013 at 2:33 PM, Aaron Ballman <[email protected]>wrote:
> On Tue, Sep 10, 2013 at 5:21 PM, Eli Friedman <[email protected]> > wrote: > > On Tue, Sep 10, 2013 at 2:03 PM, Aaron Ballman <[email protected]> > > 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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