Re: [PATCH] xen/public: fix flexible array definitions

[Juergen Gross <jgross@xxxxxxxx>]

On 26.07.23 07:52, Jan Beulich wrote:
> On 25.07.2023 18:59, Andrew Cooper wrote:
> > On 25/07/2023 5:16 pm, Jan Beulich wrote:
> > > On 25.07.2023 15:55, Juergen Gross wrote:
> > > > Flexible arrays in public headers can be problematic with some
> > > > compilers.
> > > >
> > > > Replace them with arr[XEN_FLEX_ARRAY_DIM] in order to avoid compilation
> > > > errors.
> > > >
> > > > This includes arrays defined as "arr[1]", as seen with a recent Linux
> > > > kernel [1].
> > > >
> > > > [1]: https://bugzilla.kernel.org/show_bug.cgi?id=217693
> > > >
> > > > Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> > > I think we need to be careful here: What if someone somewhere applies
> > > sizeof() to any of the types you alter?
> > Then the code was most likely wrong already.
> That's possible to judge only when seeing the code in question.
>
> > >   The resulting value would
> > > change with the changes you propose, which we cannot allow to happen
> > > in a stable interface. Therefore imo it can only be an opt-in feature
> > > to have these arrays no longer be one-element ones.
> > I don't consider this an issue.
> >
> > If people take an update to the headers and their code stops compiling,
> > then of course they fix the compilation issue.  That's normal.
> The code may continue to compile fine, and even appear to work initially.
>
> > It's unreasonable to take opt-in features to a set of headers intended
> > to be vendored in the first place, to work around a corner case that's
> > likely buggy already.
> The original intention clearly was to allow use of these headers as is.
> Anyway, I've voiced my view, yet if there are enough people agreeing
> with you, then so be it.
Any further thoughts?

I have checked the code in the Linux kernel meanwhile. There should be no
fallout resulting from this change, but I think there are some user mode
backends outside of qemu which are probably using affected structs.


Juergen

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