[Mesa-dev] question about container_of

[Pekka Paalanen]

On Mon, 27 Feb 2017 13:26:11 +0000
Emil Velikov <emil.l.velikov at gmail.com> wrote:

> Hi Julien,
> 
> On 27 February 2017 at 12:08, Julien Isorce <julien.isorce at gmail.com> wrote:
> > Hi,
> >
> > Since 2012 commit ccff74971203b533bf16b46b49a9e61753f75e6c it is said:
> > "sample must be initialized, or else the result is undefined" in the
> > description of mesa/src/util/list.h::container_of .
> >
> > But I can find a few places where it is used without initializing that
> > second parameter, i.e. like:
> >
> > struct A a;
> > container_of(ptr, a, member);
> >
> > Then I can add the "= NULL" but should it be just
> > container_of(ptr, struct A, member);
> > like in the kernel and some other places in mesa ?
> >  
> Strictly peaking these are toolchain (ASAN iirc) bugs, since there is
> no pointer deref, as we're doing pointer arithmetic.

Hi Emil,

that's what people would usually think. It used to work with GCC. Then
came Clang.

> Afaict the general decision was to to merge the patch(es) since they
> will make actual bugs stand out amongst the noise. In the long run,
> it's better to fix the tool (ASAN/other) than trying to "fix" all the
> cases in mesa and dozens of other projects. But until then patches are
> safe enough ;-)
> 
> That's my take on it, at least.

It depends on how container_of() has been defined. For more details,
see e.g.:
https://cgit.freedesktop.org/wayland/wayland/commit/?id=a18e34417ba3fefeb81d891235e8ebf394a20a74

The comment in the code in Mesa is correct for the fallback
implementation it has. Maybe things rely on the fallback implementation
never being used when compiling with Clang.

Julien, some alternatives for container_of use a pointer, others expect
a type instead. I believe one must use the correct form.


Thanks,
pq
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