Re: mainline build failure due to f1e4c916f97f ("drm/edid: add EDID block count and size helpers")

[Arnd Bergmann]

On Tue, May 31, 2022 at 8:26 AM Julia Lawall <Julia.Lawall@xxxxxxxx> wrote:
> > On 30 May 2022, at 15:27, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> > On Mon, May 30, 2022 at 4:08 PM Jani Nikula <jani.nikula@xxxxxxxxx> wrote:
> >>> On Mon, 30 May 2022, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> >>> struct my_driver_priv {
> >>>       struct device dev;
> >>>       u8 causes_misalignment;
> >>>       spinlock_t lock;
> >>>       atomic_t counter;
> >>> } __packed; /* this annotation is harmful because it breaks the atomics */
> >>
> >> I wonder if this is something that could be caught with coccinelle. Or
> >> sparse. Are there any cases where this combo is necessary? (I can't
> >> think of any, but it's a low bar. ;)
> >>
...
> >>> or if the annotation does not change the layout like
> >>>
> >>> struct my_dma_descriptor {
> >>>     __le64 address;
> >>>     __le64 length;
> >>> } __packed; /* does not change layout but makes access slow on some
> >>> architectures */
> >>
> >> Why is this the case, though? I'd imagine the compiler could figure this
> >> out.
> >
> > When you annotate the entire structure as __packed without an
> > extra __aligned() annotation, the compiler has to assume that the
> > structure itself is unaligned as well. On many of the older architectures,
> > this will result in accessing the values one byte at a time. Marking
> > the structure as "__packed __aligned(8)" instead would be harmless.
> >
> > When I have a structure with a few misaligned members, I generally
> > prefer to only annotate the members that are not naturally aligned,
> > but this approach is not very common.
>
> Searching for specific types in a packed structure would be easy.

As an experiment: what kind of results would we get when looking
for packed structures and unions that contain any of these:

- spinlock_t
- atomic_t
- dma_addr_t
- phys_addr_t
- size_t
- any pointer
- any enum
- struct mutex
- struct device

This is just a list of common data types that are used in a lot of
structures but that one should never find in hardware specific
types. If the output from coccinelle is 90% actual bugs, this would
be really helpful. OTOH if there is no output at all, or all
false-positives, we don't need to look for additional types.

> Coccinelle could duplicate the structure without the packed and see if
> any offsets change, using build bug on, but that would be architecture
> specific so maybe not useful.

I would consider this a separate issue. The first one above is for identifying
structures that are marked as packed but should not be packed at
all, regardless of whether that changes the layout.

       Arnd