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

[Jani Nikula <jani.nikula@intel.com>]

On Sat, 28 May 2022, Linus Torvalds <torvalds@linux-foundation.org> wrote:
> On Sat, May 28, 2022 at 11:59 AM Arnd Bergmann <arnd@arndb.de> wrote:
>>
>> It's CONFIG_ARM_AEABI, which is normally set everywhere. Without this
>> option, you the kernel is built for the old 'OABI' that forces all non-packed
>> struct members to be at least 16-bit aligned.
>
> Looks like forced word (32 bit) alignment to me.
>
> I wonder how many other structures that messes up, but I committed the
> EDID fix for now.

Thanks for the fix, and the thorough commit message!

> This has presumably been broken for a long time, but maybe the
> affected targets don't typically use EDID and kernel modesetting, and
> only use some fixed display setup instead.
>
> Those structure definitions go back a _loong_ time (from a quick 'git
> blame' I see November 2008).
>
> But despite that, I did not mark my fix 'cc:stable' because I don't
> know if any of those machines affected by this bad arm ABI issue could
> possibly care.
>
> At least my tree hopefully now builds on them, with the BUILD_BUG_ON()
> that uncovered this.

Indeed the bug is ancient. I just threw in the BUILD_BUG_ON() on a whim
as an extra sanity check when doing pointer arithmetics on struct edid
*.

If there are affected machines, buffer overflows are the real danger due
to edid->extensions indicating the number of extensions.


BR,
Jani.

-- 
Jani Nikula, Intel Open Source Graphics Center