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

["Russell King (Oracle)" <linux@armlinux.org.uk>]

On Mon, May 30, 2022 at 12:33:17PM +0300, Jani Nikula wrote:
> On Mon, 30 May 2022, Jani Nikula <jani.nikula@intel.com> wrote:
> > 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.
> 
> That is, for EDID. Makes you wonder about all the other packed structs
> with enum members across the kernel.

enum should not be used in structures if the layout of the struct
matters. ISTR there was a proposal for EABI to make enums just about
big enough to hold their enumerated constants - so you'd end up with
8-bit, 16-bit etc according to the largest enumerated value that the
compiler thinks it could hold.

That's a latent disaster when enums get used in structs where the
layout matters, __packed or not.

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