On Mon, May 30, 2022 at 12:33:17PM +0300, Jani Nikula wrote: > On Mon, 30 May 2022, Jani Nikula <jani.nikula@xxxxxxxxx> wrote: > > On Sat, 28 May 2022, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > >> On Sat, May 28, 2022 at 11:59 AM Arnd Bergmann <arnd@xxxxxxxx> 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. -- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!
