Re: Linux 5.12-rc5

[Rob Herring]

n Mon, Mar 29, 2021 at 1:05 PM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Sun, Mar 28, 2021 at 7:07 PM Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
> >
> > This is not really a new problem. I enabled devicetree unit tests
> > in the openrisc kernel and was rewarded with a crash.
> > https://lore.kernel.org/lkml/20210327224116.69309-1-linux@xxxxxxxxxxxx/
> > has all the glorious details.
>
> Hmm.
>
> I'm not sure I love that patch.
>
> I don't think the patch is _wrong_ per se, but if that "require 8 byte
> alignment" is a problem, then this seems to be papering over the issue
> rather than fixing it.
>
> So your patch protects from a NULL pointer dereference, but the
> underlying issue seems to be a regression, and the fix sounds like the
> kernel shouldn't be so strict about alignment requirements.

In the interest of the DT unittests not panicking and halting boot, I
think we should handle NULL pointer.

> I guess we could make ARCH_SLAB_MINALIGN be at least 8 (perhaps only
> if the allocations is >= 8) but honestly, I don't think libfdt merits
> making such a big change. Small allocations are actually not uncommon
> in the kernel, and on 32-bit architectures I think 4-byte allocations
> are normal.
>
> So I'd be inclined to just remove the new
>
>         /* The device tree must be at an 8-byte aligned address */
>         if ((uintptr_t)fdt & 7)
>                 return -FDT_ERR_ALIGNMENT;
>
> check in scripts/dtc/libfdt/fdt.c which I assume is the source of the
> problem. Rob?

That is the source, but I'd rather not remove it as we try to avoid
any modifications from upstream. And we've found a couple of cases of
not following documented alignment requirements.

> Your patch to then avoid the NULL pointer dereference seems to be then
> an additional safety, but not fixing the actual regression.

I think the right fix is not using kmemdup which copies the unittest dtb.

Rob