On 3/29/21 1:41 PM, Rob Herring wrote: > n Mon, Mar 29, 2021 at 1:05 PM Linus Torvalds > <torva...@linux-foundation.org> wrote: >> >> On Sun, Mar 28, 2021 at 7:07 PM Guenter Roeck <li...@roeck-us.net> 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-li...@roeck-us.net/ >>> 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.
Agreed. > >> 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. Agreed to not remove. We can be properly aligned without changing kmemdup(). > >> 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.A This is not the only place a kmemdup() is used by overlays. I'll create a patch this week to fix all of the kmemdup() locations and add the null pointer check. -Frank > > Rob >
