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From: Rob Herring <robh@kernel.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Guenter Roeck <linux@roeck-us.net>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Frank Rowand <frowand.list@gmail.com>
Subject: Re: Linux 5.12-rc5
Date: Mon, 29 Mar 2021 13:41:42 -0500	[thread overview]
Message-ID: <CAL_JsqKxLy3Gc8d1Q23AQaWTKLmc_a28tokZZ08rHnV2qU0iew@mail.gmail.com> (raw)
In-Reply-To: <CAHk-=wiMoP9PifpuUnQ3xmAM_LmGARr+fxFuOSX1rvh2mz35Mw@mail.gmail.com>

n Mon, Mar 29, 2021 at 1:05 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Sun, Mar 28, 2021 at 7:07 PM Guenter Roeck <linux@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-linux@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.

> 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

  reply	other threads:[~2021-03-29 18:43 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-03-28 23:05 Linus Torvalds
2021-03-29  2:07 ` Guenter Roeck
2021-03-29 18:05   ` Linus Torvalds
2021-03-29 18:41     ` Rob Herring [this message]
2021-03-29 19:04       ` Frank Rowand
2021-03-29 19:42     ` Guenter Roeck

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