linux-kernel - Re: [PATCH 1/3] drm/panfrost: Simplify lock

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Date:   Mon, 23 Aug 2021 17:09:08 -0400
From:   Alyssa Rosenzweig <alyssa@...labora.com>
To:     Steven Price <steven.price@....com>
Cc:     Alyssa Rosenzweig <alyssa.rosenzweig@...labora.com>,
        dri-devel@...ts.freedesktop.org, Rob Herring <robh@...nel.org>,
        Tomeu Vizoso <tomeu.vizoso@...labora.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>, linux-kernel@...r.kernel.org,
        Chris Morgan <macromorgan@...mail.com>, stable@...r.kernel.org
Subject: Re: [PATCH 1/3] drm/panfrost: Simplify lock_region calculation

> > In lock_region, simplify the calculation of the region_width parameter.
> > This field is the size, but encoded as log2(ceil(size)) - 1.
> > log2(ceil(size)) may be computed directly as fls(size - 1). However, we
> > want to use the 64-bit versions as the amount to lock can exceed
> > 32-bits.
> > 
> > This avoids undefined behaviour when locking all memory (size ~0),
> > caught by UBSAN.
> 
> It might have been useful to mention what it is that UBSAN specifically
> picked up (it took me a while to spot) - but anyway I think there's a
> bigger issue with it being completely wrong when size == ~0 (see below).

Indeed. I've updated the commit message in v2 to explain what goes
wrong (your analysis was spot on, but a mailing list message is more
ephermal than a commit message). I'll send out v2 tomorrow assuming
nobody objects to v1 in the mean time.

Thanks for the review.

> There is potentially a third bug which kbase only recently attempted to
> fix. The lock address is effectively rounded down by the hardware (the
> bottom bits are ignored). So if you have mask=(1<<region_width)-1 but
> (iova & mask) != ((iova + size) & mask) then you are potentially failing
> to lock the end of the intended region. kbase has added some code to
> handle this:
> 
> > 	/* Round up if some memory pages spill into the next region. */
> > 	region_frame_number_start = pfn >> (lockaddr_size_log2 - PAGE_SHIFT);
> > 	region_frame_number_end =
> > 	    (pfn + num_pages - 1) >> (lockaddr_size_log2 - PAGE_SHIFT);
> > 
> > 	if (region_frame_number_start < region_frame_number_end)
> > 		lockaddr_size_log2 += 1;
> 
> I guess we should too?

Oh, I missed this one. Guess we have 4 bugs with this code instead of
just 3, yikes. How could such a short function be so deeply and horribly
broken? 😃

Should I add a fourth patch to the series to fix this?