[PATCH] drm/i915: Prefer struct_size over open coded arithmetic

[Daniel Vetter]

On Wed, Oct 13, 2021 at 02:24:05PM +0300, Jani Nikula wrote:
> On Mon, 11 Oct 2021, Len Baker <len.baker at gmx.com> wrote:
> > Hi,
> >
> > On Sun, Oct 03, 2021 at 12:42:58PM +0200, Len Baker wrote:
> >> As noted in the "Deprecated Interfaces, Language Features, Attributes,
> >> and Conventions" documentation [1], size calculations (especially
> >> multiplication) should not be performed in memory allocator (or similar)
> >> function arguments due to the risk of them overflowing. This could lead
> >> to values wrapping around and a smaller allocation being made than the
> >> caller was expecting. Using those allocations could lead to linear
> >> overflows of heap memory and other misbehaviors.
> >>
> >> In this case these are not actually dynamic sizes: all the operands
> >> involved in the calculation are constant values. However it is better to
> >> refactor them anyway, just to keep the open-coded math idiom out of
> >> code.
> >>
> >> So, add at the end of the struct i915_syncmap a union with two flexible
> >> array members (these arrays share the same memory layout). This is
> >> possible using the new DECLARE_FLEX_ARRAY macro. And then, use the
> >> struct_size() helper to do the arithmetic instead of the argument
> >> "size + count * size" in the kmalloc and kzalloc() functions.
> >>
> >> Also, take the opportunity to refactor the __sync_seqno and __sync_child
> >> making them more readable.
> >>
> >> This code was detected with the help of Coccinelle and audited and fixed
> >> manually.
> >>
> >> [1] https://www.kernel.org/doc/html/latest/process/deprecated.html#open-coded-arithmetic-in-allocator-arguments
> >>
> >> Signed-off-by: Len Baker <len.baker at gmx.com>
> >> ---
> >>  drivers/gpu/drm/i915/i915_syncmap.c | 12 ++++++++----
> >>  1 file changed, 8 insertions(+), 4 deletions(-)
> >
> > I received a mail telling that this patch doesn't build:
> >
> > == Series Details ==
> >
> > Series: drm/i915: Prefer struct_size over open coded arithmetic
> > URL   : https://patchwork.freedesktop.org/series/95408/
> > State : failure
> >
> > But it builds without error against linux-next (tag next-20211001). Against
> > which tree and branch do I need to build?
> 
> drm-tip [1]. It's a sort of linux-next for graphics. I think there are
> still some branches that don't feed to linux-next.

Yeah we need to get gt-next in linux-next asap. Joonas promised to send
out his patch to make that happen in dim.
-Daniel

> 
> BR,
> Jani.
> 
> 
> [1] https://cgit.freedesktop.org/drm/drm-tip
> 
> 
> >
> > Regards,
> > Len
> 
> -- 
> Jani Nikula, Intel Open Source Graphics Center

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch