Re: [PATCH 1/1] kernel/fork.c: avoid division by zero

[Guenter Roeck]

On 02/16/2015 10:45 AM, Heinrich Schuchardt wrote:
> I am resending this mail because the linux-kernel list was not CCed.
Yes, sorry, that got lost. The "forward" feature on lkml.org doesn't work
as well as it used to, unfortunately, and I keep forgetting.

> On 16.02.2015 19:23, Guenter Roeck wrote:
> > On Mon, Feb 16, 2015 at 07:00:44PM +0100, Heinrich Schuchardt wrote:
> > > PAGE_SIZE is not guaranteed to be equal to or less than 8 times the
> > > THREAD_SIZE.
> > >
> > > E.g. architecture hexagon may have page size 1M and thread size 4096.
> > >
> > > This would lead to a division by zero.
> > >
> > > Signed-off-by: Heinrich Schuchardt <xypron.glpk@xxxxxx>
> > > ---
> > >   kernel/fork.c | 3 ++-
> > >   1 file changed, 2 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/kernel/fork.c b/kernel/fork.c
> > > index cf65139..71e2583 100644
> > > --- a/kernel/fork.c
> > > +++ b/kernel/fork.c
> > > @@ -273,7 +273,8 @@ void __init fork_init(unsigned long mempages)
> > >   	 * value: the thread structures can take up at most half
> > >   	 * of memory.
> > >   	 */
> > > -	max_threads = mempages / (8 * THREAD_SIZE / PAGE_SIZE);
> > > +	max_threads = (u64) PAGE_SIZE * (u64) mempages
> > > +		    / (u64) THREAD_SIZE / 8UL;
> >
> > How about
> > 	max_threads = mempages / (8 * THREAD_SIZE) * PAGE_SIZE;
> >
> > instead ? That would avoid the u64.
>
> We have to consider the case mempages < 8 * THREAD_SIZE.
>
> Let
>    mempages = 1024
>    THREAD_SIZE = 4096
>    PAGE_SIZE = 1M
>
> Your code gives 0.
> Mine gives 8192.
Makes sense.

Thanks,
Guenter


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