Re: [PATCH 1/1 v2] kernel/fork.c: avoid division by zero

Guenter Roeck
2015-02-18T19:39:15Z

On Tue, Feb 17, 2015 at 03:15:11PM -0800, Andrew Morton wrote:
> On Tue, 17 Feb 2015 20:01:38 +0100 Heinrich Schuchardt <xypron.glpk@gmx.de> 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.
> >
> > The futex implementation assumes that tids fit into the FUTEX_TID_MASK.
> > This limits the number of allowable threads.
> >
> > ...
> >
> > --- a/kernel/fork.c
> > +++ b/kernel/fork.c
> > @@ -74,6 +74,7 @@
> > #include <linux/uprobes.h>
> > #include <linux/aio.h>
> > #include <linux/compiler.h>
> > +#include <linux/math64.h>
> >
> > #include <asm/pgtable.h>
> > #include <asm/pgalloc.h>
> > @@ -255,6 +256,8 @@ void __init __weak arch_task_cache_init(void) { }
> >
> > void __init fork_init(unsigned long mempages)
> > {
> > + u64 temp;
>
> That's a really poor name. We should always try to make names
> meaningful. Here, something like "threads" would be better.
>
> > #ifndef CONFIG_ARCH_TASK_STRUCT_ALLOCATOR
> > #ifndef ARCH_MIN_TASKALIGN
> > #define ARCH_MIN_TASKALIGN L1_CACHE_BYTES
> > @@ -273,7 +276,16 @@ 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);
> > + temp = div64_u64((u64) mempages * (u64) PAGE_SIZE,
> > + (u64) THREAD_SIZE * 8UL);
> > +
> > + /*
> > + * The futex code assumes that tids fit into the FUTEX_TID_MASK.
> > + */
> > + if (temp < FUTEX_TID_MASK)
> > + max_threads = temp;
> > + else
> > + max_threads = FUTEX_TID_MASK;
>
> Seems rather complicated. How about
>
> max_threads = mempages / (8 * THREAD_SIZE);

Apparently it is possible that
mempages < 8 * THREAD_SIZE
which would result in max_threads == 0.

How about the following ?

max_threads = mempages / DIV_ROUND_UP(8 * THREAD_SIZE, PAGE_SIZE);

Guenter