Messages in this thread | | | Date | Wed, 18 Feb 2015 11:38:14 -0800 | From | Guenter Roeck <> | Subject | Re: [PATCH 1/1 v2] kernel/fork.c: avoid division by zero |
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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
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