Messages in this thread | | | Date | Wed, 18 Feb 2015 12:23:55 -0800 | From | Guenter Roeck <> | Subject | Re: [PATCH 1/1 v2] kernel/fork.c: avoid division by zero |
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On Wed, Feb 18, 2015 at 08:50:21PM +0100, Heinrich Schuchardt wrote: > On 18.02.2015 20:38, Guenter Roeck wrote: > > 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); > > For PAGE_SIZE 2MB and THREAD_SIZE 4kB this is wrong by factor 64. > That really depends on the definition of "wrong", doesn't it ? It would ensure that max_threads is never larger than mempages, which is more defensive.
One could also use max_threads = mempages / (max(1, 8 * THREAD_SIZE / PAGE_SIZE)); to solve the original problem without introducing 64 bit operations and without affecting the result for more common architectures.
Guenter
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