Re: [PATCH 2/3 v5] kernel/fork.c: avoid division by zero

David Rientjes
2015-02-24T21:14:38Z

On Tue, 24 Feb 2015, 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 in the calculation of max_threads.
>

This should only appear in one patch in the series, and I think this is
the appropriate patch for that to happen.

> With 32-bit calculation there is no solution which delivers valid results
> for all possible combinations of the parameters.
> The code is only called once.
> Hence a 64-bit calculation can be used as solution.
>
> Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
> ---
> kernel/fork.c | 38 +++++++++++++++++++++++++++-----------
> 1 file changed, 27 insertions(+), 11 deletions(-)
>
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 460b044..880c78d 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -88,6 +88,16 @@
> #include <trace/events/task.h>
>
> /*
> + * Minimum number of threads to boot the kernel
> + */
> +#define MIN_THREADS 20
> +
> +/*
> + * Maximum number of threads
> + */
> +#define MAX_THREADS FUTEX_TID_MASK
> +
> +/*
> * Protected counters by write_lock_irq(&tasklist_lock)
> */
> unsigned long total_forks; /* Handle normal Linux uptimes. */
> @@ -254,23 +264,29 @@ EXPORT_SYMBOL_GPL(__put_task_struct);
> void __init __weak arch_task_cache_init(void) { }
>
> /*
> - * set_max_threads
> - * The argument is ignored.
> + * set_max_threads tries to set the default limit to the suggested value.

I'm not sure that is true. At this point in the patch series,
set_max_threads() is only called with UINT_MAX and that's not what the
implementation tries to set max_threads to.

> */
> static void set_max_threads(unsigned int max_threads_suggested)
> {
> - /*
> - * The default maximum number of threads is set to a safe
> - * value: the thread structures can take up at most half
> - * of memory.
> - */
> - max_threads = totalram_pages / (8 * THREAD_SIZE / PAGE_SIZE);
> + u64 threads;
>
> /*
> - * we need to allow at least 20 threads to boot a system
> + * The number of threads shall be limited such that the thread
> + * structures may only consume a small part of the available memory.
> */
> - if (max_threads < 20)
> - max_threads = 20;
> + threads = div64_u64((u64) totalram_pages * (u64) PAGE_SIZE,
> + (u64) THREAD_SIZE * 8UL);

The artithmetic works here, but I'm wondering what guarantee is in place
to ensure that totalram_pages * PAGE_SIZE does not overflow on 64-bit
arches?

> +
> + if (threads > max_threads_suggested)
> + threads = max_threads_suggested;

Ok, so now the function argument just shows an implementation issue.
You're capping threads at UINT_MAX because you need max_threads to be an
int and relying on the caller to enforce that. set_max_threads() should
be handling all of these details itself.

> +
> + if (threads > MAX_THREADS)
> + threads = MAX_THREADS;
> +
> + if (threads < MIN_THREADS)
> + threads = MIN_THREADS;
> +
> + max_threads = (int) threads;
> }
>
> void __init fork_init(void)