Re: [PATCH 3/3] kernel/sysctl.c: threads-max observe limits

David Rientjes
2015-02-24T21:17:10Z

On Tue, 24 Feb 2015, Heinrich Schuchardt wrote:

> Users can change the maximum number of threads by writing to
> /proc/sys/kernel/threads-max.
>
> With the patch the value entered is checked against the same
> limits that apply when fork_init is called.
>

Correct me if I'm wrong, but this is a change in functionality (without
update to Documentation/sysctl/kernel.txt which describes threads-max)
since it does not allow the value to be lowered as before from the
calculation involving totalram_pages. The value passed to
set_max_threads() only caps the value.

> Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
> ---
> include/linux/sysctl.h | 3 +++
> kernel/fork.c | 24 ++++++++++++++++++++++++
> kernel/sysctl.c | 6 ++----
> 3 files changed, 29 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
> index b7361f8..795d5fe 100644
> --- a/include/linux/sysctl.h
> +++ b/include/linux/sysctl.h
> @@ -212,4 +212,7 @@ static inline void setup_sysctl_set(struct ctl_table_set *p,
>
> #endif /* CONFIG_SYSCTL */
>
> +int sysctl_max_threads(struct ctl_table *table, int write,
> + void __user *buffer, size_t *lenp, loff_t *ppos);
> +
> #endif /* _LINUX_SYSCTL_H */
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 880c78d..52a07c7 100644
> --- 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/sysctl.h>
>
> #include <asm/pgtable.h>
> #include <asm/pgalloc.h>
> @@ -2024,3 +2025,26 @@ int unshare_files(struct files_struct **displaced)
> task_unlock(task);
> return 0;
> }
> +
> +int sysctl_max_threads(struct ctl_table *table, int write,
> + void __user *buffer, size_t *lenp, loff_t *ppos)
> +{
> + struct ctl_table t;
> + int ret;
> + int threads = max_threads;
> + int min = MIN_THREADS;
> + int max = MAX_THREADS;
> +
> + t = *table;
> + t.data = &threads;
> + t.extra1 = &min;
> + t.extra2 = &max;
> +
> + ret = proc_dointvec_minmax(&t, write, buffer, lenp, ppos);
> + if (ret || !write)
> + return ret;
> +
> + set_max_threads(threads);
> +
> + return 0;
> +}
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index 137c7f6..2e195ae 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -92,11 +92,9 @@
> #include <linux/nmi.h>
> #endif
>
> -
> #if defined(CONFIG_SYSCTL)
>
> /* External variables not in a header file. */
> -extern int max_threads;
> extern int suid_dumpable;
> #ifdef CONFIG_COREDUMP
> extern int core_uses_pid;
> @@ -709,10 +707,10 @@ static struct ctl_table kern_table[] = {
> #endif
> {
> .procname = "threads-max",
> - .data = &max_threads,
> + .data = NULL,
> .maxlen = sizeof(int),
> .mode = 0644,
> - .proc_handler = proc_dointvec,
> + .proc_handler = sysctl_max_threads,
> },
> {
> .procname = "random",