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

Heinrich Schuchardt
2015-02-24T21:31:41Z

On 24.02.2015 22:17, David Rientjes wrote:
> 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.

threads_max is set to the value the user inputs if it is inside the
interval [20, FUTEX_TID_MASK] and it is capped by the value calculated
from totalram_pages.

So lowering and raising is still possible (inside the limits).

>
>> 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",
>
Best regards

Heinrich