>On Tue, Jun 09, 2020 at 11:08:01AM +0800, Weilong Chen wrote:
>> The check is only there to stop root fork bombs.
>>
>> BUG: KCSAN: data-race in copy_process / copy_process
>>
>> write to 0xffffffff86f87d20 of 4 bytes by task 7121 on cpu 5:
>> copy_process+0x2e1a/0x3af0 kernel/fork.c:2285
>> _do_fork+0xf7/0x790 kernel/fork.c:2430
>> __do_sys_clone+0xf9/0x130 kernel/fork.c:2585 __se_sys_clone
>> kernel/fork.c:2566 [inline]
>> __x64_sys_clone+0x6c/0x80 kernel/fork.c:2566
>> do_syscall_64+0xc7/0x3b0 arch/x86/entry/common.c:295
>> entry_SYSCALL_64_after_hwframe+0x44/0xa9
>>
>> read to 0xffffffff86f87d20 of 4 bytes by task 7125 on cpu 3:
>> copy_process+0x9eb/0x3af0 kernel/fork.c:1967
>> _do_fork+0xf7/0x790 kernel/fork.c:2430
>> __do_sys_clone+0xf9/0x130 kernel/fork.c:2585 __se_sys_clone
>> kernel/fork.c:2566 [inline]
>> __x64_sys_clone+0x6c/0x80 kernel/fork.c:2566
>> do_syscall_64+0xc7/0x3b0 arch/x86/entry/common.c:295
>> entry_SYSCALL_64_after_hwframe+0x44/0xa9
>>
>> Signed-off-by: Weilong Chen <[email protected]>
>
>Plumbing data_race() in there just to taper over this seems ugly.
>Before we do that we should probably simply make nr_threads atomic_t.
Will using atomic_t cause performance degradation ? I don’t know why atomic was
not used in the beginning.
>Also, where's the link to the syzbot/kcsan report? Or did you get this report
>from somewhere else?
I got this from local test.
>
>diff --git a/kernel/exit.c b/kernel/exit.c index c300253a7b8e..42e1cf640b20
>100644
>--- a/kernel/exit.c
>+++ b/kernel/exit.c
>@@ -71,7 +71,7 @@
>
> static void __unhash_process(struct task_struct *p, bool group_dead) {
>- nr_threads--;
>+ atomic_dec(&nr_threads);
> detach_pid(p, PIDTYPE_PID);
> if (group_dead) {
> detach_pid(p, PIDTYPE_TGID); diff --git a/kernel/fork.c
> b/kernel/fork.c index cefe8745c46e..c8355448d7c6 100644
>--- a/kernel/fork.c
>+++ b/kernel/fork.c
>@@ -122,7 +122,7 @@
> * Protected counters by write_lock_irq(&tasklist_lock)
> */
> unsigned long total_forks; /* Handle normal Linux uptimes. */
>-int nr_threads; /* The idle threads do not count.. */
>+atomic_t nr_threads; /* The idle threads do not count.. */
>
> static int max_threads; /* tunable limit on nr_threads */
>
>@@ -1978,7 +1978,7 @@ static __latent_entropy struct task_struct *copy_process(
> * to stop root fork bombs.
> */
> retval = -EAGAIN;
>- if (nr_threads >= max_threads)
>+ if (atomic_read(&nr_threads) >= max_threads)
> goto bad_fork_cleanup_count;
>
> delayacct_tsk_init(p); /* Must remain after dup_task_struct() */ @@
> -2296,7 +2296,7 @@ static __latent_entropy struct task_struct *copy_process(
> &p->signal->thread_head);
> }
> attach_pid(p, PIDTYPE_PID);
>- nr_threads++;
>+ atomic_inc(&nr_threads);
> }
> total_forks++;
> hlist_del_init(&delayed.node);
>
>> ---
>> kernel/fork.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/kernel/fork.c b/kernel/fork.c index
>> 142b23645d82..efc5493203ae 100644
>> --- a/kernel/fork.c
>> +++ b/kernel/fork.c
>> @@ -1977,7 +1977,7 @@ static __latent_entropy struct task_struct
>> *copy_process(
>> * to stop root fork bombs.
>> */
>> retval = -EAGAIN;
>> - if (nr_threads >= max_threads)
>> + if (data_race(nr_threads >= max_threads))
>> goto bad_fork_cleanup_count;
>>
>> delayacct_tsk_init(p); /* Must remain after dup_task_struct() */
>> --
>> 2.17.1
>>
>
