Re: [patch] mm, oom: remove task_lock protecting comm printing

Sergey Senozhatsky
2015-09-23T09:57:11Z

On (09/23/15 18:50), Sergey Senozhatsky wrote:
> On (09/23/15 11:43), Michal Hocko wrote:
> [..]
> > > > the previous name was already null terminated,
> > >
> > > Yeah, but if the old name is shorter than the new one, set_task_comm()
> > > overwrites the terminating null of the old name before writing the new
> > > terminating null, so there is a short time window during which tsk->comm
> > > might be not null-terminated, no?
> >
> > Not really:
> > case PR_SET_NAME:
> > comm[sizeof(me->comm) - 1] = 0;
> > if (strncpy_from_user(comm, (char __user *)arg2,
> > sizeof(me->comm) - 1) < 0)
> > return -EFAULT;
> >
> > So it first writes the terminating 0 and only then starts copying.
>
> right.

... no right. that should have been

me->comm[sizeof(me->comm) - 1] = 0;

to be save. no?

> hm, shouldn't set_task_comm()->__set_task_comm() do the same?

or something like this instead

---

fs/exec.c | 1 +
1 file changed, 1 insertion(+)

diff --git a/fs/exec.c b/fs/exec.c
index b06623a..d7d2de0 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1072,6 +1072,7 @@ EXPORT_SYMBOL_GPL(get_task_comm);

void __set_task_comm(struct task_struct *tsk, const char *buf, bool exec)
{
+ tsk->comm[sizeof(tsk->comm) - 1] = 0;
task_lock(tsk);
trace_task_rename(tsk, buf);
strlcpy(tsk->comm, buf, sizeof(tsk->comm));