Re: [PATCH v2 4.9 00/10] fix a race in release_task when flushing the dentry

[Wen Yang]

在 2021/1/7 下午8:17, Greg Kroah-Hartman 写道:
> On Thu, Jan 07, 2021 at 03:52:12PM +0800, Wen Yang wrote:
> > The dentries such as /proc/<pid>/ns/ have the DCACHE_OP_DELETE flag, they
> > should be deleted when the process exits.
> >
> > Suppose the following race appears：
> >
> > release_task                 dput
> > -> proc_flush_task
> >                               -> dentry->d_op->d_delete(dentry)
> > -> __exit_signal
> >                               -> dentry->d_lockref.count--  and return.
> >
> > In the proc_flush_task(), if another process is using this dentry, it will
> > not be deleted. At the same time, in dput(), d_op->d_delete() can be executed
> > before __exit_signal(pid has not been hashed), d_delete returns false, so
> > this dentry still cannot be deleted.
> >
> > This dentry will always be cached (although its count is 0 and the
> > DCACHE_OP_DELETE flag is set), its parent denry will also be cached too, and
> > these dentries can only be deleted when drop_caches is manually triggered.
> >
> > This will result in wasted memory. What's more troublesome is that these
> > dentries reference pid, according to the commit f333c700c610 ("pidns: Add a
> > limit on the number of pid namespaces"), if the pid cannot be released, it
> > may result in the inability to create a new pid_ns.
> >
> > This issue was introduced by 60347f6716aa ("pid namespaces: prepare
> > proc_flust_task() to flush entries from multiple proc trees"), exposed by
> > f333c700c610 ("pidns: Add a limit on the number of pid namespaces"), and then
> > fixed by 7bc3e6e55acf ("proc: Use a list of inodes to flush from proc").
>
> Why are you just submitting a series for 4.9 and 4.19, what about 4.14?
> We can't have users move to a newer kernel and then experience old bugs,
> right?
Okay, the patches corresponding to 4.14 will be ready later.


> But the larger question is why are you backporting a whole new feature
> here?  Why is CLONE_PIDFD needed?  That feels really wrong...

The reason for backporting CLONE_PIDFD is because 7bc3e6e55acf ("proc: 
Use a list of inodes to flush from proc") relies on wait_pidfd.lock. 
There are indeed many associated modifications here. We are also testing 
it. Please check the code more.

Thanks.

--
Best wishes,
Wen