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
