Messages in this thread | | | Subject | Re: [PATCH v2 4.9 00/10] fix a race in release_task when flushing the dentry | From | Wen Yang <> | Date | Fri, 8 Jan 2021 00:21:38 +0800 |
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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
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