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From:   Al Viro <viro@zeniv.linux.org.uk>
To:     "Eric W. Biederman" <ebiederm@xmission.com>
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Subject: Re: [PATCH v2 15/24] proc/fd: In proc_readfd_common use
 task_lookup_next_fd_rcu
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On Tue, Dec 08, 2020 at 04:38:09PM -0600, Eric W. Biederman wrote:

> Is there any reason we don't simply rcu free the files_struct?
> That would remove the need for the task_lock entirely.

Umm...  Potentially interesting part here is the interaction with
close_files(); currently that can't overlap with any of those
3rd-party accesses to descriptor table, but with your changes
here it's very much possible.

OTOH, it's not like close_files() did much beyond the effects of already
possible close(2) done by one of the threads sharing that sucker.
It's _probably_ safe (at least for proc_readfd_common()), but I'll need
to look at the other places doing that kind of access.  Especially the
BPF foulness...

Oh, and in any case, the trick with repurposing ->rcu of embedded
fdtable deserves a comment.  It's not hard to explain, so...