Date: Fri, 4 Sep 2020 18:27:48 -0600
From: Alan Somers <asomers@freebsd.org>
To: FreeBSD Hackers <freebsd-hackers@freebsd.org>
Subject: panic!("docallb") in nfsrv_docallback
Message-ID: <CAOtMX2h3ZTVaZB8bdMg3QeGXCLtJLAuOCEQQm-WNTVaJz1HEDA@mail.gmail.com>
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I just saw this panic on a 12-stable machine. Unfortunately, I don't have a core dump, just a stack trace. It was serving NFS v4.0, with delegations enabled. The clients were all Debian, with Linux 3.16.0. The proximal cause of the panic seems to be that the file had a write delegation issued to an unconfirmed client. Root cause is harder to determine. Did the kernel previously issue a delegation to an unconfirmed client? Or did the client somehow change to an unconfirmed state after the delegation was issued, perhaps due to a race? It's hard to tell, but I don't see any checks for lc_flags & LCL_NEEDSCONFIRM in nfsrv_openctrl (which issues the delegations), so I'm guessing that that's the problem. If so, then the event trace would look like this: 1) Client Alice sends SETCLIENTID. The server creates a client state structure for her. _) Client Alice should've sent SETCLIENTID_CONFIRM, but doesn't. Bad Alice! 2) Client Alice sends OPEN for some file, and is issued a write delegation. The server shouldn't have issued it, because Alice's client ID is unconfirmed. Bad server! 3) Client Bob tries to do a GETATTR on that same file. 4) In nfsrv_checkgetattr, the kernel finds a write delegation for that file, owned by client Alice. 5) The kernel tries to send a NFSV4OP_CBGETATTR callback to Alice, to see if the file's attributes have changed. 6) But Alice's client ID is unconfirmed. Oh no! Panic! Does this sound plausible? Should there be a check for LCL_NEEDSCONFIRM somewhere around line 3166 in nfs_nfsdstate.c? Grateful for any help. -Alan P.S.: stack trace kdb_backtrace vpanic panic nfsrv_docallback nfsrv_checkgetattr nfsrvd_getattr nfsrvd_dorpc nfssvc_program svc_run_internal svc_thread_start fork_exit fork_trampoline
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