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Date: Sun, 17 Mar 2002 22:17:39 -0800
From: Alfred Perlstein <alfred@freebsd.org>
To: Dag-Erling Smorgrav <des@ofug.org>
Cc: Kris Kennaway <kris@obsecurity.org>, current@FreeBSD.org,
	fs@FreeBSD.org
Subject: Re: panic: bwrite: buffer is not busy???
Message-ID: <20020318061739.GB894@elvis.mu.org>
References: <20020317124958.A34008@xor.obsecurity.org> <xzpadt6r1xr.fsf@flood.ping.uio.no>
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* Dag-Erling Smorgrav <des@ofug.org> [020317 19:27] wrote:
> 
> ...the process has no open files at all, because...
> 
> (kgdb) p p->p_pid
> $4 = 10099
> (kgdb) p p->p_comm
> $5 = "wc\000oot", '\000' <repeats 13 times>
> (kgdb) p p->p_stat
> $6 = 3
> (kgdb) p/x p->p_flag
> $7 = 0x6000
> 
> ...it's exiting, and fdfree() has already run.
> 
> Solution: p->p_fd must be protected by p's proc lock; fdfree() must
> set it to NULL immediately after freeing it; checkdirs() must lock
> each process before examining its fd list.
> 
> Other problem spotted while investigating this: fdfree() can fail
> silently; fdfree() should panic if fdp->fd_refcnt is non-zero.

Please let me know if this works for you.

Index: vfs_syscalls.c
===================================================================
RCS file: /home/ncvs/src/sys/kern/vfs_syscalls.c,v
retrieving revision 1.231
diff -u -r1.231 vfs_syscalls.c
--- vfs_syscalls.c	12 Mar 2002 04:00:10 -0000	1.231
+++ vfs_syscalls.c	18 Mar 2002 06:23:41 -0000
@@ -451,10 +451,14 @@
 		return;
 	sx_slock(&allproc_lock);
 	LIST_FOREACH(p, &allproc, p_list) {
+		PROC_LOCK(p);
 		fdp = p->p_fd;
-		if (fdp == NULL)
+		if (fdp == NULL) {
+			PROC_UNLOCK(p);
 			continue;
+		}
 		FILEDESC_LOCK(fdp);
+		PROC_UNLOCK(p);
 		if (fdp->fd_cdir == olddp) {
 			VREF(newdp);
 			fdp->fd_cdir = newdp;
Index: kern_descrip.c
===================================================================
RCS file: /home/ncvs/src/sys/kern/kern_descrip.c,v
retrieving revision 1.128
diff -u -r1.128 kern_descrip.c
--- kern_descrip.c	15 Mar 2002 08:03:46 -0000	1.128
+++ kern_descrip.c	18 Mar 2002 06:23:39 -0000
@@ -1321,19 +1321,26 @@
 fdfree(td)
 	struct thread *td;
 {
-	register struct filedesc *fdp = td->td_proc->p_fd;
+	register struct filedesc *fdp;
 	struct file **fpp;
 	register int i;
 
+	PROC_LOCK(td);
+	fdp = td->td_proc->p_fd;
 	/* Certain daemons might not have file descriptors. */
-	if (fdp == NULL)
+	if (fdp == NULL) {
+		PROC_UNLOCK(td);
 		return;
+	}
 
 	FILEDESC_LOCK(fdp);
 	if (--fdp->fd_refcnt > 0) {
 		FILEDESC_UNLOCK(fdp);
+		PROC_UNLOCK(td);
 		return;
 	}
+	td->td_proc->p_fd = NULL;
+	PROC_UNLOCK(td);
 	/*
 	 * we are the last reference to the structure, we can
 	 * safely assume it will not change out from under us.

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