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[NET]: Fix race when opening a proc file while a network namespace is exiting.

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The problem:  proc_net files remember which network namespace the are
against but do not remember hold a reference count (as that would pin
the network namespace).   So we currently have a small window where
the reference count on a network namespace may be incremented when opening
a /proc file when it has already gone to zero.

To fix this introduce maybe_get_net and get_proc_net.

maybe_get_net increments the network namespace reference count only if it is
greater then zero, ensuring we don't increment a reference count after it
has gone to zero.

get_proc_net handles all of the magic to go from a proc inode to the network
namespace instance and call maybe_get_net on it.

PROC_NET the old accessor is removed so that we don't get confused and use
the wrong helper function.

Then I fix up the callers to use get_proc_net and handle the case case
where get_proc_net returns NULL.  In that case I return -ENXIO because
effectively the network namespace has already gone away so the files
we are trying to access don't exist anymore.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Acked-by: Paul E. McKenney <paulmck@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Eric W. Biederman 2007-09-13 09:18:57 +02:00 • committed by David S. Miller
parent 4fabcd7118
commit 077130c0cf
7 changed files with 39 additions and 8 deletions
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6
fs/proc/proc_net.c
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@ -51,6 +51,12 @@ void proc_net_remove(struct net *net, const char *name)
}
EXPORT_SYMBOL_GPL(proc_net_remove);
struct net *get_proc_net(const struct inode *inode)
{
return maybe_get_net(PDE_NET(PDE(inode)));
}
EXPORT_SYMBOL_GPL(get_proc_net);
static struct proc_dir_entry *proc_net_shadow;
static struct dentry *proc_net_shadow_dentry(struct dentry *parent,