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336cae73cc
Channel masquerading had a conflict with autochannel locking. When locking autochannel->channel, the channel is fetched from the autochannel and then locked. During the fetch, the autochannel -- which has no locks itself -- can be modified by someone who owns the channel lock. That means that the value of autochan->channel cannot be trusted until you hold the lock. In practice, this caused problems with Local channels getting masqueraded away while the ChanSpy attempted to get info from that channel. The old channel which was about to get removed got locked, but the new (replaced) channel got unlocked (no-op). Because the replaced channel was now locked (and would never get unlocked), it couldn't get removed from the channel list in a timely manner, and would now cause deadlocks when iterating over the channel list. This change checks the autochannel after locking the channel for changes to the autochannel. If the channel had been changed, the lock is reobtained on the new channel. In theory it seems possible that after this fix, the lock attempt on the old (wrong) channel can be on an already destroyed lock, maybe causing a crash. But that hasn't been observed in the wild and is harder induce than the current deadlock. Thanks go to Filip Frank for suggesting a fix similar to this and especially to IRC user hexanol for pointing out why this deadlock was possible and testing this fix. And to Richard for catching my rookie while loop mistake ;) ASTERISK-25321 #close Change-Id: I293ae0014e531cd0e675c3f02d1d118a98683def
134 lines
4.4 KiB
C
134 lines
4.4 KiB
C
/*
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* Asterisk -- An open source telephony toolkit.
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*
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* Copyright (C) 2009, Digium, Inc.
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*
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* Mark Michelson <mmichelson@digium.com>
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*
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* See http://www.asterisk.org for more information about
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* the Asterisk project. Please do not directly contact
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* any of the maintainers of this project for assistance;
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* the project provides a web site, mailing lists and IRC
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* channels for your use.
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*
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* This program is free software, distributed under the terms of
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* the GNU General Public License Version 2. See the LICENSE file
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* at the top of the source tree.
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*/
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/*!
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* \file
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* \brief "smart" channels that update automatically if a channel is masqueraded
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*
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* \author Mark Michelson <mmichelson@digium.com>
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*/
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#include "asterisk.h"
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#include "asterisk/linkedlists.h"
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#ifndef _ASTERISK_AUTOCHAN_H
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#define _ASTERISK_AUTOCHAN_H
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struct ast_autochan {
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struct ast_channel *chan;
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AST_LIST_ENTRY(ast_autochan) list;
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};
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/*!
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* \par Just what the $!@# is an autochan?
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*
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* An ast_autochan is a structure which contains an ast_channel. The pointer
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* inside an autochan has the ability to update itself if the channel it points
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* to is masqueraded into a different channel.
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*
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* This has a great benefit for any application or service which creates a thread
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* outside of the channel's main operating thread which keeps a pointer to said
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* channel. when a masquerade occurs on the channel, the autochan's chan pointer
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* will automatically update to point to the new channel.
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*
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* Some rules for autochans
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*
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* 1. If you are going to use an autochan, then be sure to always refer to the
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* channel using the chan pointer inside the autochan if possible, since this is
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* the pointer that will be updated during a masquerade.
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*
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* 2. If you are going to save off a pointer to the autochan's chan, then be sure
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* to save off the pointer using ast_channel_ref and to unref the channel when you
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* are finished with the pointer. If you do not do this and a masquerade occurs on
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* the channel, then it is possible that your saved pointer will become invalid.
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*
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* 3. If you want to lock the autochan->chan channel, be sure to use
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* ast_autochan_channel_lock and ast_autochan_channel_unlock. An attempt to lock
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* the autochan->chan directly may result in it being changed after you've
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* retrieved the value of chan, but before you've had a chance to lock it.
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* First when chan is locked, the autochan structure is guaranteed to keep the
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* same channel.
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*/
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#define ast_autochan_channel_lock(autochan) \
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do { \
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struct ast_channel *autochan_chan = autochan->chan; \
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ast_channel_lock(autochan_chan); \
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if (autochan->chan == autochan_chan) { \
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break; \
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} \
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ast_channel_unlock(autochan_chan); \
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} while (1)
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#define ast_autochan_channel_unlock(autochan) \
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ast_channel_unlock(autochan->chan)
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/*!
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* \brief set up a new ast_autochan structure
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*
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* \details
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* Allocates and initializes an ast_autochan, sets the
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* autochan's chan pointer to point to the chan parameter, and
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* adds the autochan to the global list of autochans. The newly-
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* created autochan is returned to the caller. This function will
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* cause the refcount of chan to increase by 1.
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*
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* \param chan The channel our new autochan will point to
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*
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* \note autochans must be freed using ast_autochan_destroy
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*
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* \retval NULL Failure
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* \retval non-NULL success
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*/
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struct ast_autochan *ast_autochan_setup(struct ast_channel *chan);
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/*!
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* \brief destroy an ast_autochan structure
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*
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* \details
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* Removes the passed-in autochan from the list of autochans and
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* unrefs the channel that is pointed to. Also frees the autochan
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* struct itself. This function will unref the channel reference
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* which was made in ast_autochan_setup
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*
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* \param autochan The autochan that you wish to destroy
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*
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* \retval void
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*/
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void ast_autochan_destroy(struct ast_autochan *autochan);
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/*!
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* \brief Switch what channel autochans point to
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*
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* \details
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* Traverses the list of autochans. All autochans which point to
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* old_chan will be updated to point to new_chan instead. Currently
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* this is only called during an ast_channel_move() operation in
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* channel.c.
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*
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* \pre Both channels must be locked before calling this function.
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*
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* \param old_chan The channel that autochans may currently point to
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* \param new_chan The channel that we want to point those autochans to now
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*
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* \retval void
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*/
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void ast_autochan_new_channel(struct ast_channel *old_chan, struct ast_channel *new_chan);
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#endif /* _ASTERISK_AUTOCHAN_H */
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